ClickHouse/programs/copier/ClusterCopier.cpp

#include "ClusterCopier.h"

#include "Internals.h"
#include "StatusAccumulator.h"

#include <Common/ZooKeeper/ZooKeeper.h>
#include <Common/ZooKeeper/KeeperException.h>
#include <Common/setThreadName.h>
#include <Interpreters/InterpreterInsertQuery.h>
#include <Interpreters/InterpreterSelectWithUnionQuery.h>
#include <Parsers/ASTFunction.h>
#include <Processors/Transforms/ExpressionTransform.h>
#include <QueryPipeline/QueryPipelineBuilder.h>
#include <QueryPipeline/Chain.h>
#include <Processors/Executors/PullingPipelineExecutor.h>
#include <Processors/Executors/PushingPipelineExecutor.h>
#include <Processors/Sources/RemoteSource.h>
#include <Processors/QueryPlan/QueryPlan.h>
#include <Processors/QueryPlan/BuildQueryPipelineSettings.h>
#include <Processors/QueryPlan/Optimizations/QueryPlanOptimizationSettings.h>

namespace DB
{

namespace ErrorCodes
{
    extern const int NOT_IMPLEMENTED;
    extern const int LOGICAL_ERROR;
    extern const int UNFINISHED;
    extern const int BAD_ARGUMENTS;
}


void ClusterCopier::init()
{
    auto zookeeper = getContext()->getZooKeeper();

    task_description_watch_callback = [this] (const Coordination::WatchResponse & response)
    {
        if (response.error != Coordination::Error::ZOK)
            return;
        UInt64 version = ++task_description_version;
        LOG_INFO(log, "Task description should be updated, local version {}", version);
    };

    task_description_path = task_zookeeper_path + "/description";
    task_cluster = std::make_unique<TaskCluster>(task_zookeeper_path, working_database_name);

    reloadTaskDescription();

    task_cluster->loadTasks(*task_cluster_current_config);
    getContext()->setClustersConfig(task_cluster_current_config, false, task_cluster->clusters_prefix);

    /// Set up shards and their priority
    task_cluster->random_engine.seed(task_cluster->random_device());
    for (auto & task_table : task_cluster->table_tasks)
    {
        task_table.cluster_pull = getContext()->getCluster(task_table.cluster_pull_name);
        task_table.cluster_push = getContext()->getCluster(task_table.cluster_push_name);
        task_table.initShards(task_cluster->random_engine);
    }

    LOG_INFO(log, "Will process {} table tasks", task_cluster->table_tasks.size());

    /// Do not initialize tables, will make deferred initialization in process()

    zookeeper->createAncestors(getWorkersPathVersion() + "/");
    zookeeper->createAncestors(getWorkersPath() + "/");
    /// Init status node
    zookeeper->createIfNotExists(task_zookeeper_path + "/status", "{}");
}

template <typename T>
decltype(auto) ClusterCopier::retry(T && func, UInt64 max_tries)
{
    std::exception_ptr exception;

    if (max_tries == 0)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Cannot perform zero retries");

    for (UInt64 try_number = 1; try_number <= max_tries; ++try_number)
    {
        try
        {
            return func();
        }
        catch (...)
        {
            exception = std::current_exception();
            if (try_number < max_tries)
            {
                tryLogCurrentException(log, "Will retry");
                std::this_thread::sleep_for(retry_delay_ms);
            }
        }
    }

    std::rethrow_exception(exception);
}


void ClusterCopier::discoverShardPartitions(const ConnectionTimeouts & timeouts, const TaskShardPtr & task_shard)
{
    TaskTable & task_table = task_shard->task_table;

    LOG_INFO(log, "Discover partitions of shard {}", task_shard->getDescription());

    auto get_partitions = [&] () { return getShardPartitions(timeouts, *task_shard); };
    auto existing_partitions_names = retry(get_partitions, 60);
    Strings filtered_partitions_names;
    Strings missing_partitions;

    /// Check that user specified correct partition names
    auto check_partition_format = [] (const DataTypePtr & type, const String & partition_text_quoted)
    {
        MutableColumnPtr column_dummy = type->createColumn();
        ReadBufferFromString rb(partition_text_quoted);

        try
        {
            type->getDefaultSerialization()->deserializeTextQuoted(*column_dummy, rb, FormatSettings());
        }
        catch (Exception & e)
        {
            throw Exception(ErrorCodes::BAD_ARGUMENTS, "Partition {} has incorrect format. {}", partition_text_quoted, e.displayText());
        }
    };

    if (task_table.has_enabled_partitions)
    {
        /// Process partition in order specified by <enabled_partitions/>
        for (const String & partition_name : task_table.enabled_partitions)
        {
            /// Check that user specified correct partition names
            check_partition_format(task_shard->partition_key_column.type, partition_name);

            auto it = existing_partitions_names.find(partition_name);

            /// Do not process partition if it is not in enabled_partitions list
            if (it == existing_partitions_names.end())
            {
                missing_partitions.emplace_back(partition_name);
                continue;
            }

            filtered_partitions_names.emplace_back(*it);
        }

        for (const String & partition_name : existing_partitions_names)
        {
            if (!task_table.enabled_partitions_set.contains(partition_name))
            {
                LOG_INFO(log, "Partition {} will not be processed, since it is not in enabled_partitions of {}", partition_name, task_table.table_id);
            }
        }
    }
    else
    {
        for (const String & partition_name : existing_partitions_names)
            filtered_partitions_names.emplace_back(partition_name);
    }

    for (const String & partition_name : filtered_partitions_names)
    {
        const size_t number_of_splits = task_table.number_of_splits;
        task_shard->partition_tasks.emplace(partition_name, ShardPartition(*task_shard, partition_name, number_of_splits));
        task_shard->checked_partitions.emplace(partition_name, true);

        auto shard_partition_it = task_shard->partition_tasks.find(partition_name);
        PartitionPieces & shard_partition_pieces = shard_partition_it->second.pieces;

        for (size_t piece_number = 0; piece_number < number_of_splits; ++piece_number)
        {
            bool res = checkPresentPartitionPiecesOnCurrentShard(timeouts, *task_shard, partition_name, piece_number);
            shard_partition_pieces.emplace_back(shard_partition_it->second, piece_number, res);
        }
    }

    if (!missing_partitions.empty())
    {
        WriteBufferFromOwnString ss;
        for (const String & missing_partition : missing_partitions)
            ss << " " << missing_partition;

        LOG_WARNING(log, "There are no {} partitions from enabled_partitions in shard {} :{}", missing_partitions.size(), task_shard->getDescription(), ss.str());
    }

    LOG_INFO(log, "Will copy {} partitions from shard {}", task_shard->partition_tasks.size(), task_shard->getDescription());
}

void ClusterCopier::discoverTablePartitions(const ConnectionTimeouts & timeouts, TaskTable & task_table, UInt64 num_threads)
{
    /// Fetch partitions list from a shard
    {
        ThreadPool thread_pool(num_threads ? num_threads : 2 * getNumberOfPhysicalCPUCores());

        for (const TaskShardPtr & task_shard : task_table.all_shards)
            thread_pool.scheduleOrThrowOnError([this, timeouts, task_shard]()
            {
                setThreadName("DiscoverPartns");
                discoverShardPartitions(timeouts, task_shard);
            });

        LOG_INFO(log, "Waiting for {} setup jobs", thread_pool.active());
        thread_pool.wait();
    }
}

void ClusterCopier::uploadTaskDescription(const std::string & task_path, const std::string & task_file, const bool force)
{
    auto local_task_description_path = task_path + "/description";

    String task_config_str;
    {
        ReadBufferFromFile in(task_file);
        readStringUntilEOF(task_config_str, in);
    }
    if (task_config_str.empty())
        return;

    auto zookeeper = getContext()->getZooKeeper();

    zookeeper->createAncestors(local_task_description_path);
    auto code = zookeeper->tryCreate(local_task_description_path, task_config_str, zkutil::CreateMode::Persistent);
    if (code != Coordination::Error::ZOK && force)
        zookeeper->createOrUpdate(local_task_description_path, task_config_str, zkutil::CreateMode::Persistent);

    LOG_INFO(log, "Task description {} uploaded to {} with result {} ({})",
        ((code != Coordination::Error::ZOK && !force) ? "not " : ""), local_task_description_path, code, Coordination::errorMessage(code));
}

void ClusterCopier::reloadTaskDescription()
{
    auto zookeeper = getContext()->getZooKeeper();
    task_description_watch_zookeeper = zookeeper;

    Coordination::Stat stat{};

    /// It will throw exception if such a node doesn't exist.
    auto task_config_str = zookeeper->get(task_description_path, &stat);

    LOG_INFO(log, "Loading task description");
    task_cluster_current_config = getConfigurationFromXMLString(task_config_str);

    /// Setup settings
    task_cluster->reloadSettings(*task_cluster_current_config);
    getContext()->setSettings(task_cluster->settings_common);
}

void ClusterCopier::updateConfigIfNeeded()
{
    UInt64 version_to_update = task_description_version;
    bool is_outdated_version = task_description_current_version != version_to_update;
    bool is_expired_session  = !task_description_watch_zookeeper || task_description_watch_zookeeper->expired();

    if (!is_outdated_version && !is_expired_session)
        return;

    LOG_INFO(log, "Updating task description");
    reloadTaskDescription();

    task_description_current_version = version_to_update;
}

void ClusterCopier::process(const ConnectionTimeouts & timeouts)
{
    for (TaskTable & task_table : task_cluster->table_tasks)
    {
        LOG_INFO(log, "Process table task {} with {} shards, {} of them are local ones", task_table.table_id, task_table.all_shards.size(), task_table.local_shards.size());

        if (task_table.all_shards.empty())
            continue;

        /// Discover partitions of each shard and total set of partitions
        if (!task_table.has_enabled_partitions)
        {
            /// If there are no specified enabled_partitions, we must discover them manually
            discoverTablePartitions(timeouts, task_table);

