I have a web application driven primarily by javascript/ajax, somewhat similar to how google docs work; all people viewing a page will be seeing the same information in relative real-time. It's not crucial that the information is actually real-time, a second or so is fine.
Currently, the application is ajaxing the server every 5 seconds. I was researching server-sent events and they sound like exactly what I need... but this is my understanding: server-sent events essentially just move the polling to the server. The PHP script doing the server-sent events will check the database for changes every X seconds, and send an update to the application when it finds one.
Checking once per second would probably be adequate, but since I'm on shared hosting I want to avoid any unnecessary load possible. Is there way I can subscribe to updates to the database? Or is there a way I can notify the script from other PHP scripts that make changes to the database?
With PHP, polling the DB is the typical way to do this. You could also use TCP/IP sockets to connect to some kind of application server, that sits in front of your database, and knows about all writers and all consumers. I.e. when a write comes in, it both broadcasts it to all consumers and writes it to the DB. The consumers in that examples are the PHP scripts (one per SSE client).
If you use WebSockets, then you need exactly the same architecture, because PHP is single-threaded: each SSE connection is an independent PHP process.
If you switch to using, say, node.js, then that application server can be built-in. (Again, it would work the same way, whether SSE or WebSockets.)
But, you mention you intend to use shared hosting. SSE (and WebSockets, and comet technologies) hold a socket open, which interferes with the economics of shared hosting. So your sockets are likely to get closed regularly. My advice would be to stick with ajax (and therefore DB) polling every 5 seconds, instead of SSE, until your application is worth enough that the $10-$100/month for a real host is not an issue. Then consider using SSE to optimize the latency.
P.S. The decision between SSE and WebSockets is all about write frequency. My guideline is if your clients write data, on average, once a second or more frequently, web sockets are better, because it is keeping the write channel open. If once every 5+ seconds then web sockets does not bring much, compared to just using an Ajax post each time you have data to write. An SSE back-end is simpler to deal with than a WebSockets back-end. (Writes every 1-5 seconds is the grey area.)
What I would recommend is instead of polling the database for changes, you will know when there is going to be a database change because your application will be making that change. I would use web sockets (https://developer.mozilla.org/en-US/docs/WebSockets) and simply push an update to all active clients when any member makes a change.
Here is the difference between Server Send Events and Web Sockets. (In your case Web Sockets are the way to go)
Websockets and SSE (Server Sent Events) are both capable of pushing data to browsers, however they are not competing technologies.
Websockets connections can both send data to the browser and receive data from the browser. A good example of an application that could use websockets is a chat application.
SSE connections can only push data to the browser. Online stock quotes, or twitters updating timeline or feed are good examples of an application that could benefit from SSE.
In practice since everything that can be done with SSE can also be done with Websockets, Websockets is getting a lot more attention and love, and many more browsers support Websockets than SSE.
However, it can be overkill for some types of application, and the backend could be easier to implement with a protocol such as SSE.
Related
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 ?
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.
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.
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.
I am writing Web chat where you have several one-on-one conversations with people on the screen at the same time. (Basically, like a personal messenger, without group chats).
My technology options seem to be Long Polling and WebSockets, and I'm trying to choose.
The upside with Long Polling is that's it's very easy to implement, and I can return whatever data i want (a customized JSON-object with the data required to update the page).
What I'm afraid of with WebSockets is that there's no native library for it in PHP, so you have to shop between different 3rd party ones, and the concepts seem more complicated, what with channels and subscriptions and what have you.
Browser compatibility is not an issue for me.
Is the performance of Long Polling much poorer than with Websockets? If no, then my decision is easy!
Is there a really simple Websocket server for PHP? Or is the concept so simple I could write my own? (Mozilla has a really simple tutorial on writing a client, but not on a server).
Assuming that your long-polling scheme involves an endpoint hosted by the same web server as your frontend, this will mean two active connections for every user of the application, so you will basically cut the number of users you can support in half. Your websocket server would run on a different port and can bypass your web server, so the connections are a lot of saved overhead with websockets.
