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ClickHouse/src/Interpreters/DDLWorker.cpp
3ldar-nasyrov 37a17749ea
fixed exceeded amount of tries typo 
'retires' -> 'retries'
2021-03-16 23:17:07 +03:00

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#include <filesystem>
#include <Interpreters/DDLWorker.h>
#include <Interpreters/DDLTask.h>
#include <Parsers/ASTAlterQuery.h>
#include <Parsers/ASTDropQuery.h>
#include <Parsers/ASTOptimizeQuery.h>
#include <Parsers/ASTQueryWithOnCluster.h>
#include <Parsers/ASTQueryWithTableAndOutput.h>
#include <Parsers/ParserQuery.h>
#include <Parsers/parseQuery.h>
#include <Parsers/queryToString.h>
#include <IO/WriteHelpers.h>
#include <IO/ReadHelpers.h>
#include <IO/ReadBufferFromString.h>
#include <Storages/IStorage.h>
#include <Interpreters/executeQuery.h>
#include <Interpreters/Cluster.h>
#include <Interpreters/Context.h>
#include <Common/setThreadName.h>
#include <Common/randomSeed.h>
#include <Common/ZooKeeper/ZooKeeper.h>
#include <Common/ZooKeeper/KeeperException.h>
#include <Common/isLocalAddress.h>
#include <Storages/StorageReplicatedMergeTree.h>
#include <Poco/Timestamp.h>
#include <common/sleep.h>
#include <common/getFQDNOrHostName.h>
#include <common/logger_useful.h>
#include <random>
#include <pcg_random.hpp>
namespace fs = std::filesystem;
namespace DB
{
namespace ErrorCodes
{
extern const int LOGICAL_ERROR;
extern const int TIMEOUT_EXCEEDED;
extern const int UNFINISHED;
extern const int NOT_A_LEADER;
extern const int KEEPER_EXCEPTION;
extern const int CANNOT_ASSIGN_ALTER;
extern const int CANNOT_ALLOCATE_MEMORY;
extern const int MEMORY_LIMIT_EXCEEDED;
}
constexpr const char * TASK_PROCESSED_OUT_REASON = "Task has been already processed";
namespace
{
/** Caveats: usage of locks in ZooKeeper is incorrect in 99% of cases,
* and highlights your poor understanding of distributed systems.
*
* It's only correct if all the operations that are performed under lock
* are atomically checking that the lock still holds
* or if we ensure that these operations will be undone if lock is lost
* (due to ZooKeeper session loss) that's very difficult to achieve.
*
* It's Ok if every operation that we perform under lock is actually operation in ZooKeeper.
*
* In 1% of cases when you can correctly use Lock, the logic is complex enough, so you don't need this class.
*
* TLDR: Don't use this code.
* We only have a few cases of it's usage and it will be removed.
*/
class ZooKeeperLock
{
public:
/// lock_prefix - path where the ephemeral lock node will be created
/// lock_name - the name of the ephemeral lock node
ZooKeeperLock(
const zkutil::ZooKeeperPtr & zookeeper_,
const std::string & lock_prefix_,
const std::string & lock_name_,
const std::string & lock_message_ = "")
:
zookeeper(zookeeper_),
lock_path(fs::path(lock_prefix_) / lock_name_),
lock_message(lock_message_),
log(&Poco::Logger::get("zkutil::Lock"))
{
zookeeper->createIfNotExists(lock_prefix_, "");
}
~ZooKeeperLock()
{
try
{
unlock();
}
catch (...)
{
DB::tryLogCurrentException(__PRETTY_FUNCTION__);
}
}
void unlock()
{
Coordination::Stat stat;
std::string dummy;
bool result = zookeeper->tryGet(lock_path, dummy, &stat);
if (result && stat.ephemeralOwner == zookeeper->getClientID())
zookeeper->remove(lock_path, -1);
else
LOG_WARNING(log, "Lock is lost. It is normal if session was expired. Path: {}/{}", lock_path, lock_message);
}
bool tryLock()
{
std::string dummy;
Coordination::Error code = zookeeper->tryCreate(lock_path, lock_message, zkutil::CreateMode::Ephemeral, dummy);
if (code == Coordination::Error::ZNODEEXISTS)
{
return false;
}
else if (code == Coordination::Error::ZOK)
{
return true;
}
else
{
throw Coordination::Exception(code);
}
}
private:
zkutil::ZooKeeperPtr zookeeper;
std::string lock_path;
std::string lock_message;
Poco::Logger * log;
};
std::unique_ptr<ZooKeeperLock> createSimpleZooKeeperLock(
const zkutil::ZooKeeperPtr & zookeeper, const String & lock_prefix, const String & lock_name, const String & lock_message)
{
return std::make_unique<ZooKeeperLock>(zookeeper, lock_prefix, lock_name, lock_message);
}
}
DDLWorker::DDLWorker(int pool_size_, const std::string & zk_root_dir, const Context & context_, const Poco::Util::AbstractConfiguration * config, const String & prefix,
const String & logger_name, const CurrentMetrics::Metric * max_entry_metric_)
: context(context_)
, log(&Poco::Logger::get(logger_name))
, pool_size(pool_size_)
, max_entry_metric(max_entry_metric_)
{
if (max_entry_metric)
CurrentMetrics::set(*max_entry_metric, 0);
if (1 < pool_size)
{
LOG_WARNING(log, "DDLWorker is configured to use multiple threads. "
"It's not recommended because queries can be reordered. Also it may cause some unknown issues to appear.");