            /// After partitions of each shard are initialized, initialize cluster partitions
            for (const TaskShardPtr & task_shard : task_table.all_shards)
            {
                for (const auto & partition_elem : task_shard->partition_tasks)
                {
                    const String & partition_name = partition_elem.first;
                    task_table.cluster_partitions.emplace(partition_name, ClusterPartition{});
                }
            }

            for (auto & partition_elem : task_table.cluster_partitions)
            {
                const String & partition_name = partition_elem.first;

                for (const TaskShardPtr & task_shard : task_table.all_shards)
                    task_shard->checked_partitions.emplace(partition_name);

                task_table.ordered_partition_names.emplace_back(partition_name);
            }
        }
        else
        {
            /// If enabled_partitions are specified, assume that each shard has all partitions
            /// We will refine partition set of each shard in future

            for (const String & partition_name : task_table.enabled_partitions)
            {
                task_table.cluster_partitions.emplace(partition_name, ClusterPartition{});
                task_table.ordered_partition_names.emplace_back(partition_name);
            }
        }

        task_table.watch.restart();

        /// Retry table processing
        bool table_is_done = false;
        for (UInt64 num_table_tries = 1; num_table_tries <= max_table_tries; ++num_table_tries)
        {
            if (tryProcessTable(timeouts, task_table))
            {
                table_is_done = true;
                break;
            }
        }

        if (!table_is_done)
        {
            throw Exception(ErrorCodes::UNFINISHED, "Too many tries to process table {}. Abort remaining execution",
                            task_table.table_id);
        }
    }
}

/// Protected section


/*
 * Creates task worker node and checks maximum number of workers not to exceed the limit.
 * To achieve this we have to check version of workers_version_path node and create current_worker_path
 * node atomically.
 * */

zkutil::EphemeralNodeHolder::Ptr ClusterCopier::createTaskWorkerNodeAndWaitIfNeed(
    const zkutil::ZooKeeperPtr & zookeeper,
    const String & description,
    bool unprioritized)
{
    std::chrono::milliseconds current_sleep_time = retry_delay_ms;
    static constexpr std::chrono::milliseconds max_sleep_time(30000); // 30 sec

    if (unprioritized)
        std::this_thread::sleep_for(current_sleep_time);

    String workers_version_path = getWorkersPathVersion();
    String workers_path         = getWorkersPath();
    String current_worker_path  = getCurrentWorkerNodePath();

    UInt64 num_bad_version_errors = 0;

    while (true)
    {
        updateConfigIfNeeded();

        Coordination::Stat stat;
        zookeeper->get(workers_version_path, &stat);
        auto version = stat.version;
        zookeeper->get(workers_path, &stat);

        if (static_cast<UInt64>(stat.numChildren) >= task_cluster->max_workers)
        {
            LOG_INFO(log, "Too many workers ({}, maximum {}). Postpone processing {}", stat.numChildren, task_cluster->max_workers, description);

            if (unprioritized)
                current_sleep_time = std::min(max_sleep_time, current_sleep_time + retry_delay_ms);

            std::this_thread::sleep_for(current_sleep_time);
            num_bad_version_errors = 0;
        }
        else
        {
            Coordination::Requests ops;
            ops.emplace_back(zkutil::makeSetRequest(workers_version_path, description, version));
            ops.emplace_back(zkutil::makeCreateRequest(current_worker_path, description, zkutil::CreateMode::Ephemeral));
            Coordination::Responses responses;
            auto code = zookeeper->tryMulti(ops, responses);

            if (code == Coordination::Error::ZOK || code == Coordination::Error::ZNODEEXISTS)
                return std::make_shared<zkutil::EphemeralNodeHolder>(current_worker_path, *zookeeper, false, false, description);

            if (code == Coordination::Error::ZBADVERSION)
            {
                ++num_bad_version_errors;

                /// Try to make fast retries
                if (num_bad_version_errors > 3)
                {
                    LOG_INFO(log, "A concurrent worker has just been added, will check free worker slots again");
                    std::chrono::milliseconds random_sleep_time(std::uniform_int_distribution<int>(1, 1000)(task_cluster->random_engine));
                    std::this_thread::sleep_for(random_sleep_time);
                    num_bad_version_errors = 0;
                }
            }
            else
                throw Coordination::Exception(code);
        }
    }
}


bool ClusterCopier::checkPartitionPieceIsClean(
        const zkutil::ZooKeeperPtr & zookeeper,
        const CleanStateClock & clean_state_clock,
        const String & task_status_path)
{
    LogicalClock task_start_clock;

    Coordination::Stat stat{};
    if (zookeeper->exists(task_status_path, &stat))
        task_start_clock = LogicalClock(stat.mzxid);

    return clean_state_clock.is_clean() && (!task_start_clock.hasHappened() || clean_state_clock.discovery_zxid <= task_start_clock);
}


bool ClusterCopier::checkAllPiecesInPartitionAreDone(const TaskTable & task_table, const String & partition_name, const TasksShard & shards_with_partition)
{
    bool answer = true;
    for (size_t piece_number = 0; piece_number < task_table.number_of_splits; ++piece_number)
    {
        bool piece_is_done = checkPartitionPieceIsDone(task_table, partition_name, piece_number, shards_with_partition);
        if (!piece_is_done)
            LOG_INFO(log, "Partition {} piece {} is not already done.", partition_name, piece_number);
        answer &= piece_is_done;
    }

    return answer;
}


/* The same as function above
 * Assume that we don't know on which shards do we have partition certain piece.
 * We'll check them all (I mean shards that contain the whole partition)
 * And shards that don't have certain piece MUST mark that piece is_done true.
 * */
bool ClusterCopier::checkPartitionPieceIsDone(const TaskTable & task_table, const String & partition_name,
                               size_t piece_number, const TasksShard & shards_with_partition)
{
    LOG_INFO(log, "Check that all shards processed partition {} piece {} successfully", partition_name, piece_number);

    auto zookeeper = getContext()->getZooKeeper();

    /// Collect all shards that contain partition piece number piece_number.
    Strings piece_status_paths;
    for (const auto & shard : shards_with_partition)
    {
        ShardPartition & task_shard_partition = shard->partition_tasks.find(partition_name)->second;
        ShardPartitionPiece & shard_partition_piece = task_shard_partition.pieces[piece_number];
        piece_status_paths.emplace_back(shard_partition_piece.getShardStatusPath());
    }

    std::vector<int64_t> zxid1, zxid2;

    try
    {
        std::vector<zkutil::ZooKeeper::FutureGet> get_futures;
        for (const String & path : piece_status_paths)
            get_futures.emplace_back(zookeeper->asyncGet(path));

        // Check that state is Finished and remember zxid
        for (auto & future : get_futures)
        {
            auto res = future.get();

            TaskStateWithOwner status = TaskStateWithOwner::fromString(res.data);
            if (status.state != TaskState::Finished)
            {
                LOG_INFO(log, "The task {} is being rewritten by {}. Partition piece will be rechecked", res.data, status.owner);
                return false;
            }

            zxid1.push_back(res.stat.pzxid);
        }

        const String piece_is_dirty_flag_path = task_table.getCertainPartitionPieceIsDirtyPath(partition_name, piece_number);
        const String piece_is_dirty_cleaned_path = task_table.getCertainPartitionPieceIsCleanedPath(partition_name, piece_number);
        const String piece_task_status_path = task_table.getCertainPartitionPieceTaskStatusPath(partition_name, piece_number);

        CleanStateClock clean_state_clock (zookeeper, piece_is_dirty_flag_path, piece_is_dirty_cleaned_path);

        const bool is_clean = checkPartitionPieceIsClean(zookeeper, clean_state_clock, piece_task_status_path);


        if (!is_clean)
        {
            LOG_INFO(log, "Partition {} become dirty", partition_name);
            return false;
        }

        get_futures.clear();
        for (const String & path : piece_status_paths)
            get_futures.emplace_back(zookeeper->asyncGet(path));

        // Remember zxid of states again
        for (auto & future : get_futures)
        {
            auto res = future.get();
            zxid2.push_back(res.stat.pzxid);
        }
    }
    catch (const Coordination::Exception & e)
    {
        LOG_INFO(log, "A ZooKeeper error occurred while checking partition {} piece number {}. Will recheck the partition. Error: {}", partition_name, toString(piece_number), e.displayText());
        return false;
    }

    // If all task is finished and zxid is not changed then partition could not become dirty again
    for (UInt64 shard_num = 0; shard_num < piece_status_paths.size(); ++shard_num)
    {
        if (zxid1[shard_num] != zxid2[shard_num])
        {
            LOG_INFO(log, "The task {} is being modified now. Partition piece will be rechecked", piece_status_paths[shard_num]);
            return false;
        }
    }

    LOG_INFO(log, "Partition {} piece number {} is copied successfully", partition_name, toString(piece_number));
    return true;
}


TaskStatus ClusterCopier::tryMoveAllPiecesToDestinationTable(const TaskTable & task_table, const String & partition_name)
{
    bool inject_fault = false;
    if (move_fault_probability > 0)
    {
        double value = std::uniform_real_distribution<>(0, 1)(task_table.task_cluster.random_engine);
        inject_fault = value < move_fault_probability;
    }

    LOG_INFO(log, "Try to move {} to destination table", partition_name);

    auto zookeeper = getContext()->getZooKeeper();

    const auto current_partition_attach_is_active = task_table.getPartitionAttachIsActivePath(partition_name);
    const auto current_partition_attach_is_done   = task_table.getPartitionAttachIsDonePath(partition_name);

