kranthiB/socket.io-realtime-messaging

socket.io-realtime-messaging

Why redis-store? (Pub / Sub)

• To handle multi-node instances of NodeJS servers
• Let's suppose , if emit the events directly through socket.io with out redis in-mid, only the connected clients of that particular instance will receive the events.
• As redis ensures that the same message will get received by all subscribers , this will overcome the above specified problem.

Issue while running the app in multiple instances

• To have high availabiltiy, requests between client and server will get handled by selecting one of instances of the cluster randomly based on load-balancing algorithm.
• As there will be repeated change of server instances, the socket.io-client connection will get re-connected frequently. If at this re-connection stage, subscriber emit the events, there will be possibilty that the corresponding client may loose the events.
• To overcome this, we need to maintain session affinity or sticky session

  •     var expressSession = require('express-session');
        var cookieParser = require('cookie-parser');
        var RedisStore = require('connect-redis')(expressSession);
        var redisClient = redis.createClient(redisConnectionUri);
        app.use(cookieParser());
        app.use(expressSession({ key: 'JSESSIONID', secret: 'whatever',
                               store: new RedisStore({ client: redisClient})}));
    

Sample output (after deploying the app in IBM-Bluemix - 2 instances)