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redlock

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Redlock

This is a node.js implementation of the redlock algorithm for distributed redis locks. It provides strong guarantees in both single-redis and multi-redis environments, and provides fault tolerance through use of multiple independent redis instances or clusters.

High-Availability Recommendations

  • Use at least 3 independent servers or clusters
  • Use an odd number of independent redis servers for most installations
  • Use an odd number of independent redis clusters for massive installations
  • When possible, distribute redis nodes across different physical machines

Using Cluster/Sentinel

It is completely possible to use a single redis cluster or sentinal configuration by passing one preconfigured client to redlock. While you do gain high availability and vastly increased throughput under this scheme, the failure modes are a bit different, and it becomes theoretically possible that a lock is acquired twice:

Assume you are using eventually-consistent redis replication, and you acquire a lock for a resource. Immediately after acquiring your lock, the redis master for that shard crashes. Redis does its thing and fails over to the slave which hasn't yet synced your lock. If another process attempts to acquire a lock for the same resource, it will succeed!

This is why redlock allows you to specify multiple independent nodes/clusters: by requiring consensus between them, we can safely take out or fail-over a minority of nodes without invalidating active locks.

To learn more about the the algorithm, check out the redis distlock page.

How do I check if something is locked?

Redlock cannot tell you with certainty if a resource is currently locked. For example, if you are on the smaller side of a network partition you will fail to acquire a lock, but you don't know if the lock exists on the other side; all you know is that you can't guarantee exclusivity on yours.

That said, for many tasks it's sufficient to attempt a lock with retryCount=0, and treat a failure as the resource being "locked" or (more correctly) "unavailable",

Installation

npm install --save redlock

Configuration

Redlock can use node redis, ioredis or any other compatible redis library to keep its client connections.

A redlock object is instantiated with an array of at least one redis client and an optional options object. Properties of the Redlock object should NOT be changed after it is firstused, as doing so could have unintended consequences for live locks.

var client1 = require('redis').createClient(6379, 'redis1.example.com');
var client2 = require('redis').createClient(6379, 'redis2.example.com');
var client3 = require('redis').createClient(6379, 'redis3.example.com');
var Redlock = require('redlock');
 
var redlock = new Redlock(
    // you should have one client for each redis node 
    // in your cluster 
    [client1, client2, client3],
    {
        // the expected clock drift; for more details 
        // see http://redis.io/topics/distlock 
        driftFactor: 0.01, // time in ms 
 
        // the max number of times Redlock will attempt 
        // to lock a resource before erroring 
        retryCount:  10,
 
        // the time in ms between attempts 
        retryDelay:  200, // time in ms 
 
        // the max time in ms randomly added to retries 
        // to improve performance under high contention 
        // see https://www.awsarchitectureblog.com/2015/03/backoff.html 
        retryJitter:  200 // time in ms 
    }
);

Error Handling

Because redlock is designed for high availability, it does not care if a minority of redis instances/clusters fail at an operation. If you want to write logs or take another action when a redis client fails, you can listen for the clientError event:

 
// ... 
 
redlock.on('clientError', function(err) {
    console.error('A redis error has occurred:', err);
});
 
// ... 
 

Usage (promise style)

Locking & Unlocking

 
// the string identifier for the resource you want to lock 
var resource = 'locks:account:322456';
 
// the maximum amount of time you want the resource locked, 
// keeping in mind that you can extend the lock up until 
// the point when it expires 
var ttl = 1000;
 
redlock.lock(resource, ttl).then(function(lock) {
 
    // ...do something here... 
 
    // unlock your resource when you are done 
    return lock.unlock()
    .catch(function(err) {
        // we weren't able to reach redis; your lock will eventually 
        // expire, but you probably want to log this error 
        console.error(err);
    });
});
 

Locking and Extending

redlock.lock('locks:account:322456', 1000).then(function(lock) {
 
    // ...do something here... 
 