worker_pool = std::make_unique<ThreadPool>(pool_size);
}
queue_dir = zk_root_dir;
if (queue_dir.back() == '/')
queue_dir.resize(queue_dir.size() - 1);
if (config)
{
task_max_lifetime = config->getUInt64(prefix + ".task_max_lifetime", static_cast<UInt64>(task_max_lifetime));
cleanup_delay_period = config->getUInt64(prefix + ".cleanup_delay_period", static_cast<UInt64>(cleanup_delay_period));
max_tasks_in_queue = std::max<UInt64>(1, config->getUInt64(prefix + ".max_tasks_in_queue", max_tasks_in_queue));
if (config->has(prefix + ".profile"))
context.setSetting("profile", config->getString(prefix + ".profile"));
}
if (context.getSettingsRef().readonly)
{
LOG_WARNING(log, "Distributed DDL worker is run with readonly settings, it will not be able to execute DDL queries Set appropriate system_profile or distributed_ddl.profile to fix this.");
}
host_fqdn = getFQDNOrHostName();
host_fqdn_id = Cluster::Address::toString(host_fqdn, context.getTCPPort());
}
void DDLWorker::startup()
{
main_thread = ThreadFromGlobalPool(&DDLWorker::runMainThread, this);
cleanup_thread = ThreadFromGlobalPool(&DDLWorker::runCleanupThread, this);
}
void DDLWorker::shutdown()
{
bool prev_stop_flag = stop_flag.exchange(true);
if (!prev_stop_flag)
{
queue_updated_event->set();
cleanup_event->set();
main_thread.join();
cleanup_thread.join();
worker_pool.reset();
}
}
DDLWorker::~DDLWorker()
{
DDLWorker::shutdown();
}
ZooKeeperPtr DDLWorker::tryGetZooKeeper() const
{
std::lock_guard lock(zookeeper_mutex);
return current_zookeeper;
}
ZooKeeperPtr DDLWorker::getAndSetZooKeeper()
{
std::lock_guard lock(zookeeper_mutex);
if (!current_zookeeper || current_zookeeper->expired())
current_zookeeper = context.getZooKeeper();
return current_zookeeper;
}
DDLTaskPtr DDLWorker::initAndCheckTask(const String & entry_name, String & out_reason, const ZooKeeperPtr & zookeeper)
{
String node_data;
String entry_path = fs::path(queue_dir) / entry_name;
auto task = std::make_unique<DDLTask>(entry_name, entry_path);
if (!zookeeper->tryGet(entry_path, node_data))
{
/// It is Ok that node could be deleted just now. It means that there are no current host in node's host list.
out_reason = "The task was deleted";
return {};
}
auto write_error_status = [&](const String & host_id, const String & error_message, const String & reason)
{
LOG_ERROR(log, "Cannot parse DDL task {}: {}. Will try to send error status: {}", entry_name, reason, error_message);
createStatusDirs(entry_path, zookeeper);
zookeeper->tryCreate(fs::path(entry_path) / "finished" / host_id, error_message, zkutil::CreateMode::Persistent);
};
try
{
/// Stage 1: parse entry
task->entry.parse(node_data);
}
catch (...)
{
/// What should we do if we even cannot parse host name and therefore cannot properly submit execution status?
/// We can try to create fail node using FQDN if it equal to host name in cluster config attempt will be successful.
/// Otherwise, that node will be ignored by DDLQueryStatusInputStream.
out_reason = "Incorrect task format";
write_error_status(host_fqdn_id, ExecutionStatus::fromCurrentException().serializeText(), out_reason);
return {};
}
/// Stage 2: resolve host_id and check if we should execute query or not
/// Multiple clusters can use single DDL queue path in ZooKeeper,
/// So we should skip task if we cannot find current host in cluster hosts list.
if (!task->findCurrentHostID(context, log))
{
out_reason = "There is no a local address in host list";
return {};
}
try
{
/// Stage 3.1: parse query
task->parseQueryFromEntry(context);
/// Stage 3.2: check cluster and find the host in cluster
task->setClusterInfo(context, log);
}
catch (...)
{
out_reason = "Cannot parse query or obtain cluster info";
write_error_status(task->host_id_str, ExecutionStatus::fromCurrentException().serializeText(), out_reason);
return {};
}
if (zookeeper->exists(task->getFinishedNodePath()))
{
out_reason = TASK_PROCESSED_OUT_REASON;
return {};
}
/// Now task is ready for execution
return task;
}
static void filterAndSortQueueNodes(Strings & all_nodes)
{
all_nodes.erase(std::remove_if(all_nodes.begin(), all_nodes.end(), [] (const String & s) { return !startsWith(s, "query-"); }), all_nodes.end());
std::sort(all_nodes.begin(), all_nodes.end());
}
void DDLWorker::scheduleTasks(bool reinitialized)
{
LOG_DEBUG(log, "Scheduling tasks");
auto zookeeper = tryGetZooKeeper();
/// Main thread of DDLWorker was restarted, probably due to lost connection with ZooKeeper.
/// We have some unfinished tasks.
/// To avoid duplication of some queries we should try to write execution status again.
/// To avoid skipping of some entries which were not executed we should be careful when choosing begin_node to start from.