    /// Create ephemeral node to mark that we are active and process the partition
    zookeeper->createAncestors(current_partition_attach_is_active);
    zkutil::EphemeralNodeHolderPtr partition_attach_node_holder;
    try
    {
        partition_attach_node_holder = zkutil::EphemeralNodeHolder::create(current_partition_attach_is_active, *zookeeper, host_id);
    }
    catch (const Coordination::Exception & e)
    {
        if (e.code == Coordination::Error::ZNODEEXISTS)
        {
            LOG_INFO(log, "Someone is already moving pieces {}", current_partition_attach_is_active);
            return TaskStatus::Active;
        }

        throw;
    }


    /// Exit if task has been already processed;
    /// create blocking node to signal cleaning up if it is abandoned
    {
        String status_data;
        if (zookeeper->tryGet(current_partition_attach_is_done, status_data))
        {
            TaskStateWithOwner status = TaskStateWithOwner::fromString(status_data);
            if (status.state == TaskState::Finished)
            {
                LOG_INFO(log, "All pieces for partition from this task {} has been successfully moved to destination table by {}", current_partition_attach_is_active, status.owner);
                return TaskStatus::Finished;
            }

            /// Task is abandoned, because previously we created ephemeral node, possibly in other copier's process.
            /// Initialize DROP PARTITION
            LOG_INFO(log, "Moving piece for partition {} has not been successfully finished by {}. Will try to move by myself.", current_partition_attach_is_active, status.owner);

            /// Remove is_done marker.
            zookeeper->remove(current_partition_attach_is_done);
        }
    }


    /// Try start processing, create node about it
    {
        String start_state = TaskStateWithOwner::getData(TaskState::Started, host_id);
        zookeeper->create(current_partition_attach_is_done, start_state, zkutil::CreateMode::Persistent);
    }


    /// Try to drop destination partition in original table
    if (task_table.allow_to_drop_target_partitions)
    {
        DatabaseAndTableName original_table = task_table.table_push;

        WriteBufferFromOwnString ss;
        ss << "ALTER TABLE " << getQuotedTable(original_table) << ((partition_name == "'all'") ? " DROP PARTITION ID " : " DROP PARTITION ") << partition_name;

        UInt64 num_shards_drop_partition = executeQueryOnCluster(task_table.cluster_push, ss.str(), task_cluster->settings_push, ClusterExecutionMode::ON_EACH_SHARD);

        LOG_INFO(log, "Drop partition {} in original table {} have been executed successfully on {} shards of {}",
            partition_name, getQuotedTable(original_table), num_shards_drop_partition, task_table.cluster_push->getShardCount());
    }

    /// Move partition to original destination table.
    for (size_t current_piece_number = 0; current_piece_number < task_table.number_of_splits; ++current_piece_number)
    {
        LOG_INFO(log, "Trying to move partition {} piece {} to original table", partition_name, toString(current_piece_number));

        ASTPtr query_alter_ast;
        String query_alter_ast_string;

        DatabaseAndTableName original_table = task_table.table_push;
        DatabaseAndTableName helping_table = DatabaseAndTableName(original_table.first,
                                                                  original_table.second + "_piece_" +
                                                                  toString(current_piece_number));

        Settings settings_push = task_cluster->settings_push;
        ClusterExecutionMode execution_mode = ClusterExecutionMode::ON_EACH_NODE;

        if (settings_push.alter_sync == 1)
            execution_mode = ClusterExecutionMode::ON_EACH_SHARD;

        query_alter_ast_string += " ALTER TABLE " + getQuotedTable(original_table) +
                                  ((partition_name == "'all'") ? " ATTACH PARTITION ID " : " ATTACH PARTITION ") + partition_name +
                                  " FROM " + getQuotedTable(helping_table);

        LOG_INFO(log, "Executing ALTER query: {}", query_alter_ast_string);

        try
        {
            /// Try attach partition on each shard
            UInt64 num_nodes = executeQueryOnCluster(
                task_table.cluster_push,
                query_alter_ast_string,
                task_cluster->settings_push,
                execution_mode);

            if (settings_push.alter_sync == 1)
            {
                LOG_INFO(
                    log,
                    "Destination tables {} have been executed alter query successfully on {} shards of {}",
                    getQuotedTable(task_table.table_push),
                    num_nodes,
                    task_table.cluster_push->getShardCount());

                if (num_nodes != task_table.cluster_push->getShardCount())
                    return TaskStatus::Error;
            }
            else
            {
                LOG_INFO(log, "Number of nodes that executed ALTER query successfully : {}", toString(num_nodes));
            }
        }
        catch (...)
        {
            LOG_INFO(log, "Error while moving partition {} piece {} to original table", partition_name, toString(current_piece_number));
            LOG_WARNING(log, "In case of non-replicated tables it can cause duplicates.");
            throw;
        }

        if (inject_fault)
            throw Exception(ErrorCodes::UNFINISHED, "Copy fault injection is activated");
    }

    /// Create node to signal that we finished moving
    /// Also increment a counter of processed partitions
    {
        const auto state_finished = TaskStateWithOwner::getData(TaskState::Finished, host_id);
        const auto task_status = task_zookeeper_path + "/status";

        /// Try until success
        while (true)
        {
            Coordination::Stat stat;
            auto status_json = zookeeper->get(task_status, &stat);
            auto statuses = StatusAccumulator::fromJSON(status_json);

            /// Increment status for table.
            (*statuses)[task_table.name_in_config].processed_partitions_count += 1;
            auto statuses_to_commit = StatusAccumulator::serializeToJSON(statuses);

            Coordination::Requests ops;
            ops.emplace_back(zkutil::makeSetRequest(current_partition_attach_is_done, state_finished, 0));
            ops.emplace_back(zkutil::makeSetRequest(task_status, statuses_to_commit, stat.version));

            Coordination::Responses responses;
            Coordination::Error code = zookeeper->tryMulti(ops, responses);

            if (code == Coordination::Error::ZOK)
                break;
        }
    }

    return TaskStatus::Finished;
}

/// This is needed to create internal Distributed table
/// Removes column's TTL expression from `CREATE` query
/// Removes MATEREALIZED or ALIAS columns not to copy additional and useless data over the network.
/// Removes data skipping indices.
ASTPtr ClusterCopier::removeAliasMaterializedAndTTLColumnsFromCreateQuery(const ASTPtr & query_ast, bool allow_to_copy_alias_and_materialized_columns)
{
    const ASTs & column_asts = query_ast->as<ASTCreateQuery &>().columns_list->columns->children;
    auto new_columns = std::make_shared<ASTExpressionList>();

    for (const ASTPtr & column_ast : column_asts)
    {
        const auto & column = column_ast->as<ASTColumnDeclaration &>();

        /// Skip this columns
        if (!column.default_specifier.empty() && !allow_to_copy_alias_and_materialized_columns)
        {
            ColumnDefaultKind kind = columnDefaultKindFromString(column.default_specifier);
            if (kind == ColumnDefaultKind::Materialized || kind == ColumnDefaultKind::Alias)
                continue;
        }

        /// Remove TTL on columns definition.
        auto new_column_ast = column_ast->clone();
        auto & new_column = new_column_ast->as<ASTColumnDeclaration &>();
        if (new_column.ttl)
            new_column.ttl.reset();

        new_columns->children.emplace_back(new_column_ast);
    }

    ASTPtr new_query_ast = query_ast->clone();
    auto & new_query = new_query_ast->as<ASTCreateQuery &>();

    auto new_columns_list = std::make_shared<ASTColumns>();
    new_columns_list->set(new_columns_list->columns, new_columns);

    /// Skip indices and projections are not needed, because distributed table doesn't support it.

    new_query.replace(new_query.columns_list, new_columns_list);

    return new_query_ast;
}

/// Replaces ENGINE and table name in a create query
std::shared_ptr<ASTCreateQuery> rewriteCreateQueryStorage(const ASTPtr & create_query_ast,
                                                          const DatabaseAndTableName & new_table,
                                                          const ASTPtr & new_storage_ast)
{
    const auto & create = create_query_ast->as<ASTCreateQuery &>();
    auto res = std::make_shared<ASTCreateQuery>(create);

    if (create.storage == nullptr || new_storage_ast == nullptr)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Storage is not specified");

    res->setDatabase(new_table.first);
    res->setTable(new_table.second);

    res->children.clear();
    res->set(res->columns_list, create.columns_list->clone());
    res->set(res->storage, new_storage_ast->clone());
    /// Just to make it better and don't store additional flag like `is_table_created` somewhere else
    res->if_not_exists = true;

    return res;
}


bool ClusterCopier::tryDropPartitionPiece(
        ShardPartition & task_partition,
        const size_t current_piece_number,
        const zkutil::ZooKeeperPtr & zookeeper,
        const CleanStateClock & clean_state_clock)
{
    if (is_safe_mode)
        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "DROP PARTITION is prohibited in safe mode");

    TaskTable & task_table = task_partition.task_shard.task_table;
    ShardPartitionPiece & partition_piece = task_partition.pieces[current_piece_number];

    const String current_shards_path                  = partition_piece.getPartitionPieceShardsPath();
    const String current_partition_active_workers_dir = partition_piece.getPartitionPieceActiveWorkersPath();
    const String is_dirty_flag_path                   = partition_piece.getPartitionPieceIsDirtyPath();
    const String dirty_cleaner_path                   = partition_piece.getPartitionPieceCleanerPath();
    const String is_dirty_cleaned_path                = partition_piece.getPartitionPieceIsCleanedPath();

    zkutil::EphemeralNodeHolder::Ptr cleaner_holder;
    try
    {
        cleaner_holder = zkutil::EphemeralNodeHolder::create(dirty_cleaner_path, *zookeeper, host_id);
    }
    catch (const Coordination::Exception & e)
    {
        if (e.code == Coordination::Error::ZNODEEXISTS)
        {
            LOG_INFO(log, "Partition {} piece {} is cleaning now by somebody, sleep", task_partition.name, toString(current_piece_number));
            std::this_thread::sleep_for(retry_delay_ms);
            return false;
        }

        throw;
    }

    Coordination::Stat stat{};
    if (zookeeper->exists(current_partition_active_workers_dir, &stat))
    {
        if (stat.numChildren != 0)
        {
            LOG_INFO(log, "Partition {} contains {} active workers while trying to drop it. Going to sleep.", task_partition.name, stat.numChildren);
            std::this_thread::sleep_for(retry_delay_ms);
            return false;
        }
        else
        {
            zookeeper->remove(current_partition_active_workers_dir);
        }
    }

