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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::Base.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 connection specification name to the handler, in order to look up the correct connection pool.

Methods
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Class Public methods

new()

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 992
def initialize
  # These caches are keyed by pool_config.connection_specification_name (PoolConfig#connection_specification_name).
  @owner_to_pool_manager = Concurrent::Map.new(initial_capacity: 2)

  # Backup finalizer: if the forked child skipped Kernel#fork the early discard has not occurred
  ObjectSpace.define_finalizer self, FINALIZER
end

Instance Public methods

active_connections?(role = ActiveRecord::Base.current_role)

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

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1078
def active_connections?(role = ActiveRecord::Base.current_role)
  connection_pool_list(role).any?(&:active_connection?)
end

all_connection_pools()

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1034
def all_connection_pools
  owner_to_pool_manager.values.flat_map { |m| m.pool_configs.map(&:pool) }
end

clear_active_connections!(role = ActiveRecord::Base.current_role)

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.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1085
def clear_active_connections!(role = ActiveRecord::Base.current_role)
  connection_pool_list(role).each(&:release_connection)
end

clear_all_connections!(role = ActiveRecord::Base.current_role)

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1096
def clear_all_connections!(role = ActiveRecord::Base.current_role)
  connection_pool_list(role).each(&:disconnect!)
end

clear_reloadable_connections!(role = ActiveRecord::Base.current_role)

Clears the cache which maps classes.

See ConnectionPool#clear_reloadable_connections! for details.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1092
def clear_reloadable_connections!(role = ActiveRecord::Base.current_role)
  connection_pool_list(role).each(&:clear_reloadable_connections!)
end

connected?(spec_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

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

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1133
def connected?(spec_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  pool = retrieve_connection_pool(spec_name, role: role, shard: shard)
  pool && pool.connected?
end

connection_pool_list(role = ActiveRecord::Base.current_role)

Also aliased as: connection_pools

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1038
def connection_pool_list(role = ActiveRecord::Base.current_role)
  owner_to_pool_manager.values.flat_map { |m| m.pool_configs(role).map(&:pool) }
end

connection_pools(role = ActiveRecord::Base.current_role)

Alias for: connection_pool_list

establish_connection(config, owner_name: Base, role: ActiveRecord::Base.current_role, shard: Base.current_shard)

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1043
def establish_connection(config, owner_name: Base, role: ActiveRecord::Base.current_role, shard: Base.current_shard)
  owner_name = config.to_s if config.is_a?(Symbol)

  pool_config = resolve_pool_config(config, owner_name)
  db_config = pool_config.db_config

  # Protects the connection named `ActiveRecord::Base` from being removed
  # if the user calls `establish_connection :primary`.
  if owner_to_pool_manager.key?(pool_config.connection_specification_name)
    remove_connection_pool(pool_config.connection_specification_name, role: role, shard: shard)
  end

  message_bus = ActiveSupport::Notifications.instrumenter
  payload = {}
  if pool_config
    payload[:spec_name] = pool_config.connection_specification_name
    payload[:shard] = shard
    payload[:config] = db_config.configuration_hash
  end

  if ActiveRecord::Base.legacy_connection_handling
    owner_to_pool_manager[pool_config.connection_specification_name] ||= LegacyPoolManager.new
  else
    owner_to_pool_manager[pool_config.connection_specification_name] ||= PoolManager.new
  end
  pool_manager = get_pool_manager(pool_config.connection_specification_name)
  pool_manager.set_pool_config(role, shard, pool_config)

  message_bus.instrument("!connection.active_record", payload) do
    pool_config.pool
  end
end

flush_idle_connections!(role = ActiveRecord::Base.current_role)

Disconnects all currently idle connections.

See ConnectionPool#flush! for details.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1103
def flush_idle_connections!(role = ActiveRecord::Base.current_role)
  connection_pool_list(role).each(&:flush!)
end

remove_connection(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

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.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1142
def remove_connection(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  remove_connection_pool(owner, role: role, shard: shard)&.configuration_hash
end

remove_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1147
def remove_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  if pool_manager = get_pool_manager(owner)
    pool_config = pool_manager.remove_pool_config(role, shard)

    if pool_config
      pool_config.disconnect!
      pool_config.db_config
    end
  end
end

retrieve_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

Retrieving the connection pool happens a lot, so we cache it in @owner_to_pool_manager. This makes retrieving the connection pool O(1) once the process is warm. When a connection is established or removed, we invalidate the cache.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1161
def retrieve_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  pool_config = get_pool_manager(owner)&.get_pool_config(role, shard)
  pool_config&.pool
end

while_preventing_writes(enabled = true)

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.

This method does not provide the same protection as a readonly user and is meant to be a safeguard against accidental writes.

See `READ_QUERY` for the queries that are blocked by this method.

Source: show | on GitHub 

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 1019
def while_preventing_writes(enabled = true)
  unless ActiveRecord::Base.legacy_connection_handling
    raise NotImplementedError, "`while_preventing_writes` is only available on the connection_handler with legacy_connection_handling"
  end

  original, self.prevent_writes = self.prevent_writes, enabled
  yield
ensure
  self.prevent_writes = original
end