    // if you need more time, you can continue to extend 
    // the lock as long as you never let it expire 
    return lock.extend(1000).then(function(lock){
 
        // ...do something here... 
 
        // unlock your resource when you are done 
        return lock.unlock()
        .catch(function(err) {
            // we weren't able to reach redis; your lock will eventually 
            // expire, but you probably want to log this error 
            console.error(err);
        });
    });
});
 

Usage (disposer style)

Locking & Unlocking

var using = require('bluebird').using;
 
// the string identifier for the resource you want to lock 
var resource = 'locks:account:322456';
 
// the maximum amount of time you want the resource locked, 
// keeping in mind that you can extend the lock up until 
// the point when it expires 
var ttl = 1000;
 
// if we weren't able to reach redis, your lock will eventually 
// expire, but you probably want to do something like log that 
// an error occurred; if you don't pass a handler, this error 
// will be ignored 
function unlockErrorHandler(err) {
    console.error(err);
}
 
using(redlock.disposer(resource, ttl, unlockErrorHandler), function(lock) {
 
    // ...do something here... 
 
}); // <-- unlock is automatically handled by bluebird 
 

Locking and Extending

using(redlock.disposer('locks:account:322456', 1000, unlockErrorHandler), function(lock) {
 
    // ...do something here... 
 
    // if you need more time, you can continue to extend 
    // the lock until it expires 
    return lock.extend(1000).then(function(extended){
 
        // Note that redlock modifies the original lock, 
        // so the vars `lock` and `extended` point to the 
        // exact same object 
 
        // ...do something here... 
 
    });
}); // <-- unlock is automatically handled by bluebird 
 

Usage (callback style)

Locking & Unlocking

 
// the string identifier for the resource you want to lock 
var resource = 'locks:account:322456';
 
// the maximum amount of time you want the resource locked, 
// keeping in mind that you can extend the lock up until 
// the point when it expires 
var ttl = 1000;
 
redlock.lock(resource, ttl, function(err, lock) {
 
    // we failed to lock the resource 
    if(err) {
        // ... 
    }
 
    // we have the lock 
    else {
 
 
        // ...do something here... 
 
 
        // unlock your resource when you are done 
        lock.unlock(function(err) {
            // we weren't able to reach redis; your lock will eventually 
            // expire, but you probably want to log this error 
            console.error(err);
        });
    }
});
 

Locking and Extending

redlock.lock('locks:account:322456', 1000, function(err, lock) {
 
    // we failed to lock the resource 
    if(err) {
        // ... 
    }
 
    // we have the lock 
    else {
 
 
        // ...do something here... 
 
 
        // if you need more time, you can continue to extend 
        // the lock until it expires 
        lock.extend(1000, function(err, lock){
 
            // we failed to extend the lock on the resource 
            if(err) {
                // ... 
            }
 
 
            // ...do something here... 
 
 
            // unlock your resource when you are done 
            lock.unlock();
        }
    }
});
 

API Docs

Redlock.lock(resource, ttl, ?callback)

  • resource (string) resource to be locked
  • ttl (number) time in ms until the lock expires
  • callback (function) callback returning:
    • err (Error)
    • lock (Lock)

Redlock.unlock(lock, ?callback)

  • lock (Lock) lock to be released
  • callback (function) callback returning:
    • err (Error)

Redlock.extend(lock, ttl, ?callback)

  • lock (Lock) lock to be extended
  • ttl (number) time in ms to extend the lock's expiration
  • callback (function) callback returning:
    • err (Error)
    • lock (Lock)

Redlock.disposer(resource, ttl, ?unlockErrorHandler)

  • resource (string) resource to be locked
  • ttl (number) time in ms to extend the lock's expiration
  • callback (function) error handler called with:
    • err (Error)

Lock.unlock(?callback)

  • callback (function) callback returning:
    • err (Error)

Lock.extend(ttl, ?callback)

  • ttl (number) time in ms to extend the lock's expiration
  • callback (function) callback returning:
    • err (Error)
    • lock (Lock)