ruby 59 lines · 8 steps

Building a thread-safe connection pool in Ruby

A bounded pool that reuses connections, lazily creates them up to a cap, and blocks callers until one frees up or a deadline passes.

Explained by highlit

Advanced concurrency resource-pooling mutex condition-variable timeouts

1require "thread"
2 
3class ConnectionPool
class TimeoutError < StandardError; end
5 
def initialize(size: 5, timeout: 5, &factory)
  raise ArgumentError, "block required to build connections" unless factory
8 
  @factory = factory
  @timeout = timeout
  @mutex = Mutex.new
  @resource = ConditionVariable.new
  @available = Queue.new
  @created = 0
  @max = size
end
17 
def with
  conn = checkout
  begin
    yield conn
  ensure
    checkin(conn)
  end
end
26 
def checkout
  deadline = clock + @timeout
29 
  @mutex.synchronize do
    loop do
      return @available.pop(true) unless @available.empty?
33 
      if @created < @max
        @created += 1
        return @factory.call
      end
38 
      remaining = deadline - clock
      raise TimeoutError, "waited #{@timeout}s for a connection" if remaining <= 0
41 
      @resource.wait(@mutex, remaining)
    end
  end
end
46 
def checkin(conn)
  @mutex.synchronize do
    @available.push(conn)
    @resource.signal
  end
end
53 
private
55 
def clock
  Process.clock_gettime(Process::CLOCK_MONOTONIC)
end
59end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1A condition variable lets waiting threads sleep until a resource is returned, avoiding busy-loops while holding a lock.
2Lazily creating resources up to a cap balances startup cost against a hard limit on concurrent connections.
3Wrapping checkout and checkin in an ensure block guarantees resources return to the pool even when the caller raises.

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