/// NOTE: It does not protect from all cases of query duplication, see also comments in processTask(...)
if (reinitialized)
{
if (current_tasks.empty())
LOG_TRACE(log, "Don't have unfinished tasks after restarting");
else
LOG_INFO(log, "Have {} unfinished tasks, will check them", current_tasks.size());
assert(current_tasks.size() <= pool_size + (worker_pool != nullptr));
auto task_it = current_tasks.begin();
while (task_it != current_tasks.end())
{
auto & task = *task_it;
if (task->completely_processed)
{
assert(task->was_executed);
/// Status must be written (but finished/ node may not exist if entry was deleted).
/// If someone is deleting entry concurrently, then /active status dir must not exist.
assert(zookeeper->exists(task->getFinishedNodePath()) || !zookeeper->exists(fs::path(task->entry_path) / "active"));
++task_it;
}
else if (task->was_executed)
{
/// Connection was lost on attempt to write status. Will retry.
bool status_written = zookeeper->exists(task->getFinishedNodePath());
/// You might think that the following condition is redundant, because status_written implies completely_processed.
/// But it's wrong. It's possible that (!task->completely_processed && status_written)
/// if ZooKeeper successfully received and processed our request
/// but we lost connection while waiting for the response.
/// Yeah, distributed systems is a zoo.
if (status_written)
task->completely_processed = true;
else
processTask(*task, zookeeper);
++task_it;
}
else
{
/// We didn't even executed a query, so let's just remove it.
/// We will try to read the task again and execute it from the beginning.
/// NOTE: We can safely compare entry names as Strings, because they are padded.
/// Entry name always starts with "query-" and contain exactly 10 decimal digits
/// of log entry number (with leading zeros).
if (!first_failed_task_name || task->entry_name < *first_failed_task_name)
first_failed_task_name = task->entry_name;
task_it = current_tasks.erase(task_it);
}
}
}
Strings queue_nodes = zookeeper->getChildren(queue_dir, nullptr, queue_updated_event);
filterAndSortQueueNodes(queue_nodes);
if (max_tasks_in_queue < queue_nodes.size())
cleanup_event->set();
/// Detect queue start, using:
/// - skipped tasks
/// - in memory tasks (that are currently active or were finished recently)
/// - failed tasks (that should be processed again)
auto begin_node = queue_nodes.begin();
if (first_failed_task_name)
{
/// If we had failed tasks, then we should start from the first failed task.
assert(reinitialized);
begin_node = std::lower_bound(queue_nodes.begin(), queue_nodes.end(), first_failed_task_name);
}
else
{
/// We had no failed tasks. Let's just choose the maximum entry we have previously seen.
String last_task_name;
if (!current_tasks.empty())
last_task_name = current_tasks.back()->entry_name;
if (last_skipped_entry_name && last_task_name < *last_skipped_entry_name)
last_task_name = *last_skipped_entry_name;
begin_node = std::upper_bound(queue_nodes.begin(), queue_nodes.end(), last_task_name);
}
if (begin_node == queue_nodes.end())
LOG_DEBUG(log, "No tasks to schedule");
else
LOG_DEBUG(log, "Will schedule {} tasks starting from {}", std::distance(begin_node, queue_nodes.end()), *begin_node);
/// Let's ensure that it's exactly the first task we should process.
/// Maybe such asserts are too paranoid and excessive,
/// but it's easy enough to break DDLWorker in a very unobvious way by making some minor change in code.
[[maybe_unused]] bool have_no_tasks_info = !first_failed_task_name && current_tasks.empty() && !last_skipped_entry_name;
assert(have_no_tasks_info || queue_nodes.end() == std::find_if(queue_nodes.begin(), queue_nodes.end(), [&](const String & entry_name)
{
/// We should return true if some invariants are violated.
String reason;
auto task = initAndCheckTask(entry_name, reason, zookeeper);
bool maybe_currently_processing = current_tasks.end() != std::find_if(current_tasks.begin(), current_tasks.end(), [&](const auto & t)
{
return t->entry_name == entry_name;
});
/// begin_node is something like a log pointer
if (begin_node == queue_nodes.end() || entry_name < *begin_node)
{
/// Return true if entry should be scheduled.
/// There is a minor race condition: initAndCheckTask(...) may return not null
/// if someone is deleting outdated entry right now (including finished/ nodes), so we also check active/ status dir.
bool maybe_concurrently_deleting = task && !zookeeper->exists(fs::path(task->entry_path) / "active");
return task && !maybe_concurrently_deleting && !maybe_currently_processing;
}
else
{
/// Return true if entry should not be scheduled.
bool processed = !task && reason == TASK_PROCESSED_OUT_REASON;
return processed || maybe_currently_processing;
}
}));
for (auto it = begin_node; it != queue_nodes.end() && !stop_flag; ++it)
{
String entry_name = *it;
LOG_TRACE(log, "Checking task {}", entry_name);
String reason;
auto task = initAndCheckTask(entry_name, reason, zookeeper);
if (!task)
{
LOG_DEBUG(log, "Will not execute task {}: {}", entry_name, reason);
updateMaxDDLEntryID(entry_name);
last_skipped_entry_name.emplace(entry_name);
continue;
}
auto & saved_task = saveTask(std::move(task));
if (worker_pool)
{
worker_pool->scheduleOrThrowOnError([this, &saved_task, zookeeper]()
{
setThreadName("DDLWorkerExec");
processTask(saved_task, zookeeper);
});
}
else
{
processTask(saved_task, zookeeper);
}
}
}
DDLTaskBase & DDLWorker::saveTask(DDLTaskPtr && task)
{
current_tasks.remove_if([](const DDLTaskPtr & t) { return t->completely_processed.load(); });
/// Tasks are scheduled and executed in main thread <==> Parallel execution is disabled
assert((worker_pool != nullptr) == (1 < pool_size));
/// Parallel execution is disabled ==> All previous tasks are failed to start or finished,
/// so current tasks list must be empty when we are ready to process new one.
assert(worker_pool || current_tasks.empty());
/// Parallel execution is enabled ==> Not more than pool_size tasks are currently executing.
