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class ActiveRecord::ConnectionAdapters::ConnectionHandler

Parent:: Object

ConnectionHandler is a collection of ConnectionPool objects. It is used for keeping separate connection pools 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.

The ConnectionHandler class is not coupled with the Active models, as it has no knowledge about the model. The model needs to pass a specification name to the handler, in order to look up the correct connection pool.

Public Class Methods

new() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1008
def initialize
  # These caches are keyed by spec.name (ConnectionSpecification#name).
  @owner_to_pool = ConnectionHandler.create_owner_to_pool

  # Backup finalizer: if the forked child never needed a pool, the above
  # early discard has not occurred
  ObjectSpace.define_finalizer self, ConnectionHandler.unowned_pool_finalizer(@owner_to_pool)
end

Public Instance Methods

active_connections?() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1066
def active_connections?
  connection_pool_list.any?(&:active_connection?)
end

Returns true if there are any active connections among the connection pools that the ConnectionHandler is managing.

clear_active_connections!() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1073
def clear_active_connections!
  connection_pool_list.each(&:release_connection)
end

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.

clear_all_connections!() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1084
def clear_all_connections!
  connection_pool_list.each(&:disconnect!)
end

clear_reloadable_connections!() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1080
def clear_reloadable_connections!
  connection_pool_list.each(&:clear_reloadable_connections!)
end

Clears the cache which maps classes.

See ActiveRecord::ConnectionAdapters::ConnectionPool#clear_reloadable_connections! for details.

connected?(spec_name) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1116
def connected?(spec_name)
  pool = retrieve_connection_pool(spec_name)
  pool && pool.connected?
end

Returns true if a connection that's accessible to this class has already been opened.

connection_pool_list() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1037
def connection_pool_list
  owner_to_pool.values.compact
end

Also aliased as: connection_pools

connection_pools()

Alias for: connection_pool_list

establish_connection(config) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1042
def establish_connection(config)
  resolver = ConnectionSpecification::Resolver.new(Base.configurations)
  spec = resolver.spec(config)

  remove_connection(spec.name)

  message_bus = ActiveSupport::Notifications.instrumenter
  payload = {
    connection_id: object_id
  }
  if spec
    payload[:spec_name] = spec.name
    payload[:config] = spec.config
  end

  message_bus.instrument("!connection.active_record", payload) do
    owner_to_pool[spec.name] = ConnectionAdapters::ConnectionPool.new(spec)
  end

  owner_to_pool[spec.name]
end

flush_idle_connections!() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1091
def flush_idle_connections!
  connection_pool_list.each(&:flush!)
end

Disconnects all currently idle connections.

See ActiveRecord::ConnectionAdapters::ConnectionPool#flush! for details.

remove_connection(spec_name) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1125
def remove_connection(spec_name)
  if pool = owner_to_pool.delete(spec_name)
    pool.automatic_reconnect = false
    pool.disconnect!
    pool.spec.config
  end
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.

retrieve_connection_pool(spec_name) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1136
def retrieve_connection_pool(spec_name)
  owner_to_pool.fetch(spec_name) do
    # Check if a connection was previously established in an ancestor process,
    # which may have been forked.
    if ancestor_pool = pool_from_any_process_for(spec_name)
      # A connection was established in an ancestor process that must have
      # subsequently forked. We can't reuse the connection, but we can copy
      # the specification and establish a new connection with it.
      establish_connection(ancestor_pool.spec.to_hash).tap do |pool|
        pool.schema_cache = ancestor_pool.schema_cache if ancestor_pool.schema_cache
      end
    else
      owner_to_pool[spec_name] = nil
    end
  end
end

Retrieving the connection pool happens a lot, so we cache it in @owner_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.

while_preventing_writes(enabled = true) { || ... } Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1030
def while_preventing_writes(enabled = true)
  original, self.prevent_writes = self.prevent_writes, enabled
  yield
ensure
  self.prevent_writes = original
end

Prevent writing to the database regardless of role.

In some cases you may want to prevent writes to the database even if you are on a database that can write. `while_preventing_writes` will prevent writes to the database for the duration of the block.