Generic Pool

About

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 accomodate. 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.

History

The history has been moved to the CHANGELOG

Installation

$ npm install generic-pool [--save]

Example

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

var genericPool = require('generic-pool');
var DbDriver = require('some-db-driver');

/**
 * Step 1 - Create pool using a factory object
 */
const factory = {
    create: function(){
		 return new Promise(function(resolve, reject){
	        var client = DbDriver.createClient()
	        client.on('connected', function(){
	            resolve(client)
	        })
	    })
    },
    destroy: function(client){
        return new Promise(function(resolve){
          client.on('end', function(){
            resolve()
          })
          client.disconnect()
        })
    }
}

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

var 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()

resourcePromise.then(function(client) {
	client.query("select * from foo", [], function() {
	    // return object back to pool
	    myPool.release(client);
	});
})
.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() {
    myPool.clear();
});

Documentation

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.

createPool

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 the of the Pool

const genericPool = require('generic-pool')
const pool = genericPool.createPool(factory, opts)

factory

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 resourse for whatever.
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.

opts

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 either factory.validate or factory.validateAsync to be 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 etc once the constructor is called, (default true)
evictionRunIntervalMillis: How often to run eviction checks. Default: 0 (does not run).
numTestsPerRun: 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).

pool.acquire

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

pool.acquire().then(onfulfilled)
//or
const priority = 2
pool.acquire(priority).then(onfulfilled)

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

pool.release

pool.release(resource)

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.

pool.destroy

pool.destroy(resource)

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

pool.on

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.

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) {
    pool.release(client);
});

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

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

// etc..

Draining

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
pool.spareResourceCapacity

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

// returns number of unused resources in the pool
pool.available

// number of resources that are currently acquired by userland code
pool.borrowed

// returns number of callers waiting to acquire a resource
pool.pending

// returns number of maxixmum number of resources allowed by ppol
pool.max

// returns number of minimum number of resources allowed by ppol
pool.min

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).

Linting

We use eslint and the standard ruleset.

License

(The MIT License)

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.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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