/// Note: If current_tasks.size() == pool_size, then all worker threads are busy,
/// so we will wait on worker_pool->scheduleOrThrowOnError(...)
assert(!worker_pool || current_tasks.size() <= pool_size);
current_tasks.emplace_back(std::move(task));
if (first_failed_task_name && *first_failed_task_name == current_tasks.back()->entry_name)
first_failed_task_name.reset();
return *current_tasks.back();
}
bool DDLWorker::tryExecuteQuery(const String & query, DDLTaskBase & task, const ZooKeeperPtr & zookeeper)
{
/// Add special comment at the start of query to easily identify DDL-produced queries in query_log
String query_prefix = "/* ddl_entry=" + task.entry_name + " */ ";
String query_to_execute = query_prefix + query;
ReadBufferFromString istr(query_to_execute);
String dummy_string;
WriteBufferFromString ostr(dummy_string);
std::optional<CurrentThread::QueryScope> query_scope;
try
{
auto query_context = task.makeQueryContext(context, zookeeper);
if (!task.is_initial_query)
query_scope.emplace(*query_context);
executeQuery(istr, ostr, !task.is_initial_query, *query_context, {});
if (auto txn = query_context->getZooKeeperMetadataTransaction())
{
/// Most queries commit changes to ZooKeeper right before applying local changes,
/// but some queries does not support it, so we have to do it here.
if (!txn->isExecuted())
txn->commit();
}
}
catch (const DB::Exception & e)
{
if (task.is_initial_query)
throw;
task.execution_status = ExecutionStatus::fromCurrentException();
tryLogCurrentException(log, "Query " + query + " wasn't finished successfully");
/// We use return value of tryExecuteQuery(...) in tryExecuteQueryOnLeaderReplica(...) to determine
/// if replica has stopped being leader and we should retry query.
/// However, for the majority of exceptions there is no sense to retry, because most likely we will just
/// get the same exception again. So we return false only for several special exception codes,
/// and consider query as executed with status "failed" and return true in other cases.
bool no_sense_to_retry = e.code() != ErrorCodes::KEEPER_EXCEPTION &&
e.code() != ErrorCodes::NOT_A_LEADER &&
e.code() != ErrorCodes::CANNOT_ASSIGN_ALTER &&
e.code() != ErrorCodes::CANNOT_ALLOCATE_MEMORY &&
e.code() != ErrorCodes::MEMORY_LIMIT_EXCEEDED;
return no_sense_to_retry;
}
catch (...)
{
if (task.is_initial_query)
throw;
task.execution_status = ExecutionStatus::fromCurrentException();
tryLogCurrentException(log, "Query " + query + " wasn't finished successfully");
/// We don't know what exactly happened, but maybe it's Poco::NetException or std::bad_alloc,
/// so we consider unknown exception as retryable error.
return false;
}
task.execution_status = ExecutionStatus(0);
LOG_DEBUG(log, "Executed query: {}", query);
return true;
}
void DDLWorker::updateMaxDDLEntryID(const String & entry_name)
{
UInt64 id = DDLTaskBase::getLogEntryNumber(entry_name);
auto prev_id = max_id.load(std::memory_order_relaxed);
while (prev_id < id)
{
if (max_id.compare_exchange_weak(prev_id, id))
{
if (max_entry_metric)
CurrentMetrics::set(*max_entry_metric, id);
break;
}
}
}
void DDLWorker::processTask(DDLTaskBase & task, const ZooKeeperPtr & zookeeper)
{
LOG_DEBUG(log, "Processing task {} ({})", task.entry_name, task.entry.query);
assert(!task.completely_processed);
String active_node_path = task.getActiveNodePath();
String finished_node_path = task.getFinishedNodePath();
/// Step 1: Create ephemeral node in active/ status dir.
/// It allows other hosts to understand that task is currently executing (useful for system.distributed_ddl_queue)
/// and protects from concurrent deletion or the task.
/// It will tryRemove(...) on exception
auto active_node = zkutil::EphemeralNodeHolder::existing(active_node_path, *zookeeper);
/// Try fast path
auto create_active_res = zookeeper->tryCreate(active_node_path, {}, zkutil::CreateMode::Ephemeral);
if (create_active_res != Coordination::Error::ZOK)
{
if (create_active_res != Coordination::Error::ZNONODE && create_active_res != Coordination::Error::ZNODEEXISTS)
{
assert(Coordination::isHardwareError(create_active_res));
throw Coordination::Exception(create_active_res, active_node_path);
}
/// Status dirs were not created in enqueueQuery(...) or someone is removing entry
if (create_active_res == Coordination::Error::ZNONODE)
{
assert(dynamic_cast<DatabaseReplicatedTask *>(&task) == nullptr);
if (task.was_executed)
{
/// Special case:
/// Task was executed (and we are trying to write status after connection loss) ==> Status dirs were previously created.
/// (Status dirs were previously created AND active/ does not exist) ==> Task was removed.
/// We cannot write status, but it's not required anymore, because no one will try to execute it again.
