ClickHouse/src/Storages/StorageExternalDistributed.h

#pragma once

#include "config_core.h"

#include <Storages/IStorage.h>


namespace DB
{

struct ExternalDataSourceConfiguration;

/// Storages MySQL and PostgreSQL use ConnectionPoolWithFailover and support multiple replicas.
/// This class unites multiple storages with replicas into multiple shards with replicas.
/// A query to external database is passed to one replica on each shard, the result is united.
/// Replicas on each shard have the same priority, traversed replicas are moved to the end of the queue.
/// Similar approach is used for URL storage.
class StorageExternalDistributed final : public DB::IStorage
{
public:
    enum class ExternalStorageEngine
    {
        MySQL,
        PostgreSQL,
        URL
    };

    StorageExternalDistributed(
        const StorageID & table_id_,
        ExternalStorageEngine table_engine,
        const String & cluster_description,
        const ExternalDataSourceConfiguration & configuration,
        const ColumnsDescription & columns_,
        const ConstraintsDescription & constraints_,
        const String & comment,
        ContextPtr context_);

    StorageExternalDistributed(
        const String & addresses_description,
        const StorageID & table_id,
        const String & format_name,
        const std::optional<FormatSettings> & format_settings,
        const String & compression_method,
        const ColumnsDescription & columns,
        const ConstraintsDescription & constraints,
        ContextPtr context);

    std::string getName() const override { return "ExternalDistributed"; }

    Pipe read(
        const Names & column_names,
        const StorageSnapshotPtr & storage_snapshot,
        SelectQueryInfo & query_info,
        ContextPtr context,
        QueryProcessingStage::Enum processed_stage,
        size_t max_block_size,
        unsigned num_streams) override;

private:
    using Shards = std::unordered_set<StoragePtr>;
    Shards shards;
};

}