Websocket无限循环循环。 它真的比ajax好吗？

Question

I am trying understand websockets.

I have seen 2 examples here in doc and also here .

Both examples are using endless loop cycling, listening for when a new client connects, when they do something interesting and when they are disconnected.

My question is: Is using websockets (with endless loop cycling) better than an ajax solution with http requests per x time ?

Answer 1

AJAX and WebSockets are vastly different. Asking if one is better than the other is like asking if a screwdriver is better than a hammer.

WebSockets are used for real time, interactive communication. Both sides of a WebSocket connection can send data and it will be received within milliseconds by the other end. The connection stays open, reducing latency due to connection negotiation.

However, it only sort of plays nicely with HTTP. That is, it plays nicely with proxies that are WebSocket aware, and with firewalls. WebSocket traffic is most definitely not HTTP traffic, except for the client's first packet, which requests switching from HTTP to the WebSocket protocol.

AJAX, on the other hand, is pure HTTP. The only difference between AJAX and a standard web request is that an AJAX request is initiated by client side scripts and the response is available to that same script rather than reloading the page.

In both AJAX and WebSockets, the client scripts can receive data and use it within that same script. That's where the similarities end.

WebSockets set up a permanent connection and both sides can send data at any time, or sit quietly at any time. With AJAX, the client makes a request and the server responds.

For instance, if you were to set up a new message notification system, if you were using WebSockets, then as soon as a new message is available, the server sends it straight to the browser. If there are no new messages, the server stays quiet. If you were using AJAX, the client would periodically send a request to the server, which would always respond, either saying there were no new messages, or delivering the notifications that are pending. There is no way for the server to initiate things on its end, it must wait for the AJAX request.

Server side, things diverge from the traditional PHP web development paradigms. A typical WebSocket server will be a stand alone, CLI application running as a daemon. (If that last sentence doesn't make sense, please spend a while taking the time to really understanding how to administer a server.)

This means that multiple clients will be connecting to the same script, and superglobal variables like $_GET and $_SESSION will be absolutely meaningless. It seems easy to conceptualize in a small use case, but remember that you will most likely want to get information from other parts of your site, which often means using libraries and frameworks that have absolutely no concept of accessing data outside of the HTTP request/response model.

Thus, for simplicity, you'll usually want to stick with AJAX requests and periodic polling, unless you have the means to rethink the network data and possibly re-implement things that your libraries automate, if you're looking to update standard web traffic.

As for the server's loop:

It is not a busy loop, it is an IO blocked loop.

If the server tries to read network data and none is available, the operating system will block (pause) the script and go off to do whatever else needs to be done. In my WS server, I block waiting for network traffic for at most 1 second at a time, before the script returns to check and see if anything else new happened that I should notify my clients of. Typically, this is barely a few milliseconds before the server goes right back to its IO blocked state waiting for new data on the wire. Some others have implemented my server using LibEv, which allows them to respond to events outside of the network IO without having to wait for the block to timeout.

This is the way nearly every server does things. This is why you can have Apache actively listening and serving web traffic without every server that runs Apache being pegged at 100% CPU usage even when there is no traffic.

In closing, WebSockets is a wonderful technology, but web libraries and frameworks are simply not built to use them. Thus, unless you're working in a system where waiting 3 seconds for a full AJAX request is far, far too long, it's probably best to use AJAX. If you're writing a multiplayer interactive game or a chat system, then you've found a perfect use for WebSockets.

I do heartily encourage everyone to learn WebSockets... but it's not a magic bullet, and few parts of the web are designed in ways where people can get real use out of it.

Answer 2

Yes, sockets are better in many cases.

It's not forever loop with 100% cpu utilizing , it's just liveloop, which exists in each daemon application.

Sync accept operation is where 99.99% of time we are.

Ajax heartbeat is more traffic, more server CPU and memory.

Answer 3

I too am in the learning phase. I have built a php-based websocket server and have it communicating with web pages. Perhaps my 2c perspective is useful.

Getting the websocket server (wss) working using available sources as a starting point is not that difficult, but what to do with it all next is.

The wss runs in CLI version of php. Late model browser loads a normal http or https page containing a request to the wss, along with anything else that page needs to do, a handshake occurs. Communication is then possible directly between browser and wss at the whim of either end. This is low overhead and hence fast and simple. Very cool. What is said over that link needs to be understood by both ends - subprotocol agreement. You may have to roll your own in php and in javascript. No more http headers, urls, etc etc.

The wss is a long-lived, stateful instance of php (very unlike apache etc which forget you on sending the page). An entire app can be run in the wss instance, keeping state for itself and each connected client. It used to be said that php was too leaky for long life but I don't hear that much any more. But I believe you still have to be careful with memory.

However, being a single php instance there is not the usual separation between client instances. For example statics in classes are shared with every class instance and hence every client. So for a single user style app sharing data with a heap of clients this is great. I can see that Ajax type calls can be replaced in this way, but if the app still had to rebuild state to service each client, and then release it to save resources, that seems to lessen the advantage.

Going a step further and keeping truly stateful instances for clients seems like a possible next step. Replicating the traditional session based system is one possibility, alternatively fork new php interpreters and look after communications between parent and children via sockets or suchlike. But this would require resources per client that would be severely limiting for any non-trivial app.

Or perhaps it is possible to put the bulk of the app in the parent and let the children just do the very client specific stuff. Or break the app design into small independent units that can communicate directly via sockets. Socket communication does seem to be catching on nowadays.

As Ghedpunk says in so many ways, the real world does not yet seem ready to realise the full potential of the web socket concept but it can certainly replace Ajax. The added advantage of the server sending without being asked opens up new possibilities previously too difficult to consider.