/// So we consider task as completely processed.
LOG_WARNING(log, "Task {} is executed, but looks like entry {} was deleted, cannot write status", task.entry_name, task.entry_path);
task.completely_processed = true;
return;
}
createStatusDirs(task.entry_path, zookeeper);
}
if (create_active_res == Coordination::Error::ZNODEEXISTS)
{
/// Connection has been lost and now we are retrying,
/// but our previous ephemeral node still exists.
zkutil::EventPtr eph_node_disappeared = std::make_shared<Poco::Event>();
String dummy;
if (zookeeper->tryGet(active_node_path, dummy, nullptr, eph_node_disappeared))
{
constexpr int timeout_ms = 30 * 1000;
if (!eph_node_disappeared->tryWait(timeout_ms))
throw Exception(ErrorCodes::LOGICAL_ERROR, "Ephemeral node {} still exists, "
"probably it's owned by someone else", active_node_path);
}
}
zookeeper->create(active_node_path, {}, zkutil::CreateMode::Ephemeral);
}
/// Step 2: Execute query from the task.
if (!task.was_executed)
{
/// If table and database engine supports it, they will execute task.ops by their own in a single transaction
/// with other zk operations (such as appending something to ReplicatedMergeTree log, or
/// updating metadata in Replicated database), so we make create request for finished_node_path with status "0",
/// which means that query executed successfully.
task.ops.emplace_back(zkutil::makeRemoveRequest(active_node_path, -1));
task.ops.emplace_back(zkutil::makeCreateRequest(finished_node_path, ExecutionStatus(0).serializeText(), zkutil::CreateMode::Persistent));
try
{
String rewritten_query = queryToString(task.query);
LOG_DEBUG(log, "Executing query: {}", rewritten_query);
StoragePtr storage;
if (auto * query_with_table = dynamic_cast<ASTQueryWithTableAndOutput *>(task.query.get()); query_with_table)
{
if (!query_with_table->table.empty())
{
/// It's not CREATE DATABASE
auto table_id = context.tryResolveStorageID(*query_with_table, Context::ResolveOrdinary);
storage = DatabaseCatalog::instance().tryGetTable(table_id, context);
}
task.execute_on_leader = storage && taskShouldBeExecutedOnLeader(task.query, storage) && !task.is_circular_replicated;
}
if (task.execute_on_leader)
{
tryExecuteQueryOnLeaderReplica(task, storage, rewritten_query, task.entry_path, zookeeper);
}
else
{
storage.reset();
tryExecuteQuery(rewritten_query, task, zookeeper);
}
}
catch (const Coordination::Exception &)
{
throw;
}
catch (...)
{
if (task.is_initial_query)
throw;
tryLogCurrentException(log, "An error occurred before execution of DDL task: ");
task.execution_status = ExecutionStatus::fromCurrentException("An error occurred before execution");
}
if (task.execution_status.code != 0)
{
bool status_written_by_table_or_db = task.ops.empty();
if (status_written_by_table_or_db)
{
throw Exception(ErrorCodes::UNFINISHED, "Unexpected error: {}", task.execution_status.serializeText());
}
else
{
/// task.ops where not executed by table or database engine, so DDLWorker is responsible for
/// writing query execution status into ZooKeeper.
task.ops.emplace_back(zkutil::makeSetRequest(finished_node_path, task.execution_status.serializeText(), -1));
}
}
/// We need to distinguish ZK errors occurred before and after query executing
task.was_executed = true;
}
updateMaxDDLEntryID(task.entry_name);
/// Step 3: Create node in finished/ status dir and write execution status.
/// FIXME: if server fails right here, the task will be executed twice. We need WAL here.
/// NOTE: If ZooKeeper connection is lost here, we will try again to write query status.
/// NOTE: If both table and database are replicated, task is executed in single ZK transaction.
bool status_written = task.ops.empty();
if (!status_written)
{
zookeeper->multi(task.ops);
task.ops.clear();
}
/// Active node was removed in multi ops
active_node->setAlreadyRemoved();
task.completely_processed = true;
}
bool DDLWorker::taskShouldBeExecutedOnLeader(const ASTPtr & ast_ddl, const StoragePtr storage)
{
/// Pure DROP queries have to be executed on each node separately
if (auto * query = ast_ddl->as<ASTDropQuery>(); query && query->kind != ASTDropQuery::Kind::Truncate)
return false;
if (!ast_ddl->as<ASTAlterQuery>() && !ast_ddl->as<ASTOptimizeQuery>() && !ast_ddl->as<ASTDropQuery>())
return false;
if (auto * alter = ast_ddl->as<ASTAlterQuery>())
{
// Setting alters should be executed on all replicas
if (alter->isSettingsAlter())
return false;
if (alter->isFreezeAlter())
return false;
}
return storage->supportsReplication();
}
bool DDLWorker::tryExecuteQueryOnLeaderReplica(
DDLTaskBase & task,
StoragePtr storage,
const String & rewritten_query,
const String & /*node_path*/,
const ZooKeeperPtr & zookeeper)
{
StorageReplicatedMergeTree * replicated_storage = dynamic_cast<StorageReplicatedMergeTree *>(storage.get());
/// If we will develop new replicated storage
if (!replicated_storage)
throw Exception(ErrorCodes::LOGICAL_ERROR, "Storage type '{}' is not supported by distributed DDL", storage->getName());
String shard_path = task.getShardNodePath();
String is_executed_path = fs::path(shard_path) / "executed";
String tries_to_execute_path = fs::path(shard_path) / "tries_to_execute";
zookeeper->createAncestors(fs::path(shard_path) / ""); /* appends "/" at the end of shard_path */
/// Leader replica creates is_executed_path node on successful query execution.
