redis-sentinel-client

Redis Sentinel Client for Node.js

Supplements node_redis with Redis Sentinel support.

From the Sentinel docs:

Redis Sentinel is a system designed to help managing Redis instances. It performs the following three tasks:
Monitoring. Sentinel constantly check if your master and slave instances are working as expected.
Notification. Sentinel can notify the system administrator, or another computer program, via an API, that something is wrong with one of the monitored Redis instances.
Automatic failover. If a master is not working as expected, Sentinel can start a failover process where a slave is promoted to master, the other additional slaves are reconfigured to use the new master, and the applications using the Redis server informed about the new address to use when connecting.

Goals

1. Transparent, drop-in replacement for RedisClient, handling connections to master, slave(s), and sentinel in the background.
2. Handles all RedisClient operations (including pub/sub).
3. No loss of data during failover.

This was originally part of a fork of node_redis, and has been subsequently split to its own module. (However, it still currently requires changes to node_redis to work, so it still depends on the fork.)

See related thread about different approaches to Sentinel support: https://github.com/mranney/node_redis/issues/302

Concepts

• connects to a single sentinel, which is watching a single master/slave(s) cluster
• maintains an activeMasterClient in the background, automatically updates on failover (using psub)
• behaves exactly like a single RedisClient
• passes thru all client commands, behaves transparently like a RedisClient

Usage

npm install redis-sentinel-client

var RedisSentinel = require('redis-sentinel-client');
var sentinelClient = RedisSentinel.createClient(options);
// or
var sentinelClient = RedisSentinel.createClient(PORT, HOST);

Now use sentinelClient as a regular client: set, get, hmset, etc.

Instantiation options

• masterName: Which master the sentinel is listening to. Defaults to 'mymaster'. (If a sentinel is listening to multiple masters, create multiple SentinelClients.)
• logger: pass in winston or another custom logger, otherwises uses console. (Expects a log method.)
• debug: verbose output (to logger about internal ops)

Methods

• getMaster(): returns a reference to the sentinel client's activeMasterClient (also available directly)
• reconnect(): used on instantiation and on psub events, this checks if the master has changed and connects to the new master.
• send_command() (and any other RedisClient command): command is passed to the master client.

Events

• sentinel message (msg): passed up from the sentinel's channel listener. Note, messages can be about other masters, does not differentiate.
• failover-start: corresponds to sentinel's +failover-triggered message.
• failover-end: corresponds to sentinel's +failover-end message.
• disconnected: old master disconnected.
• reconnected (newMasterClient): new master reconnected. (In theory should be instant swapover, so no lag.) Passes new client ref for convenience.
  • (Use new ref for pub/sub and other long-running operations, otherwise let client pass through commands.)
• error (error): An error occured. (Currently does not differentiate errors in different components.) Will fire errors during a failover, before new master is connected.

Tests

There are currently 2 tests, run with Mocha:

1. I/O test: Basic usage, compatibility with RedisClient in functional state.

• Run with npm test.

1. Failover test: With a master on 5379, slave on 5380, and sentinel on 8379, this verifies that the SentinelClient connects to the new master, and buffers all IO, including pub/sub, not losing any data integrity during the failover.

• Run with ./node_modules/.bin/mocha --ui tdd --reporter spec --bail test/test-failover
  (The failover test is isolated from the other, so it doesn't break the normal IO tests.)

Limitations

• Unlike RedisClient, SentinelClient is not / does not need to be a stream
• Sentinel docs don't specify a default host+port, so option-less implementations of createClient() won't be compatible.
• Have not put any time into multi support, unknown status.

Possible roadmap

• Multiple master/slave(s) clusters per sentinel
  • But thinking not: Just create multiple sentinel clients, one per cluster.

Credits

Created by the Node.js team at DocuSign (in particular Ben Buckman and Derek Bredensteiner).

License

MIT