What is the disadvantage of using websocket/socket.io where ajax will do?

Question

Similar questions have been asked before and they all reached the conclusion that AJAX will not become obsolete. But in what ways is ajax better than websockets?

With socket.io, it's easy to fall back to flash or long polling, so browser compatibility seems to be a non-issue.

Websockets are bidirectional. Where ajax would make an asynchronous request, websocket client would send a message to the server. The POST/GET parameters can be encoded in JSON.

So what is wrong with using 100% websockets? If every visitor maintains a persistent websocket connection to the server, would that be more wasteful than making a few ajax requests throughout the visit session?

Answer 1

I think it would be more wasteful. For every connected client you need some sort of object/function/code/whatever on the server paired up with that one client. A socket handler, or a file descriptor, or however your server is setup to handle the connections.

With AJAX you don't need a 1:1 mapping of server side resource to client. Your # of clients can scale less dependently than your server-side resources. Even node.js has its limitations to how many connections it can handle and keep open.

The other thing to consider is that certain AJAX responses can be cached too. As you scale up you can add an HTTP cache to help reduce the load from frequent AJAX requests.

Answer 2

Short Answer
Keeping a websocket active has a cost, for both the client and the server, whether Ajax will have a cost only once, depending on what you're doing with it.

Long Answer
Websockets are often misunderstood because of this whole "Hey, use Ajax, that will do !". No, Websockets are not a replacement for Ajax. They can potentially be applied to the same fields, but there are cases where using Websocket is absurd.

Let's take a simple example : A dynamic page which loads data after the page is loaded on the client side. It's simple, make an Ajax call. We only need one direction, from the server to the client. The client will ask for these data, the server will send them to the client, done. Why would you implement websockets for such a task ? You don't need your connection to be opened all the time, you don't need the client to constantly ask the server, you don't need the server to notify the client. The connection will stay open, it will waste resources, because to keep a connection open you need to constantly check it.

Now for a chat application things are totally different. You need your client to be notified by the server instead of forcing the client to ask the server every x seconds or milliseconds if something is new. It would make no sense.

To understand better, see that as two persons. One of the two is the server, the over is the client. Ajax is like sending a letter. The client sends a letter, the server responds with another letter. The fact is that, for a chat application the conversation would be like that :
"Hey Server, got something for me ?
- No.
- Hey Server, got something for me ?
- No.
- Hey Server, got something for me ?
- Yes, here it is."
The server can't actually send a letter to the client, if the client never asked for an answer. It's a huge waste of resources. Because for every Ajax request, even if it's cached, you need to make an operation on the server side.

Now the case I discussed earlier with the data loaded with Ajax. Imagine the client is on the phone with the server. Keeping the connection active has a cost. It costs electricity and you have to pay your operator. Now why would you need to call someone and keep him on phone for an hour, if you just want that person to tell you 3 words ? Send a goddamn letter.

In conclusion Websockets are not a total replacement for Ajax !
Sometimes you will need Ajax where Websocket usage is absurd.

Edit : The SSE case
That technology isn't used very widely but it can be useful. As its name states it, Server-Sent Events are a one-way push from the server to the client. The client doesn't request anything, the server just sends the data.

In short :
- Unidirectional from the client : Ajax
- Unidirectional from the server : SSE
- Bidirectional : Websockets