Generic resource pooling for Node.JS
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Generic Pool


Generic resource pool with Promise based API. Can be used to reuse or throttle usage of expensive resources such as database connections.

V3 upgrade warning

Version 3 contains many breaking changes. The differences are mostly minor and I hope easy to accommodate. There is a very rough and basic upgrade guide I've written, improvements and other attempts most welcome.

If you are after the older version 2 of this library you should look at the current github branch for it.


The history has been moved to the CHANGELOG


$ npm install generic-pool [--save]


Here is an example using a fictional generic database driver that doesn't implement any pooling whatsoever itself.

const genericPool = require("generic-pool");
const DbDriver = require("some-db-driver");

 * Step 1 - Create pool using a factory object
const factory = {
  create: function() {
    return DbDriver.createClient();
  destroy: function(client) {

const opts = {
  max: 10, // maximum size of the pool
  min: 2 // minimum size of the pool

const myPool = genericPool.createPool(factory, opts);

 * Step 2 - Use pool in your code to acquire/release resources

// acquire connection - Promise is resolved
// once a resource becomes available
const resourcePromise = myPool.acquire();

  .then(function(client) {
    client.query("select * from foo", [], function() {
      // return object back to pool
  .catch(function(err) {
    // handle error - this is generally a timeout or maxWaitingClients
    // error

 * Step 3 - Drain pool during shutdown (optional)
// Only call this once in your application -- at the point you want
// to shutdown and stop using this pool.
myPool.drain().then(function() {


Creating a pool

Whilst it is possible to directly instantiate the Pool class directly, it is recommended to use the createPool function exported by module as the constructor method signature may change in the future.


The createPool function takes two arguments:

  • factory : an object containing functions to create/destroy/test resources for the Pool
  • opts : an optional object/dictonary to allow configuring/altering behaviour of the Pool
const genericPool = require('generic-pool')
const pool = genericPool.createPool(factory, opts)


Can be any object/instance but must have the following properties:

  • create : a function that the pool will call when it wants a new resource. It should return a Promise that either resolves to a resource or rejects to an Error if it is unable to create a resource for whatever reason.
  • destroy: a function that the pool will call when it wants to destroy a resource. It should accept one argument resource where resource is whatever factory.create made. The destroy function should return a Promise that resolves once it has destroyed the resource.

optionally it can also have the following property:

  • validate: a function that the pool will call if it wants to validate a resource. It should accept one argument resource where resource is whatever factory.create made. Should return a Promise that resolves a boolean where true indicates the resource is still valid or false if the resource is invalid.

Note: The values returned from create, destroy, and validate are all wrapped in a Promise.resolve by the pool before being used internally.


An optional object/dictionary with the any of the following properties:

  • max: maximum number of resources to create at any given time. (default=1)
  • min: minimum number of resources to keep in pool at any given time. If this is set >= max, the pool will silently set the min to equal max. (default=0)
  • maxWaitingClients: maximum number of queued requests allowed, additional acquire calls will be callback with an err in a future cycle of the event loop.
  • testOnBorrow: boolean: should the pool validate resources before giving them to clients. Requires that factory.validate is specified.
  • acquireTimeoutMillis: max milliseconds an acquire call will wait for a resource before timing out. (default no limit), if supplied should non-zero positive integer.
  • fifo : if true the oldest resources will be first to be allocated. If false the most recently released resources will be the first to be allocated. This in effect turns the pool's behaviour from a queue into a stack. boolean, (default true)
  • priorityRange: int between 1 and x - if set, borrowers can specify their relative priority in the queue if no resources are available. see example. (default 1)
  • autostart: boolean, should the pool start creating resources, initialize the evictor, etc once the constructor is called. If false, the pool can be started by calling pool.start, otherwise the first call to acquire will start the pool. (default true)
  • evictionRunIntervalMillis: How often to run eviction checks. Default: 0 (does not run).
  • numTestsPerEvictionRun: Number of resources to check each eviction run. Default: 3.
  • softIdleTimeoutMillis: amount of time an object may sit idle in the pool before it is eligible for eviction by the idle object evictor (if any), with the extra condition that at least "min idle" object instances remain in the pool. Default -1 (nothing can get evicted)
  • idleTimeoutMillis: the minimum amount of time that an object may sit idle in the pool before it is eligible for eviction due to idle time. Supercedes softIdleTimeoutMillis Default: 30000
  • Promise: Promise lib, a Promises/A+ implementation that the pool should use. Defaults to whatever global.Promise is (usually native promises).


const onfulfilled = function(resource){
	// release/destroy/etc

const priority = 2

This function is for when you want to "borrow" a resource from the pool.

acquire takes one optional argument:

  • priority: optional, number, see Priority Queueing below.

and returns a Promise Once a resource in the pool is available, the promise will be resolved with a resource (whatever factory.create makes for you). If the Pool is unable to give a resource (e.g timeout) then the promise will be rejected with an Error



This function is for when you want to return a resource to the pool.

release takes one required argument:

  • resource: a previously borrowed resource

and returns a Promise. This promise will resolve once the resource is accepted by the pool, or reject if the pool is unable to accept the resource for any reason (e.g resource is not a resource or object that came from the pool). If you do not care the outcome it is safe to ignore this promise.



