Class: Concurrent::FixedThreadPool

ThreadPoolExecutor show all
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Failure to properly shutdown a thread pool can lead to unpredictable results. Please read Shutting Down Thread Pools for more information.

A thread pool that reuses a fixed number of threads operating off an unbounded queue. At any point, at most num_threads will be active processing tasks. When all threads are busy new tasks #post to the thread pool are enqueued until a thread becomes available. Should a thread crash for any reason the thread will immediately be removed from the pool and replaced.

The API and behavior of this class are based on Java's FixedThreadPool

Thread Pool Options

Thread pools support several configuration options:

  • idletime: The number of seconds that a thread may be idle before being reclaimed.
  • max_queue: The maximum number of tasks that may be waiting in the work queue at any one time. When the queue size reaches max_queue and no new threads can be created, subsequent tasks will be rejected in accordance with the configured fallback_policy.
  • auto_terminate: When true (default) an at_exit handler will be registered which will stop the thread pool when the application exits. See below for more information on shutting down thread pools.
  • fallback_policy: The policy defining how rejected tasks are handled.

Three fallback policies are supported:

  • :abort: Raise a RejectedExecutionError exception and discard the task.
  • :discard: Discard the task and return false.
  • :caller_runs: Execute the task on the calling thread.

Shutting Down Thread Pools

Killing a thread pool while tasks are still being processed, either by calling the #kill method or at application exit, will have unpredictable results. There is no way for the thread pool to know what resources are being used by the in-progress tasks. When those tasks are killed the impact on those resources cannot be predicted. The best practice is to explicitly shutdown all thread pools using the provided methods:

  • Call #shutdown to initiate an orderly termination of all in-progress tasks
  • Call #wait_for_termination with an appropriate timeout interval an allow the orderly shutdown to complete
  • Call #kill only when the thread pool fails to shutdown in the allotted time

On some runtime platforms (most notably the JVM) the application will not exit until all thread pools have been shutdown. To prevent applications from "hanging" on exit all thread pools include an at_exit handler that will stop the thread pool when the application exits. This handler uses a brute force method to stop the pool and makes no guarantees regarding resources being used by any tasks still running. Registration of this at_exit handler can be prevented by setting the thread pool's constructor :auto_terminate option to false when the thread pool is created. All thread pools support this option.

pool1 = # an `at_exit` handler will be registered
pool2 =, auto_terminate: false) # prevent `at_exit` handler registration

Instance Method Summary collapse

Constructor Details

#initialize(num_threads, opts = {}) ⇒ FixedThreadPool

Create a new thread pool.


  • num_threads (Integer)

    the number of threads to allocate

  • opts (Hash) (defaults to: {})

    the options defining pool behavior.

Options Hash (opts):

  • :fallback_policy (Symbol) — default: `:abort`

    the fallback policy


  • (ArgumentError)

    if num_threads is less than or equal to zero

  • (ArgumentError)

    if fallback_policy is not a known policy

See Also:

# File 'lib/concurrent/executor/fixed_thread_pool.rb', line 197

def initialize(num_threads, opts = {})
  raise'number of threads must be greater than zero') if num_threads.to_i < 1
  defaults  = { max_queue:   DEFAULT_MAX_QUEUE_SIZE,
                idletime:    DEFAULT_THREAD_IDLETIMEOUT }
  overrides = { min_threads: num_threads,
                max_threads: num_threads }