ClickHouse/dbms/src/Client/ConnectionPoolWithFailover.h

#pragma once

#include <Common/PoolWithFailoverBase.h>
#include <Client/ConnectionPool.h>


namespace DB
{

/** Connection pool with fault tolerance.
  * Initialized by several other IConnectionPools.
  * When a connection is received, it tries to create or select a live connection from a pool,
  *  fetch them in some order, using no more than the specified number of attempts.
  * Pools with fewer errors are preferred;
  *  pools with the same number of errors are tried in random order.
  *
  * Note: if one of the nested pools is blocked due to overflow, then this pool will also be blocked.
  */

/// Specifies how many connections to return from ConnectionPoolWithFailover::getMany() method.
enum class PoolMode
{
    /// Return exactly one connection.
    GET_ONE = 0,
    /// Return a number of connections, this number being determined by max_parallel_replicas setting.
    GET_MANY,
    /// Return a connection from each nested pool.
    GET_ALL
};

class ConnectionPoolWithFailover : public IConnectionPool, private PoolWithFailoverBase<IConnectionPool>
{
public:
    ConnectionPoolWithFailover(
            ConnectionPoolPtrs nested_pools_,
            LoadBalancing load_balancing,
            time_t decrease_error_period_ = DBMS_CONNECTION_POOL_WITH_FAILOVER_DEFAULT_DECREASE_ERROR_PERIOD);

    using Entry = IConnectionPool::Entry;

    /** Allocates connection to work. */
    Entry get(const ConnectionTimeouts & timeouts,
              const Settings * settings = nullptr,
              bool force_connected = true) override; /// From IConnectionPool

    /** Allocates up to the specified number of connections to work.
      * Connections provide access to different replicas of one shard.
      */
    std::vector<Entry> getMany(const ConnectionTimeouts & timeouts,
                               const Settings * settings, PoolMode pool_mode);

    /// The same as getMany(), but return std::vector<TryResult>.
    std::vector<TryResult> getManyForTableFunction(const ConnectionTimeouts & timeouts,
                                                   const Settings * settings, PoolMode pool_mode);

    using Base = PoolWithFailoverBase<IConnectionPool>;
    using TryResult = Base::TryResult;

    /// The same as getMany(), but check that replication delay for table_to_check is acceptable.
    /// Delay threshold is taken from settings.
    std::vector<TryResult> getManyChecked(
            const ConnectionTimeouts & timeouts,
            const Settings * settings,
            PoolMode pool_mode,
            const QualifiedTableName & table_to_check);

private:
    /// Get the values of relevant settings and call Base::getMany()
    std::vector<TryResult> getManyImpl(
            const Settings * settings,
            PoolMode pool_mode,
            const TryGetEntryFunc & try_get_entry);

    /// Try to get a connection from the pool and check that it is good.
    /// If table_to_check is not null and the check is enabled in settings, check that replication delay
    /// for this table is not too large.
    TryResult tryGetEntry(
            IConnectionPool & pool,
            const ConnectionTimeouts & timeouts,
            std::string & fail_message,
            const Settings * settings,
            const QualifiedTableName * table_to_check = nullptr);

private:
    std::vector<size_t> hostname_differences; /// Distances from name of this host to the names of hosts of pools.
    LoadBalancing default_load_balancing;
};

using ConnectionPoolWithFailoverPtr = std::shared_ptr<ConnectionPoolWithFailover>;
using ConnectionPoolWithFailoverPtrs = std::vector<ConnectionPoolWithFailoverPtr>;

}