Package Exports

generic-pool
generic-pool/lib/errors

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Readme

Generic Pool

About

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

Node.js Version Warning

Generic-Pool v3 requires a nodejs version of at least 4

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 Pool = require('generic-pool').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 = new Pool(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
        pool.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.
pool.drain(function() {
    pool.clear();
});

Documentation

Constructor

The Pool constructor 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

var pool = new Pool(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.

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.
refreshIdle: boolean that specifies whether idle resources at or below the min threshold should be destroyed/re-created. (default=true)
idleTimeoutMillis: max milliseconds a resource can stay unused in the pool without being borrowed before it should be destroyed (default 30000)
reapIntervalMillis: interval to check for idle resources (default 1000). (remove me!)
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)
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

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

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.

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.

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
var opts = {
  priorityRange : 3
}
var pool = new Pool(someFactory,opts);

// acquire connection - no priority specified - will go onto lowest priority queue
pool.acquire().thenfunction(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

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

To transparently handle object acquisition for a function, one can use pooled():

var privateFn, publicFn;
publicFn = pool.pooled(privateFn = function(client, arg, cb) {
    // Do something with the client and arg. Client is auto-released when cb is called
    cb(null, arg);
});

Keeping both private and public versions of each function allows for pooled functions to call other pooled functions with the same member. This is a handy pattern for database transactions:

var privateTop, privateBottom, publicTop, publicBottom;
publicBottom = pool.pooled(privateBottom = function(client, arg, cb) {
    //Use client, assumed auto-release
});

publicTop = pool.pooled(privateTop = function(client, cb) {
    // e.g., open a database transaction
    privateBottom(client, "arg", function(err, retVal) {
        if(err) { return cb(err); }
        // e.g., close a transaction
        cb();
    });
});

Pool info

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

// 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

// 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.