/// We will remove create_shard_flag from zk operations list, if current replica is just waiting for leader to execute the query.
auto create_shard_flag = zkutil::makeCreateRequest(is_executed_path, task.host_id_str, zkutil::CreateMode::Persistent);
/// Node exists, or we will create or we will get an exception
zookeeper->tryCreate(tries_to_execute_path, "0", zkutil::CreateMode::Persistent);
static constexpr int MAX_TRIES_TO_EXECUTE = 3;
static constexpr int MAX_EXECUTION_TIMEOUT_SEC = 3600;
String executed_by;
zkutil::EventPtr event = std::make_shared<Poco::Event>();
/// We must use exists request instead of get, because zookeeper will not setup event
/// for non existing node after get request
if (zookeeper->exists(is_executed_path, nullptr, event))
{
LOG_DEBUG(log, "Task {} has already been executed by replica ({}) of the same shard.", task.entry_name, zookeeper->get(is_executed_path));
return true;
}
pcg64 rng(randomSeed());
auto lock = createSimpleZooKeeperLock(zookeeper, shard_path, "lock", task.host_id_str);
Stopwatch stopwatch;
bool executed_by_us = false;
bool executed_by_other_leader = false;
/// Defensive programming. One hour is more than enough to execute almost all DDL queries.
/// If it will be very long query like ALTER DELETE for a huge table it's still will be executed,
/// but DDL worker can continue processing other queries.
while (stopwatch.elapsedSeconds() <= MAX_EXECUTION_TIMEOUT_SEC)
{
StorageReplicatedMergeTree::Status status;
// Has to get with zk fields to get active replicas field
replicated_storage->getStatus(status, true);
// Should return as soon as possible if the table is dropped.
bool replica_dropped = replicated_storage->is_dropped;
bool all_replicas_likely_detached = status.active_replicas == 0 && !DatabaseCatalog::instance().isTableExist(replicated_storage->getStorageID(), context);
if (replica_dropped || all_replicas_likely_detached)
{
LOG_WARNING(log, ", task {} will not be executed.", task.entry_name);
task.execution_status = ExecutionStatus(ErrorCodes::UNFINISHED, "Cannot execute replicated DDL query, table is dropped or detached permanently");
return false;
}
if (task.is_initial_query && !status.is_leader)
throw Exception(ErrorCodes::NOT_A_LEADER, "Cannot execute initial query on non-leader replica");
/// Any replica which is leader tries to take lock
if (status.is_leader && lock->tryLock())
{
/// In replicated merge tree we can have multiple leaders. So we can
/// be "leader" and took lock, but another "leader" replica may have
/// already executed this task.
if (zookeeper->tryGet(is_executed_path, executed_by))
{
LOG_DEBUG(log, "Task {} has already been executed by replica ({}) of the same shard.", task.entry_name, executed_by);
executed_by_other_leader = true;
break;
}
/// Checking and incrementing counter exclusively.
size_t counter = parse<int>(zookeeper->get(tries_to_execute_path));
if (counter > MAX_TRIES_TO_EXECUTE)
break;
zookeeper->set(tries_to_execute_path, toString(counter + 1));
task.ops.push_back(create_shard_flag);
SCOPE_EXIT({ if (!executed_by_us && !task.ops.empty()) task.ops.pop_back(); });
/// If the leader will unexpectedly changed this method will return false
/// and on the next iteration new leader will take lock
if (tryExecuteQuery(rewritten_query, task, zookeeper))
{
executed_by_us = true;
break;
}
lock->unlock();
}
/// Waiting for someone who will execute query and change is_executed_path node
if (event->tryWait(std::uniform_int_distribution<int>(0, 1000)(rng)))
{
LOG_DEBUG(log, "Task {} has already been executed by replica ({}) of the same shard.", task.entry_name, zookeeper->get(is_executed_path));
executed_by_other_leader = true;
break;
}
else
{
String tries_count;
zookeeper->tryGet(tries_to_execute_path, tries_count);
if (parse<int>(tries_count) > MAX_TRIES_TO_EXECUTE)
{
/// Nobody will try to execute query again
LOG_WARNING(log, "Maximum retries count for task {} exceeded, cannot execute replicated DDL query", task.entry_name);
break;
}
else
{
/// Will try to wait or execute
LOG_TRACE(log, "Task {} still not executed, will try to wait for it or execute ourselves, tries count {}", task.entry_name, tries_count);
}
}
}
assert(!(executed_by_us && executed_by_other_leader));
/// Not executed by leader so was not executed at all
if (!executed_by_us && !executed_by_other_leader)
{
/// If we failed with timeout
if (stopwatch.elapsedSeconds() >= MAX_EXECUTION_TIMEOUT_SEC)
{
LOG_WARNING(log, "Task {} was not executed by anyone, maximum timeout {} seconds exceeded", task.entry_name, MAX_EXECUTION_TIMEOUT_SEC);
task.execution_status = ExecutionStatus(ErrorCodes::TIMEOUT_EXCEEDED, "Cannot execute replicated DDL query, timeout exceeded");
}
else /// If we exceeded amount of tries
{
LOG_WARNING(log, "Task {} was not executed by anyone, maximum number of retries exceeded", task.entry_name);
task.execution_status = ExecutionStatus(ErrorCodes::UNFINISHED, "Cannot execute replicated DDL query, maximum retries exceeded");
}
return false;
}
if (executed_by_us)
LOG_DEBUG(log, "Task {} executed by current replica", task.entry_name);
else // if (executed_by_other_leader)
LOG_DEBUG(log, "Task {} has already been executed by replica ({}) of the same shard.", task.entry_name, zookeeper->get(is_executed_path));
return true;
}
void DDLWorker::cleanupQueue(Int64, const ZooKeeperPtr & zookeeper)
{
LOG_DEBUG(log, "Cleaning queue");
Strings queue_nodes = zookeeper->getChildren(queue_dir);
filterAndSortQueueNodes(queue_nodes);
for (auto it = queue_nodes.cbegin(); it < queue_nodes.cend(); ++it)
{
if (stop_flag)
