### Home Classes Methods

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- lib/active_record/connection_adapters/abstract/connection_pool.rb

### Parent

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- ::new
- #active_connection?
- #checkin
- #checkout
- #clear_cache!
- #clear_reloadable_connections!
- #clear_stale_cached_connections!
- #columns
- #columns_hash
- #connected?
- #connection
- #disconnect!
- #primary_keys
- #release_connection
- #with_connection

### Included Modules

- MonitorMixin

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No matching classes.

# ActiveRecord::ConnectionAdapters::ConnectionPool

Connection pool base class for managing Active Record database connections.

## Introduction

A connection pool synchronizes thread access to a limited number of database connections. The basic idea is that each thread checks out a database connection from the pool, uses that connection, and checks the connection back in. ConnectionPool is completely thread-safe, and will ensure that a connection cannot be used by two threads at the same time, as long as ConnectionPool's contract is correctly followed. It will also handle cases in which there are more threads than connections: if all connections have been checked out, and a thread tries to checkout a connection anyway, then ConnectionPool will wait until some other thread has checked in a connection.

## Obtaining (checking out) a connection

Connections can be obtained and used from a connection pool in several ways:

1. Simply use ActiveRecord::Base.connection as with Active Record 2.1 and earlier (pre-connection-pooling). Eventually, when you're done with the connection(s) and wish it to be returned to the pool, you call ActiveRecord::Base.clear_active_connections!. This will be the default behavior for Active Record when used in conjunction with Action Pack's request handling cycle.

2. Manually check out a connection from the pool with ActiveRecord::Base.connection_pool.checkout. You are responsible for returning this connection to the pool when finished by calling ActiveRecord::Base.connection_pool.checkin(connection).

3. Use ActiveRecord::Base.connection_pool.with_connection(&block), which obtains a connection, yields it as the sole argument to the block, and returns it to the pool after the block completes.

Connections in the pool are actually AbstractAdapter objects (or objects compatible with AbstractAdapter's interface).

## Options

There are two connection-pooling-related options that you can add to your database connection configuration:

- `pool`: number indicating size of connection pool (default 5)

- +checkout _timeout+: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds). ('wait_timeout' supported for backwards compatibility, but conflicts with key used for different purpose by mysql2 adapter).

### Attributes

automatic_reconnect[RW]
connections[R]
spec[R]

### Public Class Methods

new(spec) click to toggle source

Creates a new ConnectionPool object. `spec` is a ConnectionSpecification object which describes database connection information (e.g. adapter, host name, username, password, etc), as well as the maximum size for this ConnectionPool.

The default ConnectionPool maximum size is 5.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 74
    def initialize(spec)
      super()

      @spec = spec

      # The cache of reserved connections mapped to threads
      @reserved_connections = {}

      @queue = new_cond
      # 'wait_timeout', the backward-compatible key, conflicts with spec key
      # used by mysql2 for something entirely different, checkout_timeout
      # preferred to avoid conflict and allow independent values.
      @timeout = spec.config[:checkout_timeout] || spec.config[:wait_timeout] || 5

      # default max pool size to 5
      @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5

      @connections         = []
      @automatic_reconnect = true
    end

### Public Instance Methods

active_connection?() click to toggle source

Is there an open connection that is being used for the current thread?

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 107
    def active_connection?
      synchronize do
        @reserved_connections.fetch(current_connection_id) {
          return false
        }.in_use?
      end
    end

checkin(conn) click to toggle source

Check-in a database connection back into the pool, indicating that you no longer need this connection.

`conn`: an AbstractAdapter object, which was obtained by earlier by calling `checkout` on this pool.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 285
    def checkin(conn)
      synchronize do
        conn.run_callbacks :checkin do
          conn.expire
          @queue.signal
        end

        release conn
      end
    end

checkout() click to toggle source

Check-out a database connection from the pool, indicating that you want to use it. You should call checkin when you no longer need this.

This is done by either returning an existing connection, or by creating a new connection. If the maximum number of connections for this pool has already been reached, but the pool is empty (i.e. they're all being used), then this method will wait until a thread has checked in a connection. The wait time is bounded however: if no connection can be checked out within the timeout specified for this pool, then a ConnectionTimeoutError exception will be raised.