This function is for when you need to check if a resource has been acquired from the pool and not yet released/destroyed.

isBorrowedResource takes one required argument:

  • resource: any object which you need to test

and returns true (primitive, not Promise) if resource is currently borrowed from the pool, false otherwise.



This function is for when you want to return a resource to the pool but want it destroyed rather than being made available to other resources. E.g you may know the resource has timed out or crashed.

destroy takes one required argument:

  • resource: a previously borrowed resource

and returns a Promise. This promise will resolve once the resource is accepted by the pool, or reject if the pool is unable to accept the resource for any reason (e.g resource is not a resource or object that came from the pool). If you do not care the outcome it is safe to ignore this promise.


pool.on('factoryCreateError', function(err){
  //log stuff maybe

pool.on('factoryDestroyError', function(err){
  //log stuff maybe

The pool is an event emitter. Below are the events it emits and any args for those events

  • factoryCreateError : emitted when a promise returned by factory.create is rejected. If this event has no listeners then the error will be silently discarded

    • error: whatever reason the promise was rejected with.
  • factoryDestroyError : emitted when a promise returned by factory.destroy is rejected. If this event has no listeners then the error will be silently discarded

    • error: whatever reason the promise was rejected with.



If autostart is false then this method can be used to start the pool and therefore begin creation of resources, start the evictor, and any other internal logic.


const myTask = dbClient => {
  return new Promise( (resolve, reject) => {
    // do something with the client and resolve/reject

pool.use(myTask).then(/* a promise that will run after myTask resolves */)

This method handles acquiring a resource from the pool, handing it to your function and then calling pool.release or pool.destroy with resource after your function has finished.

use takes one required argument:

  • fn: a function that accepts a resource and returns a Promise. Once that promise resolves the resource is returned to the pool, else if it rejects then the resource is destroyed.

and returns a Promise that either resolves with the value from the user supplied fn or rejects with an error.

Idle Object Eviction

The pool has an evictor (off by default) which will inspect idle items in the pool and destroy them if they are too old.

By default the evictor does not run, to enable it you must set the evictionRunIntervalMillis option to a non-zero value. Once enable the evictor will check at most numTestsPerEvictionRun each time, this is to stop it blocking your application if you have lots of resources in the pool.

Priority Queueing

The pool supports optional priority queueing. This becomes relevant when no resources are available and the caller has to wait. acquire() accepts an optional priority int which specifies the caller's relative position in the queue. Each priority slot has it's own internal queue created for it. When a resource is available for borrowing, the first request in the highest priority queue will be given it.

Specifying a priority to acquire that is outside the priorityRange set at Pool creation time will result in the priority being converted the lowest possible priority

// create pool with priorityRange of 3
// borrowers can specify a priority 0 to 2
const opts = {
  priorityRange : 3
const pool = genericPool.createPool(someFactory,opts);

// acquire connection - no priority specified - will go onto lowest priority queue
pool.acquire().then(function(client) {

// acquire connection - high priority - will go into highest priority queue
pool.acquire(0).then(function(client) {

// acquire connection - medium priority - will go into 'mid' priority queue
pool.acquire(1).then(function(client) {

// etc..


If you are shutting down a long-lived process, you may notice that node fails to exit for 30 seconds or so. This is a side effect of the idleTimeoutMillis behavior -- the pool has a setTimeout() call registered that is in the event loop queue, so node won't terminate until all resources have timed out, and the pool stops trying to manage them.

This behavior will be more problematic when you set factory.min > 0, as the pool will never become empty, and the setTimeout calls will never end.

In these cases, use the pool.drain() function. This sets the pool into a "draining" state which will gracefully wait until all idle resources have timed out. For example, you can call:

If you do this, your node process will exit gracefully.

If you know you would like to terminate all the available resources in your pool before any timeouts they might have are reached, you can use clear() in conjunction with drain():

const p = pool.drain()
.then(function() {
    return pool.clear();

The promise returned will resolve once all waiting clients have acquired and return resources, and any available resources have been destroyed

One side-effect of calling drain() is that subsequent calls to acquire() will throw an Error.

Pooled function decoration

This has now been extracted out it's own module generic-pool-decorator

Pool info

The following properties will let you get information about the pool:

// How many many more resources can the pool manage/create

// returns number of resources in the pool regardless of
// whether they are free or in use

// returns number of unused resources in the pool

// number of resources that are currently acquired by userland code

// returns number of callers waiting to acquire a resource

// returns number of maxixmum number of resources allowed by pool

// returns number of minimum number of resources allowed by pool

Run Tests

$ npm install
$ npm test

The tests are run/written using Tap. Most are ports from the old espresso tests and are not in great condition. Most cases are inside test/generic-pool-test.js with newer cases in their own files (legacy reasons).


We use eslint combined with prettier


(The MIT License)

Copyright (c) 2010-2016 James Cooper <>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.


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