class ActiveRecord::ConnectionAdapters::ConnectionPool::Reaper

Active Record Connection Pool Reaper

The reaper is a singleton that exists in the background of the process and is responsible for general maintenance of all the connection pools.

It will reclaim connections that are leased to now-dead threads, ensuring that a bad thread can’t leak a pool slot forever. By definition, this involves touching currently-leased connections, but that is safe because the owning thread is known to be dead.

Beyond that, it manages the health of available / unleased connections:

* retiring connections that have been idle[1] for too long
* creating occasional activity on inactive[1] connections
* keeping the pool prepopulated up to its minimum size
* proactively connecting to the target database from any pooled
  connections that had lazily deferred that step
* resetting or replacing connections that are known to be broken

[1]: “idle” and “inactive” here distinguish between connections that have not been requested by the application in a while (idle) and those that have not spoken to their remote server in a while (inactive). The former is a desirable opportunity to reduce our connection count (‘idle_timeout`); the latter is a risk that the server or a firewall may drop a connection we still anticipate using (avoided by `keepalive`).

Attributes

frequency [R]

pool [R]

Public Class Methods

new (pool, frequency)

Source

# File lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb, line 36
def initialize(pool, frequency)
  @pool      = pool
  @frequency = frequency
end

Private Class Methods

spawn_thread (frequency)

Source

# File lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb, line 66
def spawn_thread(frequency)
  Thread.new(frequency) do |t|
    # Advise multi-threaded app servers to ignore this thread for
    # the purposes of fork safety warnings
    Thread.current.thread_variable_set(:fork_safe, true)
    Thread.current.name = "AR Pool Reaper"
    running = true
    while running
      sleep t

      refs = nil

      @mutex.synchronize do
        refs = @pools[frequency]

        refs.select! do |pool|
          pool.weakref_alive? && !pool.discarded?
        rescue WeakRef::RefError
        end

        if refs.empty?
          @pools.delete(frequency)
          @threads.delete(frequency)
          running = false
        end
      end

      if running
        pools(refs).each do |pool|
          pool.reaper_lock do
            pool.reap
            pool.flush
            pool.prepopulate
            pool.retire_old_connections
            pool.keep_alive
            pool.preconnect
          end
        end
      end
    end
  end
end

Public Instance Methods

run ()

Source

# File lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb, line 110
def run
  return unless frequency && frequency > 0
  self.class.register_pool(pool, frequency)
end