Simple Engineering

The reactive aspect of nodejs applications is synonymous with the nodejs runtime itself. Even though the real-time aspect may be attributed to WebSocket implementation, the realtime reactive aspect of nodejs applications heavily rely on pub/sub mechanisms — most of the time backed by datastore engines like redis. This article explores how to integrate redis datastore into a nodejs application.

In this article we will talk about:

• redis support with and without expressjs
• redis support with and without WebSocket push mechanism
• Alternatives to redis in nodejs world and beyond

Even though this blog post was designed to offer complementary materials to those who bought my Testing nodejs Applications book, the content can help any software developer to tuneup working environment. You use this link to buy the book.

Show me the code

The following code sample showcase how nodejs/redis integration can be achieved. It demonstrates that it is still possible to share sessions via a middleware between socket.io and expressjs.

var app = express();
var server = Server(app);
var sio = require("socket.io")(server),
	redis = require('redis'), 
	rhost = process.env.REDIS_HOST,
	rport = process.env.REDIS_PORT,
	pub = redis.createClient(rport, rhost), 
	sub = redis.createClient(rport, rhost);


function middleware(req, res, next){
 //session initialization thing
 next();
}

//socket.io/expressjs session sharing middleware
sio.use(function(socket, next){
 	middleware(socket.request, socket.request.res, next);
});

//express uses middleware for session management
app.use(middleware);
    
//somewhere
sio.sockets.on("connection", function(socket) {
 
 //socket.request.session 
 //Now it's available from `socket.io` sockets too! Win!
 socket.on('message', (event) => {
	 var payload = JSON.parse(event.payload || event),
	 	user = socket.handshake.user || false;
	 
	 //except when coming from pub  			
	 pub.publish(payload.conversation, payload)); 
 });

 //redis listener
 sub.on('message', function(channel, event) {
	var payload = JSON.parse(event.payload || event),
		user = socket.handshake.user || false;
	sio.sockets.in(payload.conversation).emit('message', payload);
 });

Example: excerpt source: StackOverflow

What can possibly go wrong?

When trying to figure out how to approach redis datastore integration into nodejs application for inter-process communication and real-time feature, the following points may be a challenge:

• How to decouple the WebSocket events from the redis specific (pub/sub) events. We should be able to decouple, but still provide an environment where interoperability is possible at any time.
• How to make integration modular, testable, and overall friendly to the rest of the application ecosystem

When testing this implementation, we should expect the additional challenge to emerge:

• The redis client instances (pub/sub) are created as soon as the library loads, and a redis server should be up and running by that time. The issue is when testing the application, there should be no server or any other system dependency hindering the application from being tested.
  
• getting rid of redis server with a drop-in-replacement, or stubs/mocks, is more of a dream than reality ~ hard but feasible.

There is additional information in mocking and stubbing redis data store in “How to Mock redis datastore” article.

Conclusion

In this article, we revisited how to enhance nodejs application with redis based pub/sub mechanism, critical to having a reactive real-time experience. The use of WebSocket and Pub/Sub powered by a key/value data store was especially the main focus of this article. There are additional complimentary materials in the “Testing nodejs applications” book.

References

• Testing nodejs Applications book
• Sharing session between expressjs and socket.io ~ StackOverflow Question
• examples using redis with socket.io ~ StackOverflow Question
• Using redis as PubSub over socket.io ~ StackOVerflow Question
• socket.io – redis
• redis pubsub library ~ Github Cloned Library
• Building a Chat Server with node and redis – tests ~ Matthew Daly Blog
• High Volume, low latency difficulties nodejs/pubsub/redis ~ StackOverflow Question

#snippets #nodejs #integration #redis