    {
        zkutil::EphemeralNodeHolder::Ptr active_workers_lock;
        try
        {
            active_workers_lock = zkutil::EphemeralNodeHolder::create(current_partition_active_workers_dir, *zookeeper, host_id);
        }
        catch (const Coordination::Exception & e)
        {
            if (e.code == Coordination::Error::ZNODEEXISTS)
            {
                LOG_INFO(log, "Partition {} is being filled now by somebody, sleep", task_partition.name);
                return false;
            }

            throw;
        }

        // Lock the dirty flag
        zookeeper->set(is_dirty_flag_path, host_id, clean_state_clock.discovery_version.value());
        zookeeper->tryRemove(partition_piece.getPartitionPieceCleanStartPath());
        CleanStateClock my_clock(zookeeper, is_dirty_flag_path, is_dirty_cleaned_path);

        /// Remove all status nodes
        {
            Strings children;
            if (zookeeper->tryGetChildren(current_shards_path, children) == Coordination::Error::ZOK)
                for (const auto & child : children)
                {
                    zookeeper->removeRecursive(current_shards_path + "/" + child);
                }
        }


        DatabaseAndTableName original_table = task_table.table_push;
        DatabaseAndTableName helping_table = DatabaseAndTableName(original_table.first, original_table.second + "_piece_" + toString(current_piece_number));

        String query = "ALTER TABLE " + getQuotedTable(helping_table);
        query += ((task_partition.name == "'all'") ? " DROP PARTITION ID " : " DROP PARTITION ")  + task_partition.name + "";

        /// TODO: use this statement after servers will be updated up to 1.1.54310
        // query += " DROP PARTITION ID '" + task_partition.name + "'";

        ClusterPtr & cluster_push = task_table.cluster_push;
        Settings settings_push = task_cluster->settings_push;

        /// It is important, DROP PARTITION must be done synchronously
        settings_push.alter_sync = 2;

        LOG_INFO(log, "Execute distributed DROP PARTITION: {}", query);
        /// We have to drop partition_piece on each replica
        size_t num_shards = executeQueryOnCluster(
                cluster_push, query,
                settings_push,
                ClusterExecutionMode::ON_EACH_NODE);

        LOG_INFO(log, "DROP PARTITION was successfully executed on {} nodes of a cluster.", num_shards);

        /// Update the locking node
        if (!my_clock.is_stale())
        {
            zookeeper->set(is_dirty_flag_path, host_id, my_clock.discovery_version.value());
            if (my_clock.clean_state_version)
                zookeeper->set(is_dirty_cleaned_path, host_id, my_clock.clean_state_version.value());
            else
                zookeeper->create(is_dirty_cleaned_path, host_id, zkutil::CreateMode::Persistent);
        }
        else
        {
            LOG_INFO(log, "Clean state is altered when dropping the partition, cowardly bailing");
            /// clean state is stale
            return false;
        }

        LOG_INFO(log, "Partition {} piece {} was dropped on cluster {}", task_partition.name, toString(current_piece_number), task_table.cluster_push_name);
        if (zookeeper->tryCreate(current_shards_path, host_id, zkutil::CreateMode::Persistent) == Coordination::Error::ZNODEEXISTS)
            zookeeper->set(current_shards_path, host_id);
    }

    LOG_INFO(log, "Partition {} piece {} is safe for work now.", task_partition.name, toString(current_piece_number));
    return true;
}

bool ClusterCopier::tryProcessTable(const ConnectionTimeouts & timeouts, TaskTable & task_table)
{
    /// Create destination table
    TaskStatus task_status = TaskStatus::Error;

    task_status = tryCreateDestinationTable(timeouts, task_table);
    /// Exit if success
    if (task_status != TaskStatus::Finished)
    {
        LOG_WARNING(log, "Create destination table failed ");
        return false;
    }

    /// Set all_partitions_count for table in Zookeeper
    auto zookeeper = getContext()->getZooKeeper();
    while (true)
    {
        Coordination::Stat stat;
        auto status_json = zookeeper->get(task_zookeeper_path + "/status", &stat);
        auto statuses = StatusAccumulator::fromJSON(status_json);

        /// Exit if someone already set the initial value for this table.
        if (statuses->find(task_table.name_in_config) != statuses->end())
            break;
        (*statuses)[task_table.name_in_config] = StatusAccumulator::TableStatus
        {
            /*all_partitions_count=*/task_table.ordered_partition_names.size(),
            /*processed_partition_count=*/0
        };

        auto statuses_to_commit = StatusAccumulator::serializeToJSON(statuses);
        auto error = zookeeper->trySet(task_zookeeper_path + "/status", statuses_to_commit, stat.version);
        if (error == Coordination::Error::ZOK)
            break;
    }


    /// An heuristic: if previous shard is already done, then check next one without sleeps due to max_workers constraint
    bool previous_shard_is_instantly_finished = false;

    /// Process each partition that is present in cluster
    for (const String & partition_name : task_table.ordered_partition_names)
    {
        if (!task_table.cluster_partitions.contains(partition_name))
            throw Exception(ErrorCodes::LOGICAL_ERROR, "There are no expected partition {}. It is a bug", partition_name);

        ClusterPartition & cluster_partition = task_table.cluster_partitions[partition_name];

        Stopwatch watch;
        /// We will check all the shards of the table and check if they contain current partition.
        TasksShard expected_shards;
        UInt64 num_failed_shards = 0;

        ++cluster_partition.total_tries;

        LOG_INFO(log, "Processing partition {} for the whole cluster", partition_name);

        /// Process each source shard having current partition and copy current partition
        /// NOTE: shards are sorted by "distance" to current host
        bool has_shard_to_process = false;
        for (const TaskShardPtr & shard : task_table.all_shards)
        {
            /// Does shard have a node with current partition?
            if (!shard->partition_tasks.contains(partition_name))
            {
                /// If not, did we check existence of that partition previously?
                if (!shard->checked_partitions.contains(partition_name))
                {
                    auto check_shard_has_partition = [&] () { return checkShardHasPartition(timeouts, *shard, partition_name); };
                    bool has_partition = retry(check_shard_has_partition);

                    shard->checked_partitions.emplace(partition_name);

                    if (has_partition)
                    {
                        const size_t number_of_splits = task_table.number_of_splits;
                        shard->partition_tasks.emplace(partition_name, ShardPartition(*shard, partition_name, number_of_splits));
                        LOG_INFO(log, "Discovered partition {} in shard {}", partition_name, shard->getDescription());
                        /// To save references in the future.
                        auto shard_partition_it = shard->partition_tasks.find(partition_name);
                        PartitionPieces & shard_partition_pieces = shard_partition_it->second.pieces;

                        for (size_t piece_number = 0; piece_number < number_of_splits; ++piece_number)
                        {
                            auto res = checkPresentPartitionPiecesOnCurrentShard(timeouts, *shard, partition_name, piece_number);
                            shard_partition_pieces.emplace_back(shard_partition_it->second, piece_number, res);
                        }
                    }
                    else
                    {
                        LOG_INFO(log, "Found that shard {} does not contain current partition {}", shard->getDescription(), partition_name);
                        continue;
                    }
                }
                else
                {
                    /// We have already checked that partition, but did not discover it
                    previous_shard_is_instantly_finished = true;
                    continue;
                }
            }

            auto it_shard_partition = shard->partition_tasks.find(partition_name);
            /// Previously when we discovered that shard does not contain current partition, we skipped it.
            /// At this moment partition have to be present.
            if (it_shard_partition == shard->partition_tasks.end())
                throw Exception(ErrorCodes::LOGICAL_ERROR, "There are no such partition in a shard. This is a bug.");
            auto & partition = it_shard_partition->second;

            expected_shards.emplace_back(shard);

            /// Do not sleep if there is a sequence of already processed shards to increase startup
            bool is_unprioritized_task = !previous_shard_is_instantly_finished && shard->priority.is_remote;
            task_status = TaskStatus::Error;
            bool was_error = false;
            has_shard_to_process = true;
            for (UInt64 try_num = 1; try_num <= max_shard_partition_tries; ++try_num)
            {
                task_status = tryProcessPartitionTask(timeouts, partition, is_unprioritized_task);

                /// Exit if success
                if (task_status == TaskStatus::Finished)
                    break;

                was_error = true;

                /// Skip if the task is being processed by someone
                if (task_status == TaskStatus::Active)
                    break;

                /// Repeat on errors
                std::this_thread::sleep_for(retry_delay_ms);
            }

            if (task_status == TaskStatus::Error)
                ++num_failed_shards;

            previous_shard_is_instantly_finished = !was_error;
        }

        cluster_partition.elapsed_time_seconds += watch.elapsedSeconds();

        /// Check that whole cluster partition is done
        /// Firstly check the number of failed partition tasks, then look into ZooKeeper and ensure that each partition is done
        bool partition_copying_is_done = num_failed_shards == 0;
        try
        {
            partition_copying_is_done =
                    !has_shard_to_process
                    || (partition_copying_is_done && checkAllPiecesInPartitionAreDone(task_table, partition_name, expected_shards));
        }
        catch (...)
        {
            tryLogCurrentException(log);
            partition_copying_is_done = false;
        }


        bool partition_moving_is_done = false;
        /// Try to move only if all pieces were copied.
        if (partition_copying_is_done)
        {
            for (UInt64 try_num = 0; try_num < max_shard_partition_piece_tries_for_alter; ++try_num)
            {
                try
                {
                    auto res = tryMoveAllPiecesToDestinationTable(task_table, partition_name);
                    /// Exit and mark current task is done.
                    if (res == TaskStatus::Finished)
                    {
                        partition_moving_is_done = true;
                        break;
                    }

                    /// Exit if this task is active.
                    if (res == TaskStatus::Active)
                        break;