return;
String node_name = *it;
String node_path = fs::path(queue_dir) / node_name;
Coordination::Stat stat;
String dummy;
try
{
/// Already deleted
if (!zookeeper->exists(node_path, &stat))
continue;
if (!canRemoveQueueEntry(node_name, stat))
continue;
/// At first we remove entry/active node to prevent staled hosts from executing entry concurrently
auto rm_active_res = zookeeper->tryRemove(fs::path(node_path) / "active");
if (rm_active_res != Coordination::Error::ZOK && rm_active_res != Coordination::Error::ZNONODE)
{
if (rm_active_res == Coordination::Error::ZNOTEMPTY)
LOG_DEBUG(log, "Task {} should be deleted, but there are active workers. Skipping it.", node_name);
else
LOG_WARNING(log, "Unexpected status code {} on attempt to remove {}/active", rm_active_res, node_name);
continue;
}
/// Now we can safely delete entry
LOG_INFO(log, "Task {} is outdated, deleting it", node_name);
/// We recursively delete all nodes except node_path/finished to prevent staled hosts from
/// creating node_path/active node (see createStatusDirs(...))
zookeeper->tryRemoveChildrenRecursive(node_path, "finished");
/// And then we remove node_path and node_path/finished in a single transaction
Coordination::Requests ops;
Coordination::Responses res;
ops.emplace_back(zkutil::makeCheckRequest(node_path, -1)); /// See a comment below
ops.emplace_back(zkutil::makeRemoveRequest(fs::path(node_path) / "finished", -1));
ops.emplace_back(zkutil::makeRemoveRequest(node_path, -1));
auto rm_entry_res = zookeeper->tryMulti(ops, res);
if (rm_entry_res == Coordination::Error::ZNONODE)
{
/// Most likely both node_path/finished and node_path were removed concurrently.
bool entry_removed_concurrently = res[0]->error == Coordination::Error::ZNONODE;
if (entry_removed_concurrently)
continue;
/// Possible rare case: initiator node has lost connection after enqueueing entry and failed to create status dirs.
/// No one has started to process the entry, so node_path/active and node_path/finished nodes were never created, node_path has no children.
/// Entry became outdated, but we cannot remove remove it in a transaction with node_path/finished.
assert(res[0]->error == Coordination::Error::ZOK && res[1]->error == Coordination::Error::ZNONODE);
rm_entry_res = zookeeper->tryRemove(node_path);
assert(rm_entry_res != Coordination::Error::ZNOTEMPTY);
continue;
}
zkutil::KeeperMultiException::check(rm_entry_res, ops, res);
}
catch (...)
{
LOG_INFO(log, "An error occurred while checking and cleaning task {} from queue: {}", node_name, getCurrentExceptionMessage(false));
}
}
}
bool DDLWorker::canRemoveQueueEntry(const String & entry_name, const Coordination::Stat & stat)
{
/// Delete node if its lifetime is expired (according to task_max_lifetime parameter)
constexpr UInt64 zookeeper_time_resolution = 1000;
Int64 zookeeper_time_seconds = stat.ctime / zookeeper_time_resolution;
bool node_lifetime_is_expired = zookeeper_time_seconds + task_max_lifetime < Poco::Timestamp().epochTime();
/// If too many nodes in task queue (> max_tasks_in_queue), delete oldest one
UInt32 entry_number = DDLTaskBase::getLogEntryNumber(entry_name);
bool node_is_outside_max_window = entry_number + max_tasks_in_queue < max_id.load(std::memory_order_relaxed);
return node_lifetime_is_expired || node_is_outside_max_window;
}
/// Try to create nonexisting "status" dirs for a node
void DDLWorker::createStatusDirs(const std::string & node_path, const ZooKeeperPtr & zookeeper)
{
Coordination::Requests ops;
ops.emplace_back(zkutil::makeCreateRequest(fs::path(node_path) / "active", {}, zkutil::CreateMode::Persistent));
ops.emplace_back(zkutil::makeCreateRequest(fs::path(node_path) / "finished", {}, zkutil::CreateMode::Persistent));
Coordination::Responses responses;
Coordination::Error code = zookeeper->tryMulti(ops, responses);
bool both_created = code == Coordination::Error::ZOK;
/// Failed on attempt to create node_path/active because it exists, so node_path/finished must exist too
bool both_already_exists = responses.size() == 2 && responses[0]->error == Coordination::Error::ZNODEEXISTS
&& responses[1]->error == Coordination::Error::ZRUNTIMEINCONSISTENCY;
assert(!both_already_exists || (zookeeper->exists(fs::path(node_path) / "active") && zookeeper->exists(fs::path(node_path) / "finished")));
/// Failed on attempt to create node_path/finished, but node_path/active does not exist
bool is_currently_deleting = responses.size() == 2 && responses[0]->error == Coordination::Error::ZOK
&& responses[1]->error == Coordination::Error::ZNODEEXISTS;
if (both_created || both_already_exists)
return;
if (is_currently_deleting)
{
cleanup_event->set();
throw Exception(ErrorCodes::UNFINISHED, "Cannot create status dirs for {}, "
"most likely because someone is deleting it concurrently", node_path);
}
/// Connection lost or entry was removed
assert(Coordination::isHardwareError(code) || code == Coordination::Error::ZNONODE);
zkutil::KeeperMultiException::check(code, ops, responses);
}
String DDLWorker::enqueueQuery(DDLLogEntry & entry)
{
if (entry.hosts.empty())
throw Exception("Empty host list in a distributed DDL task", ErrorCodes::LOGICAL_ERROR);
auto zookeeper = getAndSetZooKeeper();
String query_path_prefix = fs::path(queue_dir) / "query-";
zookeeper->createAncestors(query_path_prefix);
String node_path = zookeeper->create(query_path_prefix, entry.toString(), zkutil::CreateMode::PersistentSequential);
/// We cannot create status dirs in a single transaction with previous request,
/// because we don't know node_path until previous request is executed.
