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:
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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.
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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).
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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:
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pool: number indicating size of connection pool (default 5)
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wait_timeout: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds).
- A
- C
- D
- N
- R
- T
- W
[RW] | automatic_reconnect | |
[R] | spec | |
[R] | connections | |
[R] | columns | |
[R] | columns_hash | |
[R] | primary_keys | |
[R] | tables | |
[R] | column_defaults |
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 activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 71 def initialize(spec) @spec = spec # The cache of reserved connections mapped to threads @reserved_connections = {} # The mutex used to synchronize pool access @connection_mutex = Monitor.new @queue = @connection_mutex.new_cond @timeout = spec.config[:wait_timeout] || 5 # default max pool size to 5 @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5 @connections = [] @checked_out = [] @automatic_reconnect = true @tables = {} @visitor = nil @columns = Hash.new do |h, table_name| h[table_name] = with_connection do |conn| # Fetch a list of columns conn.columns(table_name, "#{table_name} Columns").tap do |columns| # set primary key information columns.each do |column| column.primary = column.name == primary_keys[table_name] end end end end @columns_hash = Hash.new do |h, table_name| h[table_name] = Hash[columns[table_name].map { |col| [col.name, col] }] end @column_defaults = Hash.new do |h, table_name| h[table_name] = Hash[columns[table_name].map { |col| [col.name, col.default] }] end @primary_keys = Hash.new do |h, table_name| h[table_name] = with_connection do |conn| table_exists?(table_name) ? conn.primary_key(table_name) : 'id' end end end
Check to see if there is an active connection in this connection pool.
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.
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:
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ConnectionTimeoutError: no connection can be obtained from the pool within the timeout period.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 258 def checkout # Checkout an available connection @connection_mutex.synchronize do loop do conn = if @checked_out.size < @connections.size checkout_existing_connection elsif @connections.size < @size checkout_new_connection end return conn if conn @queue.wait(@timeout) if(@checked_out.size < @connections.size) next else clear_stale_cached_connections! if @size == @checked_out.size raise ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout}. The max pool size is currently #{@size}; consider increasing it." end end end end end
Clears out internal caches:
* columns * columns_hash * tables
Clears the cache which maps classes.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 208 def clear_reloadable_connections! @reserved_connections.each do |name, conn| checkin conn end @reserved_connections = {} @connections.each do |conn| conn.disconnect! if conn.requires_reloading? end @connections.delete_if do |conn| conn.requires_reloading? end end
Return any checked-out connections back to the pool by threads that are no longer alive.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 232 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| checkin @reserved_connections[key] @reserved_connections.delete(key) end end
Clear out internal caches for table with table_name.
Returns true if a connection has already been opened.
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.
Disconnects all connections in the pool, and clears the pool.
Signal that the thread is finished with the current connection. release_connection releases the connection-thread association and returns the connection to the pool.
A cached lookup for table existence.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 125 def table_exists?(name) return true if @tables.key? name with_connection do |conn| conn.tables.each { |table| @tables[table] = true } @tables[name] = true if !@tables.key?(name) && conn.table_exists?(name) end @tables.key? name end
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 activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 182 def with_connection connection_id = current_connection_id fresh_connection = true unless @reserved_connections[connection_id] yield connection ensure release_connection(connection_id) if fresh_connection end