                    /// Repeat on errors.
                    std::this_thread::sleep_for(retry_delay_ms);
                }
                catch (...)
                {
                    tryLogCurrentException(log, "Some error occurred while moving pieces to destination table for partition " + partition_name);
                }
            }
        }

        if (partition_copying_is_done && partition_moving_is_done)
        {
            task_table.finished_cluster_partitions.emplace(partition_name);

            task_table.bytes_copied += cluster_partition.bytes_copied;
            task_table.rows_copied += cluster_partition.rows_copied;
            double elapsed = cluster_partition.elapsed_time_seconds;

            LOG_INFO(log, "It took {} seconds to copy partition {}: {} uncompressed bytes, {} rows and {} source blocks are copied",
                elapsed, partition_name,
                formatReadableSizeWithDecimalSuffix(cluster_partition.bytes_copied),
                formatReadableQuantity(cluster_partition.rows_copied),
                cluster_partition.blocks_copied);

            if (cluster_partition.rows_copied)
            {
                LOG_INFO(log, "Average partition speed: {} per second.", formatReadableSizeWithDecimalSuffix(cluster_partition.bytes_copied / elapsed));
            }

            if (task_table.rows_copied)
            {
                LOG_INFO(log, "Average table {} speed: {} per second.", task_table.table_id, formatReadableSizeWithDecimalSuffix(task_table.bytes_copied / elapsed));
            }
        }
    }

    UInt64 required_partitions = task_table.cluster_partitions.size();
    UInt64 finished_partitions = task_table.finished_cluster_partitions.size();
    bool table_is_done = finished_partitions >= required_partitions;

    if (!table_is_done)
    {
        LOG_INFO(log, "Table {} is not processed yet. Copied {} of {}, will retry", task_table.table_id, finished_partitions, required_partitions);
    }
    else
    {
        /// Delete helping tables in case that whole table is done
        dropHelpingTables(task_table);
    }

    return table_is_done;
}

TaskStatus ClusterCopier::tryCreateDestinationTable(const ConnectionTimeouts & timeouts, TaskTable & task_table)
{
    /// Try create original table (if not exists) on each shard

    //TaskTable & task_table = task_shard.task_table;
    const TaskShardPtr task_shard = task_table.all_shards.at(0);
    /// We need to update table definitions for each part, it could be changed after ALTER
    task_shard->current_pull_table_create_query = getCreateTableForPullShard(timeouts, *task_shard);
    try
    {
        auto create_query_push_ast
            = rewriteCreateQueryStorage(task_shard->current_pull_table_create_query, task_table.table_push, task_table.engine_push_ast);
        auto & create = create_query_push_ast->as<ASTCreateQuery &>();
        create.if_not_exists = true;
        InterpreterCreateQuery::prepareOnClusterQuery(create, getContext(), task_table.cluster_push_name);
        String query = queryToString(create_query_push_ast);

        LOG_INFO(log, "Create destination tables. Query: {}", query);
        UInt64 shards = executeQueryOnCluster(task_table.cluster_push, query, task_cluster->settings_push, ClusterExecutionMode::ON_EACH_NODE);
        LOG_INFO(
            log,
            "Destination tables {} have been created on {} shards of {}",
            getQuotedTable(task_table.table_push),
            shards,
            task_table.cluster_push->getShardCount());
    }
    catch (...)
    {
        tryLogCurrentException(log, "Error while creating original table. Maybe we are not first.");
    }

    return TaskStatus::Finished;
}

/// Job for copying partition from particular shard.
TaskStatus ClusterCopier::tryProcessPartitionTask(const ConnectionTimeouts & timeouts, ShardPartition & task_partition, bool is_unprioritized_task)
{
    TaskStatus res;

    try
    {
        res = iterateThroughAllPiecesInPartition(timeouts, task_partition, is_unprioritized_task);
    }
    catch (...)
    {
        tryLogCurrentException(log, "An error occurred while processing partition " + task_partition.name);
        res = TaskStatus::Error;
    }

    /// At the end of each task check if the config is updated
    try
    {
        updateConfigIfNeeded();
    }
    catch (...)
    {
        tryLogCurrentException(log, "An error occurred while updating the config");
    }

    return res;
}

TaskStatus ClusterCopier::iterateThroughAllPiecesInPartition(const ConnectionTimeouts & timeouts, ShardPartition & task_partition,
                                                       bool is_unprioritized_task)
{
    const size_t total_number_of_pieces = task_partition.task_shard.task_table.number_of_splits;

    TaskStatus res{TaskStatus::Finished};

    bool was_failed_pieces = false;
    bool was_active_pieces = false;

    for (size_t piece_number = 0; piece_number < total_number_of_pieces; piece_number++)
    {
        for (UInt64 try_num = 0; try_num < max_shard_partition_tries; ++try_num)
        {
            LOG_INFO(log, "Attempt number {} to process partition {} piece number {} on shard number {} with index {}.",
                try_num, task_partition.name, piece_number,
                task_partition.task_shard.numberInCluster(),
                task_partition.task_shard.indexInCluster());

            res = processPartitionPieceTaskImpl(timeouts, task_partition, piece_number, is_unprioritized_task);

            /// Exit if success
            if (res == TaskStatus::Finished)
                break;

            /// Skip if the task is being processed by someone
            if (res == TaskStatus::Active)
                break;

            /// Repeat on errors
            std::this_thread::sleep_for(retry_delay_ms);
        }

        was_active_pieces |= (res == TaskStatus::Active);
        was_failed_pieces |= (res == TaskStatus::Error);
    }

    if (was_failed_pieces)
        return TaskStatus::Error;

    if (was_active_pieces)
        return TaskStatus::Active;

    return TaskStatus::Finished;
}


TaskStatus ClusterCopier::processPartitionPieceTaskImpl(
        const ConnectionTimeouts & timeouts, ShardPartition & task_partition,
        const size_t current_piece_number, bool is_unprioritized_task)
{
    TaskShard & task_shard = task_partition.task_shard;
    TaskTable & task_table = task_shard.task_table;
    ClusterPartition & cluster_partition  = task_table.getClusterPartition(task_partition.name);
    ShardPartitionPiece & partition_piece = task_partition.pieces[current_piece_number];

    const size_t number_of_splits = task_table.number_of_splits;
    const String primary_key_comma_separated = task_table.primary_key_comma_separated;

    /// We need to update table definitions for each partition, it could be changed after ALTER
    createShardInternalTables(timeouts, task_shard, true);

    auto split_table_for_current_piece = task_shard.list_of_split_tables_on_shard[current_piece_number];

    auto zookeeper = getContext()->getZooKeeper();

    const String piece_is_dirty_flag_path          = partition_piece.getPartitionPieceIsDirtyPath();
    const String piece_is_dirty_cleaned_path       = partition_piece.getPartitionPieceIsCleanedPath();
    const String current_task_piece_is_active_path = partition_piece.getActiveWorkerPath();
    const String current_task_piece_status_path    = partition_piece.getShardStatusPath();

    /// Auxiliary functions:

    /// Creates is_dirty node to initialize DROP PARTITION
    auto create_is_dirty_node = [&] (const CleanStateClock & clock)
    {
        if (clock.is_stale())
            LOG_INFO(log, "Clean state clock is stale while setting dirty flag, cowardly bailing");
        else if (!clock.is_clean())
            LOG_INFO(log, "Thank you, Captain Obvious");
        else if (clock.discovery_version)
        {
            LOG_INFO(log, "Updating clean state clock");
            zookeeper->set(piece_is_dirty_flag_path, host_id, clock.discovery_version.value());
        }
        else
        {
            LOG_INFO(log, "Creating clean state clock");
            zookeeper->create(piece_is_dirty_flag_path, host_id, zkutil::CreateMode::Persistent);
        }
    };

    /// Returns SELECT query filtering current partition and applying user filter
    auto get_select_query = [&] (const DatabaseAndTableName & from_table, const String & fields, bool enable_splitting, String limit = "")
    {
        String query;
        query += "WITH " + task_partition.name + " AS partition_key ";
        query += "SELECT " + fields + " FROM " + getQuotedTable(from_table);

        if (enable_splitting && experimental_use_sample_offset)
            query += " SAMPLE 1/" + toString(number_of_splits) + " OFFSET " + toString(current_piece_number) + "/" + toString(number_of_splits);

        /// TODO: Bad, it is better to rewrite with ASTLiteral(partition_key_field)
        query += " WHERE (" + queryToString(task_table.engine_push_partition_key_ast) + " = partition_key)";

        if (enable_splitting && !experimental_use_sample_offset)
            query += " AND ( cityHash64(" + primary_key_comma_separated + ") %" + toString(number_of_splits) + " = " + toString(current_piece_number) + " )";

        if (!task_table.where_condition_str.empty())
            query += " AND (" + task_table.where_condition_str + ")";

        if (!limit.empty())
            query += " LIMIT " + limit;

        query += " FORMAT Native";

        ParserQuery p_query(query.data() + query.size());

        const auto & settings = getContext()->getSettingsRef();
        return parseQuery(p_query, query, settings.max_query_size, settings.max_parser_depth);
    };

    /// Load balancing
    auto worker_node_holder = createTaskWorkerNodeAndWaitIfNeed(zookeeper, current_task_piece_status_path, is_unprioritized_task);

    LOG_INFO(log, "Processing {}", current_task_piece_status_path);

    const String piece_status_path = partition_piece.getPartitionPieceShardsPath();

    CleanStateClock clean_state_clock(zookeeper, piece_is_dirty_flag_path, piece_is_dirty_cleaned_path);

    const bool is_clean = checkPartitionPieceIsClean(zookeeper, clean_state_clock, piece_status_path);