/// Se we try to create status dirs here or later when we will execute entry.
try
{
createStatusDirs(node_path, zookeeper);
}
catch (...)
{
LOG_INFO(log, "An error occurred while creating auxiliary ZooKeeper directories in {} . They will be created later. Error : {}", node_path, getCurrentExceptionMessage(true));
}
return node_path;
}
bool DDLWorker::initializeMainThread()
{
assert(!initialized);
setThreadName("DDLWorker");
LOG_DEBUG(log, "Initializing DDLWorker thread");
while (!stop_flag)
{
try
{
auto zookeeper = getAndSetZooKeeper();
zookeeper->createAncestors(fs::path(queue_dir) / "");
initialized = true;
return true;
}
catch (const Coordination::Exception & e)
{
if (!Coordination::isHardwareError(e.code))
{
/// A logical error.
LOG_ERROR(log, "ZooKeeper error: {}. Failed to start DDLWorker.", getCurrentExceptionMessage(true));
assert(false); /// Catch such failures in tests with debug build
}
tryLogCurrentException(__PRETTY_FUNCTION__);
}
catch (...)
{
tryLogCurrentException(log, "Cannot initialize DDL queue.");
}
/// Avoid busy loop when ZooKeeper is not available.
sleepForSeconds(5);
}
return false;
}
void DDLWorker::runMainThread()
{
auto reset_state = [&]()
{
initialized = false;
/// It will wait for all threads in pool to finish and will not rethrow exceptions (if any).
/// We create new thread pool to forget previous exceptions.
if (1 < pool_size)
worker_pool = std::make_unique<ThreadPool>(pool_size);
/// Clear other in-memory state, like server just started.
current_tasks.clear();
last_skipped_entry_name.reset();
max_id = 0;
LOG_INFO(log, "Cleaned DDLWorker state");
};
setThreadName("DDLWorker");
LOG_DEBUG(log, "Starting DDLWorker thread");
while (!stop_flag)
{
try
{
bool reinitialized = !initialized;
/// Reinitialize DDLWorker state (including ZooKeeper connection) if required
if (!initialized)
{
/// Stopped
if (!initializeMainThread())
break;
LOG_DEBUG(log, "Initialized DDLWorker thread");
}
cleanup_event->set();
scheduleTasks(reinitialized);
LOG_DEBUG(log, "Waiting for queue updates");
queue_updated_event->wait();
}
catch (const Coordination::Exception & e)
{
if (Coordination::isHardwareError(e.code))
{
initialized = false;
/// Wait for pending async tasks
if (1 < pool_size)
worker_pool = std::make_unique<ThreadPool>(pool_size);
LOG_INFO(log, "Lost ZooKeeper connection, will try to connect again: {}", getCurrentExceptionMessage(true));
}
else
{
LOG_ERROR(log, "Unexpected ZooKeeper error, will try to restart main thread: {}", getCurrentExceptionMessage(true));
reset_state();
}
sleepForSeconds(1);
}
catch (...)
{
tryLogCurrentException(log, "Unexpected error, will try to restart main thread:");
reset_state();
sleepForSeconds(5);
}
}
}
void DDLWorker::runCleanupThread()
{
setThreadName("DDLWorkerClnr");
LOG_DEBUG(log, "Started DDLWorker cleanup thread");
Int64 last_cleanup_time_seconds = 0;
while (!stop_flag)
{
try
{
cleanup_event->wait();
if (stop_flag)
break;
Int64 current_time_seconds = Poco::Timestamp().epochTime();
if (last_cleanup_time_seconds && current_time_seconds < last_cleanup_time_seconds + cleanup_delay_period)
{
LOG_TRACE(log, "Too early to clean queue, will do it later.");
continue;
}
/// ZooKeeper connection is recovered by main thread. We will wait for it on cleanup_event.
auto zookeeper = tryGetZooKeeper();
if (zookeeper->expired())
continue;
cleanupQueue(current_time_seconds, zookeeper);
last_cleanup_time_seconds = current_time_seconds;
}
catch (...)
{
tryLogCurrentException(log, __PRETTY_FUNCTION__);
}
}
}
}
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