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.
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Class Public methods
new() Link
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 75 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) Link
Returns true if there are any active connections among the connection pools that the ConnectionHandler
is managing.
all_connection_pools() Link
clear_active_connections!(role = ActiveRecord::Base.current_role) Link
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!(role = ActiveRecord::Base.current_role) Link
clear_reloadable_connections!(role = ActiveRecord::Base.current_role) Link
Clears the cache which maps classes.
See ConnectionPool#clear_reloadable_connections!
for details.
connected?(spec_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard) Link
Returns true if a connection that's accessible to this class has already been opened.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 216 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
establish_connection(config, owner_name: Base, role: ActiveRecord::Base.current_role, shard: Base.current_shard) Link
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 126 def establish_connection(config, owner_name: Base, role: ActiveRecord::Base.current_role, shard: Base.current_shard) owner_name = StringConnectionOwner.new(config.to_s) if config.is_a?(Symbol) pool_config = resolve_pool_config(config, owner_name, role, shard) 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.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) Link
Disconnects all currently idle connections.
See ConnectionPool#flush!
for details.
remove_connection_pool(owner, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard) Link
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 221 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) Link
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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 235 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) Link
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.
# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 102 def while_preventing_writes(enabled = true) unless ActiveRecord.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