    /// Do not start if partition piece is dirty, try to clean it
    if (is_clean)
    {
        LOG_INFO(log, "Partition {} piece {} appears to be clean", task_partition.name, current_piece_number);
        zookeeper->createAncestors(current_task_piece_status_path);
    }
    else
    {
        LOG_INFO(log, "Partition {} piece {} is dirty, try to drop it", task_partition.name, current_piece_number);

        try
        {
            tryDropPartitionPiece(task_partition, current_piece_number, zookeeper, clean_state_clock);
        }
        catch (...)
        {
            tryLogCurrentException(log, "An error occurred when clean partition");
        }

        return TaskStatus::Error;
    }

    /// Create ephemeral node to mark that we are active and process the partition
    zookeeper->createAncestors(current_task_piece_is_active_path);
    zkutil::EphemeralNodeHolderPtr partition_task_node_holder;
    try
    {
        partition_task_node_holder = zkutil::EphemeralNodeHolder::create(current_task_piece_is_active_path, *zookeeper, host_id);
    }
    catch (const Coordination::Exception & e)
    {
        if (e.code == Coordination::Error::ZNODEEXISTS)
        {
            LOG_INFO(log, "Someone is already processing {}", current_task_piece_is_active_path);
            return TaskStatus::Active;
        }

        throw;
    }

    /// Exit if task has been already processed;
    /// create blocking node to signal cleaning up if it is abandoned
    {
        String status_data;
        if (zookeeper->tryGet(current_task_piece_status_path, status_data))
        {
            TaskStateWithOwner status = TaskStateWithOwner::fromString(status_data);
            if (status.state == TaskState::Finished)
            {
                LOG_INFO(log, "Task {} has been successfully executed by {}", current_task_piece_status_path, status.owner);
                return TaskStatus::Finished;
            }

            /// Task is abandoned, because previously we created ephemeral node, possibly in other copier's process.
            /// Initialize DROP PARTITION
            LOG_INFO(log, "Task {} has not been successfully finished by {}. Partition will be dropped and refilled.", current_task_piece_status_path, status.owner);

            create_is_dirty_node(clean_state_clock);
            return TaskStatus::Error;
        }
    }


    /// Try create table (if not exists) on each shard
    /// We have to create this table even in case that partition piece is empty
    /// This is significant, because we will have simpler code
    {
        /// 1) Get columns description from any replica of destination cluster
        /// 2) Change ENGINE, database and table name
        /// 3) Create helping table on the whole destination cluster
        auto & settings_push = task_cluster->settings_push;

        auto connection = task_table.cluster_push->getAnyShardInfo().pool->get(timeouts, &settings_push, true);
        String create_query = getRemoteCreateTable(task_shard.task_table.table_push, *connection, settings_push);

        ParserCreateQuery parser_create_query;
        auto create_query_ast = parseQuery(parser_create_query, create_query, settings_push.max_query_size, settings_push.max_parser_depth);
        /// Define helping table database and name for current partition piece
        DatabaseAndTableName database_and_table_for_current_piece
        {
            task_table.table_push.first,
            task_table.table_push.second + "_piece_" + toString(current_piece_number)
        };


        auto new_engine_push_ast = task_table.engine_push_ast;
        if (task_table.isReplicatedTable())
            new_engine_push_ast = task_table.rewriteReplicatedCreateQueryToPlain();

        /// Take columns definition from destination table, new database and table name, and new engine (non replicated variant of MergeTree)
        auto create_query_push_ast = rewriteCreateQueryStorage(create_query_ast, database_and_table_for_current_piece, new_engine_push_ast);
        String query = queryToString(create_query_push_ast);

        LOG_INFO(log, "Create destination tables. Query: {}", query);
        UInt64 shards = executeQueryOnCluster(task_table.cluster_push, query, task_cluster->settings_push, ClusterExecutionMode::ON_EACH_NODE);
        LOG_INFO(
            log,
            "Destination tables {} have been created on {} shards of {}",
            getQuotedTable(task_table.table_push),
            shards,
            task_table.cluster_push->getShardCount());
    }


    /// Exit if current piece is absent on this shard. Also mark it as finished, because we will check
    /// whether each shard have processed each partitition (and its pieces).
    if (partition_piece.is_absent_piece)
    {
        String state_finished = TaskStateWithOwner::getData(TaskState::Finished, host_id);
        auto res = zookeeper->tryCreate(current_task_piece_status_path, state_finished, zkutil::CreateMode::Persistent);
        if (res == Coordination::Error::ZNODEEXISTS)
            LOG_INFO(log, "Partition {} piece {} is absent on current replica of a shard. But other replicas have already marked it as done.", task_partition.name, current_piece_number);
        if (res == Coordination::Error::ZOK)
            LOG_INFO(log, "Partition {} piece {} is absent on current replica of a shard. Will mark it as done. Other replicas will do the same.", task_partition.name, current_piece_number);
        return TaskStatus::Finished;
    }

    /// Check that destination partition is empty if we are first worker
    /// NOTE: this check is incorrect if pull and push tables have different partition key!
    String clean_start_status;
    if (!zookeeper->tryGet(partition_piece.getPartitionPieceCleanStartPath(), clean_start_status) || clean_start_status != "ok")
    {
        zookeeper->createIfNotExists(partition_piece.getPartitionPieceCleanStartPath(), "");
        auto checker = zkutil::EphemeralNodeHolder::create(partition_piece.getPartitionPieceCleanStartPath() + "/checker",
                                                           *zookeeper, host_id);
        // Maybe we are the first worker

        ASTPtr query_select_ast = get_select_query(split_table_for_current_piece, "count()", /* enable_splitting= */ true);
        UInt64 count;
        {
            auto local_context = Context::createCopy(context);
            // Use pull (i.e. readonly) settings, but fetch data from destination servers
            local_context->setSettings(task_cluster->settings_pull);
            local_context->setSetting("skip_unavailable_shards", true);

            InterpreterSelectWithUnionQuery select(query_select_ast, local_context, SelectQueryOptions{});
            QueryPlan plan;
            select.buildQueryPlan(plan);
            auto builder = std::move(*plan.buildQueryPipeline(
                QueryPlanOptimizationSettings::fromContext(local_context),
                BuildQueryPipelineSettings::fromContext(local_context)));

            Block block = getBlockWithAllStreamData(std::move(builder));
            count = (block) ? block.safeGetByPosition(0).column->getUInt(0) : 0;
        }

        if (count != 0)
        {
            LOG_INFO(log, "Partition {} piece {} is not empty. In contains {} rows.", task_partition.name, current_piece_number, count);
            Coordination::Stat stat_shards{};
            zookeeper->get(partition_piece.getPartitionPieceShardsPath(), &stat_shards);

            /// NOTE: partition is still fresh if dirt discovery happens before cleaning
            if (stat_shards.numChildren == 0)
            {
                LOG_WARNING(log, "There are no workers for partition {} piece {}, but destination table contains {} rows. Partition will be dropped and refilled.", task_partition.name, toString(current_piece_number), count);

                create_is_dirty_node(clean_state_clock);
                return TaskStatus::Error;
            }
        }
        zookeeper->set(partition_piece.getPartitionPieceCleanStartPath(), "ok");
    }
    /// At this point, we need to sync that the destination table is clean
    /// before any actual work

    /// Try start processing, create node about it
    {
        String start_state = TaskStateWithOwner::getData(TaskState::Started, host_id);
        CleanStateClock new_clean_state_clock(zookeeper, piece_is_dirty_flag_path, piece_is_dirty_cleaned_path);
        if (clean_state_clock != new_clean_state_clock)
        {
            LOG_INFO(log, "Partition {} piece {} clean state changed, cowardly bailing", task_partition.name, toString(current_piece_number));
            return TaskStatus::Error;
        }
        else if (!new_clean_state_clock.is_clean())
        {
            LOG_INFO(log, "Partition {} piece {} is dirty and will be dropped and refilled", task_partition.name, toString(current_piece_number));
            create_is_dirty_node(new_clean_state_clock);
            return TaskStatus::Error;
        }
        zookeeper->create(current_task_piece_status_path, start_state, zkutil::CreateMode::Persistent);
    }


    /// Do the copying
    {
        bool inject_fault = false;
        if (copy_fault_probability > 0)
        {
            double value = std::uniform_real_distribution<>(0, 1)(task_table.task_cluster.random_engine);
            inject_fault = value < copy_fault_probability;
        }

        // Select all fields
        ASTPtr query_select_ast = get_select_query(task_shard.table_read_shard, "*", /* enable_splitting= */ true, inject_fault ? "1" : "");

        LOG_INFO(log, "Executing SELECT query and pull from {}: {}", task_shard.getDescription(), queryToString(query_select_ast));

        ASTPtr query_insert_ast;
        {
            String query;
            query += "INSERT INTO " + getQuotedTable(split_table_for_current_piece) + " FORMAT Native  ";

            ParserQuery p_query(query.data() + query.size());
            const auto & settings = getContext()->getSettingsRef();
            query_insert_ast = parseQuery(p_query, query, settings.max_query_size, settings.max_parser_depth);

            LOG_INFO(log, "Executing INSERT query: {}", query);
        }

        try
        {
            auto context_select = Context::createCopy(context);
            context_select->setSettings(task_cluster->settings_pull);

            auto context_insert = Context::createCopy(context);
            context_insert->setSettings(task_cluster->settings_push);

            /// Custom INSERT SELECT implementation
            QueryPipeline input;
            QueryPipeline output;
            {
                BlockIO io_insert = InterpreterFactory::get(query_insert_ast, context_insert)->execute();

                InterpreterSelectWithUnionQuery select(query_select_ast, context_select, SelectQueryOptions{});
                QueryPlan plan;
                select.buildQueryPlan(plan);
                auto builder = std::move(*plan.buildQueryPipeline(
                    QueryPlanOptimizationSettings::fromContext(context_select),
                    BuildQueryPipelineSettings::fromContext(context_select)));

                output = std::move(io_insert.pipeline);

