leader-election-mongo

A leader election package which uses MongoDB. A single leader is chosen from a group of instances in a single, non-recurring election.

Use cases

This package is useful when doing a single election for tasks such as a once-daily cron job (as opposed to having a group of instances continuously trying to determine a leader). It is intended for a relatively small number of instances, such as 5 or 10, not 10,000.

Install

npm install leader-election-mongo

Example

Here is an example using promises:

const crypto = require('crypto')
const { Leader } = require('leader-election-mongo')
const { MongoClient } = require('mongodb')
 
const url = 'mongodb://localhost:27017'
 
MongoClient.connect(
    url,
    {
        useNewUrlParser: true,
        useUnifiedTopology: true
    },
    function (err, client) {
        const id = process.pid + '-' + crypto.randomBytes(8).toString('hex')
        const db = client.db('leadertest')
        const candidate = new Leader(db, { id, ttl: 10000 })
 
        candidate
            .initDatabase()
            .then(collectionExists => {
                if (collectionExists) {
                    return candidate.elect()
                }
                console.log(`${id} collection was not initialized!`)
                process.exit()
            })
            .then(isLeader => {
                if (isLeader) {
                    console.log(`${id} is the LEADER`)
                    // do your stuff here
                    return candidate.cleanup()
                } else {
                    console.log(`${id} is not the leader`)
                    process.exit()
                }
            })
            .then(() => {
                console.log(`${id} Cleanup finished`)
                process.exit()
            })
    }
)

This package can also be used with async/await:

const crypto = require('crypto')
const { Leader } = require('leader-election-mongo')
const { MongoClient } = require('mongodb')
 
const url = 'mongodb://localhost:27017'
 
MongoClient.connect(
    url,
    {
        useNewUrlParser: true,
        useUnifiedTopology: true
    },
    async function (err, client) {
        const id = process.pid + '-' + crypto.randomBytes(8).toString('hex')
        const db = client.db('leadertest')
        const candidate = new Leader(db, { id })
 
        const collectionExists = await candidate.initDatabase()
        if (!collectionExists) {
            console.log(`${id} collection was not initialized!`)
            process.exit()
        }
 
        const isLeader = await candidate.elect()
        if (!isLeader) {
            console.log(`${id} is not the leader`)
            process.exit()
        }
 
        console.log(`${id} is the LEADER`)
        // do your stuff here
 
        await candidate.cleanup()
        console.log(`${id} Cleanup finished`)
        process.exit()
    }
)

API

new Leader(db, options)

Create a new Leader class.

db is a MongoClient.Db object.

options.id is an optional ID for this instance. Default is a random hex string.

options.ttl is the lock time-to-live in milliseconds. Will be automatically released after that time. The default and minimum value is 5000.

If the provided MongoClient.Db object doesn't have admin privileges, the TTL cleanup will only happen every 60 seconds (the MongoDB default) which might cause a problem if the leader doesn't call cleanup() and you try to run multiple elections within 60 seconds.

options.key is a unique identifier for the group of instances trying to be elected as leader. Default value is 'default'

initDatabase()

Initializes the election collection. Returns a promise that resolves to a boolean indicating whether the collection exists.

elect()

Performs the election and returns a promise that resolves to true if the instance is the leader; otherwise, false.

cleanup()

Removes the election collection. Returns a promise that resolves to void.

Events

elected The event fired when the instance becomes a leader.

cleaned The event fired when the cleanup is finished.

License

See the LICENSE file (spoiler alert, it's Apache-2.0).

Credits

Inspired by the mongo-leader package, but with a modified API and different election algorithm.