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:
class Author < ActiveRecord::Base
end
class BankAccount < ActiveRecord::Base
end
class Book < ActiveRecord::Base
establish_connection "library_db"
end
class ScaryBook < Book
end
class GoodBook < Book
end
And a database.yml that looked like this:
development:
database: my_application
host: localhost
library_db:
database: library
host: some.library.org
Your primary database in the development environment is “my_application” but the Book model connects to a separate database called “library_db” (this can even be a database on a different machine).
Book, ScaryBook and GoodBook will all use the same connection pool to “library_db” while Author, BankAccount, and any other models you create will use the default connection pool to “my_application”.
The various connection pools are managed by a single instance of ConnectionHandler accessible via ActiveRecord::Core.connection_handler. All Active Record models use this handler 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 511 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 527 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.) ".squish) Hash[connection_pool_list.map { |pool| [pool.spec, pool] }] end
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 536 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 605 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