Returns: an AbstractAdapter object.

Raises:

- ConnectionTimeoutError: no connection can be obtained from the pool within the timeout period.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 238
    def checkout
      synchronize do
        waited_time = 0

        loop do
          conn = @connections.find { |c| c.lease }

          unless conn
            if @connections.size < @size
              conn = checkout_new_connection
              conn.lease
            end
          end

          if conn
            checkout_and_verify conn
            return conn
          end

          if waited_time >= @timeout
            raise ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout} (waited #{waited_time} seconds). The max pool size is currently #{@size}; consider increasing it."
          end

          # Sometimes our wait can end because a connection is available,
          # but another thread can snatch it up first. If timeout hasn't
          # passed but no connection is avail, looks like that happened --
          # loop and wait again, for the time remaining on our timeout.
          before_wait = Time.now
          @queue.wait( [@timeout - waited_time, 0].max )
          waited_time += (Time.now - before_wait)

          # Will go away in Rails 4, when we don't clean up
          # after leaked connections automatically anymore. Right now, clean
          # up after we've returned from a 'wait' if it looks like it's
          # needed, then loop and try again.
          if(active_connections.size >= @connections.size)
            clear_stale_cached_connections!
          end
        end
      end
    end

clear_cache!() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 197
    def clear_cache!
      with_connection do |c|
        c.schema_cache.clear!
      end
    end

clear_reloadable_connections!() click to toggle source

Clears the cache which maps classes.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 152
    def clear_reloadable_connections!
      synchronize do
        @reserved_connections = {}
        @connections.each do |conn|
          checkin conn
          conn.disconnect! if conn.requires_reloading?
        end
        @connections.delete_if do |conn|
          conn.requires_reloading?
        end
      end
    end

clear_stale_cached_connections!() click to toggle source

Return any checked-out connections back to the pool by threads that are no longer alive.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 206
    def clear_stale_cached_connections!
      keys = @reserved_connections.keys - Thread.list.find_all { |t|
        t.alive?
      }.map { |thread| thread.object_id }
      keys.each do |key|
        conn = @reserved_connections[key]
        ActiveSupport::Deprecation.warn(Database connections will not be closed automatically, please close yourdatabase connection at the end of the thread by calling `close` on yourconnection.  For example: ActiveRecord::Base.connection.close) if conn.in_use?
        checkin conn
        @reserved_connections.delete(key)
      end
    end

columns() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 176
    def columns
      with_connection do |c|
        c.schema_cache.columns
      end
    end

columns_hash() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 183
    def columns_hash
      with_connection do |c|
        c.schema_cache.columns_hash
      end
    end

connected?() click to toggle source

Returns true if a connection has already been opened.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 135
    def connected?
      synchronize { @connections.any? }
    end

connection() click to toggle source

Retrieve the connection associated with the current thread, or call checkout to obtain one if necessary.

connection can be called any number of times; the connection is held in a hash keyed by the thread id.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 100
    def connection
      synchronize do
        @reserved_connections[current_connection_id] ||= checkout
      end
    end

disconnect!() click to toggle source

Disconnects all connections in the pool, and clears the pool.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 140
    def disconnect!
      synchronize do
        @reserved_connections = {}
        @connections.each do |conn|
          checkin conn
          conn.disconnect!
        end
        @connections = []
      end
    end

primary_keys() click to toggle source

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 190
    def primary_keys
      with_connection do |c|
        c.schema_cache.primary_keys
      end
    end

release_connection(with_id = current_connection_id) click to toggle source

Signal that the thread is finished with the current connection. release_connection releases the connection-thread association and returns the connection to the pool.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 118
    def release_connection(with_id = current_connection_id)
      conn = synchronize { @reserved_connections.delete(with_id) }
      checkin conn if conn
    end

with_connection() click to toggle source

If a connection already exists yield it to the block. If no connection exists checkout a connection, yield it to the block, and checkin the connection when finished.

    # File lib/active_record/connection_adapters/abstract/connection_pool.rb, line 126
    def with_connection
      connection_id = current_connection_id
      fresh_connection = true unless active_connection?
      yield connection
    ensure
      release_connection(connection_id) if fresh_connection
    end

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