ConnectionHandler is a collection of ConnectionPool objects. It is used for keeping separate connection pools for Active Record models that connect to different databases.
For example, suppose that you have 5 models, with the following hierarchy:
|
+-- Book
| |
| +-- ScaryBook
| +-- GoodBook
+-- Author
+-- BankAccount
Suppose that Book is to connect to a separate database (i.e. one other than the default database). Then Book, ScaryBook and GoodBook will all use the same connection pool. Likewise, Author and BankAccount will use the same connection pool. However, the connection pool used by Author/BankAccount is not the same as the one used by Book/ScaryBook/GoodBook.
Normally there is only a single ConnectionHandler instance, accessible via ActiveRecord::Core.connection_handler. Active Record models use this to determine the connection pool that they should use.
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# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 483 def initialize # These caches are keyed by klass.name, NOT klass. Keying them by klass # alone would lead to memory leaks in development mode as all previous # instances of the class would stay in memory. @owner_to_pool = ThreadSafe::Cache.new(:initial_capacity => 2) do |h,k| h[k] = ThreadSafe::Cache.new(:initial_capacity => 2) end @class_to_pool = ThreadSafe::Cache.new(:initial_capacity => 2) do |h,k| h[k] = ThreadSafe::Cache.new end end
Returns true if there are any active connections among the connection pools that the ConnectionHandler is managing.
Returns any connections in use by the current thread back to the pool, and also returns connections to the pool cached by threads that are no longer alive.
Clears the cache which maps classes.
Returns true if a connection that's accessible to this class has already been opened.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 499 def connection_pools ActiveSupport::Deprecation.warn( "In the next release, this will return the same as #connection_pool_list. " "(An array of pools, rather than a hash mapping specs to pools.)" ) Hash[connection_pool_list.map { |pool| [pool.spec, pool] }] end
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 507 def establish_connection(owner, spec) @class_to_pool.clear raise RuntimeError, "Anonymous class is not allowed." unless owner.name owner_to_pool[owner.name] = ConnectionAdapters::ConnectionPool.new(spec) end
Remove the connection for this class. This will close the active connection and the defined connection (if they exist). The result can be used as an argument for #establish_connection, for easily re-establishing the connection.
Retrieving the connection pool happens a lot so we cache it in @class_to_pool. This makes retrieving the connection pool O(1) once the process is warm. When a connection is established or removed, we invalidate the cache.
Ideally we would use fetch here, as class_to_pool may sometimes be nil. However, benchmarking (gist.github.com/jonleighton/3552829) showed that fetch is significantly slower than []. So in the nil case, no caching will take place, but that's ok since the nil case is not the common one that we wish to optimise for.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 573 def retrieve_connection_pool(klass) class_to_pool[klass.name] ||= begin until pool = pool_for(klass) klass = klass.superclass break unless klass <= Base end class_to_pool[klass.name] = pool end end