generic-pool

What is generic-pool?

The generic-pool npm package is a resource pooling library that allows users to manage a pool of resources such as database connections, network connections, or any other resource that is expensive to create and can be reused. It helps to limit the number of resources created and manages the allocation and deallocation of these resources efficiently.

What are generic-pool's main functionalities?

Creating a pool of resources

This code sample demonstrates how to create a pool with a maximum of 10 resources and a minimum of 2. The 'create' function is used to create a new resource, and the 'destroy' function is used to clean up a resource when it is no longer needed.

const genericPool = require('generic-pool');
const pool = genericPool.createPool({
  create: () => createMyResource(),
  destroy: (resource) => destroyMyResource(resource)
}, {
  max: 10,
  min: 2
});

Acquiring and releasing resources

This code sample shows how to acquire a resource from the pool and then release it back to the pool once it is no longer needed. The 'acquire' method returns a promise that resolves with a resource when one becomes available.

pool.acquire().then(resource => {
  // use the resource
  pool.release(resource);
}).catch(err => {
  // handle error
});

Draining the pool and shutting down

This code sample illustrates how to drain the pool of all its resources and then shut it down completely. The 'drain' method returns a promise that resolves once all the resources have been returned to the pool and are no longer in use.

pool.drain().then(() => pool.clear());

Other packages similar to generic-pool

bottleneck

Bottleneck is a rate limiter that can be used to throttle function calls. It is similar to generic-pool in that it helps manage resource usage, but it focuses on limiting the rate of operations rather than managing a pool of reusable resources.

pool2

Pool2 is another resource pooling library that provides similar functionalities to generic-pool. It offers features like resource creation, destruction, and timeout handling. Pool2 is designed to be a more modern and extensible version of generic-pool with additional features like priority queuing.

README

About

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

Installation

$ npm install generic-pool

History

The history has been moved to the CHANGELOG

Example

Step 1 - Create pool using a factory object

// Create a MySQL connection pool with
// a max of 10 connections, a min of 2, and a 30 second max idle time
var Pool = require('generic-pool').Pool;
var pool = new Pool({
    name     : 'mysql',
    create   : function(callback) {
        var Client = require('mysql').Client;
        var c = new Client();
        c.user     = 'scott';
        c.password = 'tiger';
        c.database = 'mydb';
        c.connect();

        // parameter order: err, resource
        // new in 1.0.6
        callback(null, c);
    },
    destroy  : function(client) { client.end(); },
    max      : 10,
    // optional. if you set this, make sure to drain() (see step 3)
    min      : 2,
    // specifies how long a resource can stay idle in pool before being removed
    idleTimeoutMillis : 30000,
     // if true, logs via console.log - can also be a function
    log : true
});

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

// acquire connection - callback function is called
// once a resource becomes available
pool.acquire(function(err, client) {
    if (err) {
        // handle error - this is generally the err from your
        // factory.create function  
    }
    else {
        client.query("select * from foo", [], function() {
            // return object back to pool
            pool.release(client);
        });
    }
});

Step 3 - Drain pool during shutdown (optional)

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:

// Only call this once in your application -- at the point you want
// to shutdown and stop using this pool.
pool.drain(function() {
    pool.destroyAllNow();
});

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

Documentation

Pool() accepts an object with these slots:

              name : name of pool (string, optional)
            create : function that returns a new resource
                       should call callback() with the created resource
           destroy : function that accepts a resource and destroys it
               max : maximum number of resources to create at any given time
                     optional (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 factory.max - 1
                     optional (default=0)
       refreshIdle : boolean that specifies whether idle resources at or below the min threshold
                     should be destroyed/re-created.  optional (default=true)
 idleTimeoutMillis : max milliseconds a resource can go unused before it should be destroyed
                     (default 30000)
reapIntervalMillis : frequency to check for idle resources (default 1000),
      returnToHead : boolean, if true 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. optional (default false)
     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)
          validate : function that accepts a pooled resource and returns true if the resource
                     is OK to use, or false if the object is invalid.  Invalid objects will be destroyed.
                     This function is called in acquire() before returning a resource from the pool.
                     Optional.  Default function always returns true.
     validateAsync : Asynchronous validate function. Receives a callback function as its second argument, 
                     which should be called with a single boolean argument being true if the item is still 
                     valid and false if it should be removed from the pool. Called before item is acquired 
                     from pool. Default is undefined. Only one of validate/validateAsync may be specified
               log : true/false or function -
                       If a log is a function, it will be called with two parameters:
                                                - log string
                                                - log level ('verbose', 'info', 'warn', 'error')
                       Else if log is true, verbose log info will be sent to console.log()
                       Else internal log messages be ignored (this is the default)

Priority Queueing

The pool now 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.

 // create pool with priorityRange of 3
 // borrowers can specify a priority 0 to 2
 var pool = new Pool({
     name     : 'mysql',
     create   : function(callback) {
         // do something
     },
     destroy  : function(client) {
         // cleanup.  omitted for this example
     },
     max      : 10,
     idleTimeoutMillis : 30000,
     priorityRange : 3
 });

 // acquire connection - no priority - will go at front of line (same as high priority)
 pool.acquire(function(err, client) {
     pool.release(client);
 });

 // acquire connection - high priority - will go into front slot
 pool.acquire(function(err, client) {
     pool.release(client);
 }, 0);

 // acquire connection - medium priority - will go into middle slot
 pool.acquire(function(err, client) {
     pool.release(client);
 }, 1);

 // etc..

Draining

If you know would like to terminate all the resources in your pool before their timeouts have been reached, you can use destroyAllNow() in conjunction with drain():

pool.drain(function() {
    pool.destroyAllNow();
});

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 factory.name for this pool
pool.getName()

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

// returns number of unused resources in the pool
pool.availableObjectsCount()

// returns number of callers waiting to acquire a resource
pool.waitingClientsCount()

// returns number of maxixmum number of resources allowed by ppol
pool.getMaxPoolSize()

// returns number of minimum number of resources allowed by ppol
pool.getMinPoolSize()

Run Tests

$ npm install expresso
$ npm test

Linting

We use eslint and the standard ruleset. At the moment linting is not done as part of the test suite but this will probably change in the future. You should ideally lint your code before making any PR's patches etc.

Becuase the linting tools require nodejs >= 0.10 but we test against 0.8 and 0.6 installation of the tools is done via npm run lint-install. Some kind of optionalDevDependencies would be great!

$ npm run lint-install
$ npm run lint

License

(The MIT License)

Copyright (c) 2010-2016 James Cooper <[e-mail]>

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.

Changelog

[2.4.0] - January 18 2016

• Merged #118 - closes #110 - optional eslinting for test and lib using "standard" ruleset
• Merged #114 - closes #113 - "classes" now used internally instead of object literals and exports support being called as a constructor (along with old factory behaviour) (contributed by @felixfbecker)
• Move history from README.md to CHANGELOG.md and reformat
• Closes #122 - fixes trapped connection bug when destroying a connection while others are in use