lock.js

const Redis = require('redis');

const Config = require('./config.js');
const Log = require('./log.js');

let redisClient = Redis.createClient(Config.redis.uri);

// Locking utility function. This is based on https://github.com/errorception/redis-lock but we only needed a simple use-case of it,
// and redis-lock continually retries - we wanted to fail immediately if a lock was in use.
class Lock {
    static acquire(feedId) {
        return new Promise(function(resolve, reject) {
            let value = '' + new Date(Date.now() + Config.redis.lockDuration),
                key = 'hoover:lock:feed:' + feedId;

            Log.info('LOCK: Will lock with key ' + key);
            redisClient.set(key, value, 'PX', Config.redis.lockDuration, 'NX', function(err, result) {
                if (err || result === null) {
                    reject('LOCK: ' + key + ' already locked');
                }

                resolve(result);
            });
        });
    }

    static release(feedId) {
        return new Promise(function(resolve, reject) {
            let key = 'hoover:lock:feed:' + feedId;

            Log.info('LOCK: Releasing key ' + key);
            redisClient.del(key, function(err, result) {
                if (err) {
                    reject(err);
                }

                resolve(result);
            });
        });
    }
}

module.exports = Lock;

usage.js

// Lock it
const someId = '1234'; // What we want to lock
try {
    await Lock.acquire(someId);
} catch (e) {
    Log.info(`${someId} already being processed, skipping...`);
    return 0;
}

// ... Do the work...

// Release and done.
await Lock.release(someId);