                /// Add converting actions to make it possible to copy blocks with slightly different schema
                const auto & select_block = builder.getHeader();
                const auto & insert_block = output.getHeader();
                auto actions_dag = ActionsDAG::makeConvertingActions(
                        select_block.getColumnsWithTypeAndName(),
                        insert_block.getColumnsWithTypeAndName(),
                        ActionsDAG::MatchColumnsMode::Position);

                auto actions = std::make_shared<ExpressionActions>(actions_dag, ExpressionActionsSettings::fromContext(getContext()));

                builder.addSimpleTransform([&](const Block & header)
                {
                    return std::make_shared<ExpressionTransform>(header, actions);
                });
                input = QueryPipelineBuilder::getPipeline(std::move(builder));
            }

            /// Fail-fast optimization to abort copying when the current clean state expires
            std::future<Coordination::ExistsResponse> future_is_dirty_checker;

            Stopwatch watch(CLOCK_MONOTONIC_COARSE);
            constexpr UInt64 check_period_milliseconds = 500;

            /// Will asynchronously check that ZooKeeper connection and is_dirty flag appearing while copying data
            auto cancel_check = [&] ()
            {
                if (zookeeper->expired())
                    throw Exception(ErrorCodes::UNFINISHED, "ZooKeeper session is expired, cancel INSERT SELECT");

                if (!future_is_dirty_checker.valid())
                    future_is_dirty_checker = zookeeper->asyncExists(piece_is_dirty_flag_path);

                /// check_period_milliseconds should less than average insert time of single block
                /// Otherwise, the insertion will slow a little bit
                if (watch.elapsedMilliseconds() >= check_period_milliseconds)
                {
                    Coordination::ExistsResponse status = future_is_dirty_checker.get();

                    if (status.error != Coordination::Error::ZNONODE)
                    {
                        LogicalClock dirt_discovery_epoch (status.stat.mzxid);
                        if (dirt_discovery_epoch == clean_state_clock.discovery_zxid)
                            return false;
                        throw Exception(ErrorCodes::UNFINISHED, "Partition is dirty, cancel INSERT SELECT");
                    }
                }

                return false;
            };

            /// Update statistics
            /// It is quite rough: bytes_copied don't take into account DROP PARTITION.
            auto update_stats = [&cluster_partition] (const Block & block)
            {
                cluster_partition.bytes_copied += block.bytes();
                cluster_partition.rows_copied += block.rows();
                cluster_partition.blocks_copied += 1;
            };

            /// Main work is here
            PullingPipelineExecutor pulling_executor(input);
            PushingPipelineExecutor pushing_executor(output);

            Block data;
            bool is_cancelled = false;
            while (pulling_executor.pull(data))
            {
                if (cancel_check())
                {
                    is_cancelled = true;
                    pushing_executor.cancel();
                    pushing_executor.cancel();
                    break;
                }
                pushing_executor.push(data);
                update_stats(data);
            }

            if (!is_cancelled)
                pushing_executor.finish();

            // Just in case
            if (future_is_dirty_checker.valid())
                future_is_dirty_checker.get();

            if (inject_fault)
                throw Exception(ErrorCodes::UNFINISHED, "Copy fault injection is activated");
        }
        catch (...)
        {
            tryLogCurrentException(log, "An error occurred during copying, partition will be marked as dirty");
            create_is_dirty_node(clean_state_clock);
            return TaskStatus::Error;
        }
    }

    LOG_INFO(log, "Partition {} piece {} copied. But not moved to original destination table.", task_partition.name, toString(current_piece_number));

    /// Finalize the processing, change state of current partition task (and also check is_dirty flag)
    {
        String state_finished = TaskStateWithOwner::getData(TaskState::Finished, host_id);
        CleanStateClock new_clean_state_clock (zookeeper, piece_is_dirty_flag_path, piece_is_dirty_cleaned_path);
        if (clean_state_clock != new_clean_state_clock)
        {
            LOG_INFO(log, "Partition {} piece {} clean state changed, cowardly bailing", task_partition.name, toString(current_piece_number));
            return TaskStatus::Error;
        }
        else if (!new_clean_state_clock.is_clean())
        {
            LOG_INFO(log, "Partition {} piece {} became dirty and will be dropped and refilled", task_partition.name, toString(current_piece_number));
            create_is_dirty_node(new_clean_state_clock);
            return TaskStatus::Error;
        }
        zookeeper->set(current_task_piece_status_path, state_finished, 0);
    }

    return TaskStatus::Finished;
}

void ClusterCopier::dropAndCreateLocalTable(const ASTPtr & create_ast)
{
    const auto & create = create_ast->as<ASTCreateQuery &>();
    dropLocalTableIfExists({create.getDatabase(), create.getTable()});

    auto create_context = Context::createCopy(getContext());

    InterpreterCreateQuery interpreter(create_ast, create_context);
    interpreter.execute();
}

void ClusterCopier::dropLocalTableIfExists(const DatabaseAndTableName & table_name) const
{
    auto drop_ast = std::make_shared<ASTDropQuery>();
    drop_ast->if_exists = true;
    drop_ast->setDatabase(table_name.first);
    drop_ast->setTable(table_name.second);

    auto drop_context = Context::createCopy(getContext());

    InterpreterDropQuery interpreter(drop_ast, drop_context);
    interpreter.execute();
}

void ClusterCopier::dropHelpingTablesByPieceNumber(const TaskTable & task_table, size_t current_piece_number)
{
    LOG_INFO(log, "Removing helping tables piece {}", current_piece_number);

    DatabaseAndTableName original_table = task_table.table_push;
    DatabaseAndTableName helping_table
        = DatabaseAndTableName(original_table.first, original_table.second + "_piece_" + toString(current_piece_number));

    String query = "DROP TABLE IF EXISTS " + getQuotedTable(helping_table);

    const ClusterPtr & cluster_push = task_table.cluster_push;
    Settings settings_push = task_cluster->settings_push;

    LOG_INFO(log, "Execute distributed DROP TABLE: {}", query);

    /// We have to drop partition_piece on each replica
    UInt64 num_nodes = executeQueryOnCluster(cluster_push, query, settings_push, ClusterExecutionMode::ON_EACH_NODE);

    LOG_INFO(log, "DROP TABLE query was successfully executed on {} nodes.", toString(num_nodes));
}

void ClusterCopier::dropHelpingTables(const TaskTable & task_table)
{
    LOG_INFO(log, "Removing helping tables");
    for (size_t current_piece_number = 0; current_piece_number < task_table.number_of_splits; ++current_piece_number)
    {
        dropHelpingTablesByPieceNumber(task_table, current_piece_number);
    }
}

void ClusterCopier::dropParticularPartitionPieceFromAllHelpingTables(const TaskTable & task_table, const String & partition_name)
{
    LOG_INFO(log, "Try drop partition partition from all helping tables.");
    for (size_t current_piece_number = 0; current_piece_number < task_table.number_of_splits; ++current_piece_number)
    {
        DatabaseAndTableName original_table = task_table.table_push;
        DatabaseAndTableName helping_table = DatabaseAndTableName(original_table.first, original_table.second + "_piece_" + toString(current_piece_number));

        String query = "ALTER TABLE " + getQuotedTable(helping_table) + ((partition_name == "'all'") ? " DROP PARTITION ID " : " DROP PARTITION ") + partition_name;

        const ClusterPtr & cluster_push = task_table.cluster_push;
        Settings settings_push = task_cluster->settings_push;

        LOG_INFO(log, "Execute distributed DROP PARTITION: {}", query);
        /// We have to drop partition_piece on each replica
        UInt64 num_nodes = executeQueryOnCluster(
                cluster_push, query,
                settings_push,
                ClusterExecutionMode::ON_EACH_NODE);

        LOG_INFO(log, "DROP PARTITION query was successfully executed on {} nodes.", toString(num_nodes));
    }
    LOG_INFO(log, "All helping tables dropped partition {}", partition_name);
}

String ClusterCopier::getRemoteCreateTable(const DatabaseAndTableName & table, Connection & connection, const Settings & settings)
{
    auto remote_context = Context::createCopy(context);
    remote_context->setSettings(settings);

    String query = "SHOW CREATE TABLE " + getQuotedTable(table);

    QueryPipelineBuilder builder;
    builder.init(Pipe(std::make_shared<RemoteSource>(
            std::make_shared<RemoteQueryExecutor>(connection, query, InterpreterShowCreateQuery::getSampleBlock(), remote_context), false, false)));
    Block block = getBlockWithAllStreamData(std::move(builder));
    return typeid_cast<const ColumnString &>(*block.safeGetByPosition(0).column).getDataAt(0).toString();
}


ASTPtr ClusterCopier::getCreateTableForPullShard(const ConnectionTimeouts & timeouts, TaskShard & task_shard)
{
    /// Fetch and parse (possibly) new definition
    auto connection_entry = task_shard.info.pool->get(timeouts, &task_cluster->settings_pull, true);
    String create_query_pull_str = getRemoteCreateTable(
            task_shard.task_table.table_pull,
            *connection_entry,
            task_cluster->settings_pull);

    ParserCreateQuery parser_create_query;
    const auto & settings = getContext()->getSettingsRef();
    return parseQuery(parser_create_query, create_query_pull_str, settings.max_query_size, settings.max_parser_depth);
}


/// If it is implicitly asked to create split Distributed table for certain piece on current shard, we will do it.
void ClusterCopier::createShardInternalTables(const ConnectionTimeouts & timeouts,
        TaskShard & task_shard, bool create_split)
{
    TaskTable & task_table = task_shard.task_table;

    /// We need to update table definitions for each part, it could be changed after ALTER
    task_shard.current_pull_table_create_query = getCreateTableForPullShard(timeouts, task_shard);

    /// Create local Distributed tables:
    ///  a table fetching data from current shard and a table inserting data to the whole destination cluster
    String read_shard_prefix = ".read_shard_" + toString(task_shard.indexInCluster()) + ".";
    String split_shard_prefix = ".split.";
    task_shard.table_read_shard = DatabaseAndTableName(working_database_name, read_shard_prefix + task_table.table_id);
    task_shard.main_table_split_shard = DatabaseAndTableName(working_database_name, split_shard_prefix + task_table.table_id);

    for (const auto & piece_number : collections::range(0, task_table.number_of_splits))
    {
        task_shard.list_of_split_tables_on_shard[piece_number] =
                DatabaseAndTableName(working_database_name, split_shard_prefix + task_table.table_id + "_piece_" + toString(piece_number));
    }

