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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::Base.connection_handler. Active Record models use this to determine the connection pool that they should use.

Methods
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Class Public methods
new()
# 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
Instance Public methods
active_connections?()

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

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 515
def active_connections?
  connection_pool_list.any?(&:active_connection?)
end
clear_active_connections!()

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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 522
def clear_active_connections!
  connection_pool_list.each(&:release_connection)
end
clear_all_connections!()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 531
def clear_all_connections!
  connection_pool_list.each(&:disconnect!)
end
clear_reloadable_connections!()

Clears the cache which maps classes.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 527
def clear_reloadable_connections!
  connection_pool_list.each(&:clear_reloadable_connections!)
end
connected?(klass)

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 549
def connected?(klass)
  conn = retrieve_connection_pool(klass)
  conn && conn.connected?
end
connection_pool_list()
# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 495
def connection_pool_list
  owner_to_pool.values.compact
end
connection_pools()
# 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
establish_connection(owner, spec)
# 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_connection(owner)

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.

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 558
def remove_connection(owner)
  if pool = owner_to_pool.delete(owner.name)
    @class_to_pool.clear
    pool.automatic_reconnect = false
    pool.disconnect!
    pool.spec.config
  end
end
retrieve_connection_pool(klass)

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 576
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