    /// Create special cluster with single shard
    String shard_read_cluster_name = read_shard_prefix + task_table.cluster_pull_name;
    ClusterPtr cluster_pull_current_shard = task_table.cluster_pull->getClusterWithSingleShard(task_shard.indexInCluster());
    getContext()->setCluster(shard_read_cluster_name, cluster_pull_current_shard);

    auto storage_shard_ast = createASTStorageDistributed(shard_read_cluster_name, task_table.table_pull.first, task_table.table_pull.second);

    auto create_query_ast = removeAliasMaterializedAndTTLColumnsFromCreateQuery(
        task_shard.current_pull_table_create_query,
        task_table.allow_to_copy_alias_and_materialized_columns);

    auto create_table_pull_ast = rewriteCreateQueryStorage(create_query_ast, task_shard.table_read_shard, storage_shard_ast);
    dropAndCreateLocalTable(create_table_pull_ast);

    if (create_split)
    {
        auto create_table_split_piece_ast = rewriteCreateQueryStorage(
                create_query_ast,
                task_shard.main_table_split_shard,
                task_table.main_engine_split_ast);

        dropAndCreateLocalTable(create_table_split_piece_ast);

        /// Create auxiliary split tables for each piece
        for (const auto & piece_number : collections::range(0, task_table.number_of_splits))
        {
            const auto & storage_piece_split_ast = task_table.auxiliary_engine_split_asts[piece_number];

            create_table_split_piece_ast = rewriteCreateQueryStorage(
                    create_query_ast,
                    task_shard.list_of_split_tables_on_shard[piece_number],
                    storage_piece_split_ast);

            dropAndCreateLocalTable(create_table_split_piece_ast);
        }
    }

}


std::set<String> ClusterCopier::getShardPartitions(const ConnectionTimeouts & timeouts, TaskShard & task_shard)
{
    std::set<String> res;

    createShardInternalTables(timeouts, task_shard, false);

    TaskTable & task_table = task_shard.task_table;

    const String & partition_name = queryToString(task_table.engine_push_partition_key_ast);

    if (partition_name == "'all'")
    {
        res.emplace("'all'");
        return res;
    }

    String query;
    {
        WriteBufferFromOwnString wb;
        wb << "SELECT " << partition_name << " AS partition FROM "
           << getQuotedTable(task_shard.table_read_shard) << " GROUP BY partition ORDER BY partition DESC";
        query = wb.str();
    }

    ParserQuery parser_query(query.data() + query.size());
    const auto & settings = getContext()->getSettingsRef();
    ASTPtr query_ast = parseQuery(parser_query, query, settings.max_query_size, settings.max_parser_depth);

    LOG_INFO(log, "Computing destination partition set, executing query: {}", query);

    auto local_context = Context::createCopy(context);
    local_context->setSettings(task_cluster->settings_pull);
    InterpreterSelectWithUnionQuery select(query_ast, local_context, SelectQueryOptions{});
    QueryPlan plan;
    select.buildQueryPlan(plan);
    auto builder = std::move(*plan.buildQueryPipeline(
        QueryPlanOptimizationSettings::fromContext(local_context),
        BuildQueryPipelineSettings::fromContext(local_context)));

    Block block = getBlockWithAllStreamData(std::move(builder));

    if (block)
    {
        ColumnWithTypeAndName & column = block.getByPosition(0);
        task_shard.partition_key_column = column;

        for (size_t i = 0; i < column.column->size(); ++i)
        {
            WriteBufferFromOwnString wb;
            column.type->getDefaultSerialization()->serializeTextQuoted(*column.column, i, wb, FormatSettings());
            res.emplace(wb.str());
        }
    }

    LOG_INFO(log, "There are {} destination partitions in shard {}", res.size(), task_shard.getDescription());

    return res;
}

bool ClusterCopier::checkShardHasPartition(const ConnectionTimeouts & timeouts,
        TaskShard & task_shard, const String & partition_quoted_name)
{
    createShardInternalTables(timeouts, task_shard, false);

    TaskTable & task_table = task_shard.task_table;

    WriteBufferFromOwnString ss;
    ss << "WITH " + partition_quoted_name + " AS partition_key ";
    ss << "SELECT 1 FROM " << getQuotedTable(task_shard.table_read_shard);
    ss << " WHERE (" << queryToString(task_table.engine_push_partition_key_ast) << " = partition_key)";
    if (!task_table.where_condition_str.empty())
        ss << " AND (" << task_table.where_condition_str << ")";
    ss << " LIMIT 1";
    auto query = ss.str();

    ParserQuery parser_query(query.data() + query.size());
    const auto & settings = getContext()->getSettingsRef();
    ASTPtr query_ast = parseQuery(parser_query, query, settings.max_query_size, settings.max_parser_depth);

    LOG_INFO(log, "Checking shard {} for partition {} existence, executing query: {}",
        task_shard.getDescription(), partition_quoted_name, query_ast->formatForErrorMessage());

    auto local_context = Context::createCopy(context);
    local_context->setSettings(task_cluster->settings_pull);
    auto pipeline = InterpreterFactory::get(query_ast, local_context)->execute().pipeline;
    PullingPipelineExecutor executor(pipeline);
    Block block;
    executor.pull(block);
    return block.rows() != 0;
}

bool ClusterCopier::checkPresentPartitionPiecesOnCurrentShard(const ConnectionTimeouts & timeouts,
                           TaskShard & task_shard, const String & partition_quoted_name, size_t current_piece_number)
{
    createShardInternalTables(timeouts, task_shard, false);

    TaskTable & task_table = task_shard.task_table;
    const size_t number_of_splits = task_table.number_of_splits;
    const String & primary_key_comma_separated = task_table.primary_key_comma_separated;

    UNUSED(primary_key_comma_separated);

    std::string query;

    query += "WITH " + partition_quoted_name + " AS partition_key ";
    query += "SELECT 1 FROM " + getQuotedTable(task_shard.table_read_shard);

    if (experimental_use_sample_offset)
        query += " SAMPLE 1/" + toString(number_of_splits) + " OFFSET " + toString(current_piece_number) + "/" + toString(number_of_splits);

    query += " WHERE (" + queryToString(task_table.engine_push_partition_key_ast) + " = partition_key)";

    if (!experimental_use_sample_offset)
        query += " AND (cityHash64(" + primary_key_comma_separated + ") % "
                 + std::to_string(number_of_splits) + " = " + std::to_string(current_piece_number) + " )";

    if (!task_table.where_condition_str.empty())
        query += " AND (" + task_table.where_condition_str + ")";

    query += " LIMIT 1";

    LOG_INFO(log, "Checking shard {} for partition {} piece {} existence, executing query: {}", task_shard.getDescription(), partition_quoted_name, std::to_string(current_piece_number), query);

    ParserQuery parser_query(query.data() + query.size());
    const auto & settings = getContext()->getSettingsRef();
    ASTPtr query_ast = parseQuery(parser_query, query, settings.max_query_size, settings.max_parser_depth);

    auto local_context = Context::createCopy(context);
    local_context->setSettings(task_cluster->settings_pull);
    auto pipeline = InterpreterFactory::get(query_ast, local_context)->execute().pipeline;
    PullingPipelineExecutor executor(pipeline);
    Block result;
    executor.pull(result);
    if (result.rows() != 0)
        LOG_INFO(log, "Partition {} piece number {} is PRESENT on shard {}", partition_quoted_name, std::to_string(current_piece_number), task_shard.getDescription());
    else
        LOG_INFO(log, "Partition {} piece number {} is ABSENT on shard {}", partition_quoted_name, std::to_string(current_piece_number), task_shard.getDescription());
    return result.rows() != 0;
}


/** Executes simple query (without output streams, for example DDL queries) on each shard of the cluster
  * Returns number of shards for which at least one replica executed query successfully
  */
UInt64 ClusterCopier::executeQueryOnCluster(
        const ClusterPtr & cluster,
        const String & query,
        const Settings & current_settings,
        ClusterExecutionMode execution_mode) const
{
    ClusterPtr cluster_for_query = cluster;
    if (execution_mode == ClusterExecutionMode::ON_EACH_NODE)
        cluster_for_query = cluster->getClusterWithReplicasAsShards(current_settings);

    std::vector<std::shared_ptr<Connection>> connections;
    connections.reserve(cluster->getShardCount());

    std::atomic<UInt64> successfully_executed = 0;

    for (const auto & replicas : cluster_for_query->getShardsAddresses())
    {
        for (const auto & node : replicas)
        {
            try
            {
                connections.emplace_back(std::make_shared<Connection>(
                    node.host_name, node.port, node.default_database,
                    node.user, node.password, node.quota_key, node.cluster, node.cluster_secret,
                    "ClusterCopier", node.compression, node.secure
                ));

                /// We execute only Alter, Create and Drop queries.
                const auto header = Block{};

                /// For unknown reason global context is passed to IStorage::read() method
                /// So, task_identifier is passed as constructor argument. It is more obvious.
                auto remote_query_executor = std::make_shared<RemoteQueryExecutor>(
                        *connections.back(), query, header, getContext(),
                        /*throttler=*/nullptr, Scalars(), Tables(), QueryProcessingStage::Complete);

                try
                {
                    remote_query_executor->sendQuery();
                }
                catch (...)
                {
                    LOG_WARNING(log, "Node with address {} seems to be unreachable.", node.host_name);
                    continue;
                }

                while (true)
                {
                    auto block = remote_query_executor->readBlock();
                    if (!block)
                        break;
                }

                remote_query_executor->finish();
                ++successfully_executed;
                break;
            }
            catch (...)
            {
                LOG_WARNING(log, "An error occurred while processing query: {}", query);
                tryLogCurrentException(log);
                continue;
            }
        }
    }

    return successfully_executed.load();
}

}