Another place websockets save on overhead is that once your connection is established, there is no need for constant requests and responses. Zombie websocket connections are essentially free in terms of both bandwidth and CPU.
Finally, I would not think that long polling would be simpler to implement. Since websockets are designed to do exactly what you want, I think that leveraging an existing websocket package will actually save you some lines of code. I would look at Ratchet (feature-rich) or phpwebsocket (lite), if you want to use PHP.
Long Polling is definitely way much poorer than Werbsockets.
It is not recommended to use whatever websockets library with PHP, specially for chat applications.
I suggest using Python, Ruby or Node.js instead.
I am looking to create a site that will allow users to create groups and then chat/post within those groups. However, when posts/chats are made within a group, I do not want users to have to reload the page to view these new posts/chats within that group. My question boils down to this: what is your best opinion of how to do this (languages, webservices, etc.)?
I know PHP, SQL, HTML, CSS well and know XML, Javascript, AJAX not so well (I have encountered them enough to read the code and know how they work, but I am by no means skilled or confident with them. I have feeling I will need to read a book on one/all of these to build the kind of site I described.)
Any and all input would be greatly appreciated.
The Web does a great job handling the typical request/response model where a client makes a request and a server responds with a resource. However, when it comes to applications where the server must send data to the client without that client requesting the data, this is where we must get creative.
There are a few different methods that one can use to facilitate a real time Web-based application.
Polling:
Polling involves the client periodically making requests to the server in order to receive updates. There are two main problems with this approach: First, there may not be any data for the server to push for a very long time. Hence, lots of bandwidth is potentially wasted continuing to poll the server for updates. Second, the polling rate determines how real-time the application feels. While a fast polling rate will make the updates appear sooner, it wastes bandwidth. Conversely, polling in longer intervals uses less bandwidth, but the downside is that updates don't appear as quickly.
In general, this is a very poor solution to use for a chat application in the year 2012.
Comet/Reverse AJAX:
Comet is a technique that has been successfully used in the last 5 years to take the concept of request/response and use a hack to simulate a real-time effect. The general idea behind Comet is that the client makes a request to the server, and the server holds the connection open indefinitely. The server waits until there is an update to send to the clients. Once the update is ready, the server sends the response, which simulates the server making the request to the client. Once the client receives the response, it opens a new connection, and the process repeats.
This technique has been shown to scale to over 20,000 simultaneous connections on some platforms when combined with Continuations, which ensure waiting threads are freed up for other tasks.
This not only saves bandwidth, but it makes the application feel extremely real time.
Websockets:
Websockets was introduced in HTML5 as a replacement for Comet, using the ws:// protocol instead of http. However, this has not yet been widely adopted by all browser vendors, and there may still be discussions regarding the specification for the protocol. It has many of the same benefits as Comet.
For more information on Comet, check out Comet and PHP and the challenges of Comet in PHP. For client side integration, check out the Dojo Cometd Library.
I would have said AJAX is the best method of doing this. Alternatively, create an iframe which reloads
I'd like to develop a near real time web based chat system. Any suggestions on how to implement this via jQuery, any gotchas to look out for, and what is this Comet thing I keep reading about?
Ideally, I'd like to support up to about 5,000 concurrent chatters.
Comet, also known as Ajax Push, is often refered as "Reverse AJAX". Instead of pulling the information from the server in regular intervals, the data is pushed from the server to the browser when it is needed. That requires an open connection, for which there are several implementations.
I recommend that you use APE. Here is a demo: http://www.ape-project.org/demos/1/ape-real-time-chat.html
Advantage: It will be very responsive
and real-time.
Disadvantage: You need
to setup the APE server on your
webserver machine.
Comet is a "push" tecnology, created to avoiding the client (javascript code) to continously poll the server. This could cause bandwith problem, because you have to create (maybe) a new TCP connection, then contact the http server, he runs some server-side logic and then sends a response to the client. With comet, if the server decide that you should recive some information (e.g., new chat message) he directly send it to the client.
There are several different implementation, you can have a start here.
the simplest implementation tecnique is the hidden iframe, but I'd raccomend the long polling wich is much more controllable.
One more thing, thake a look at HTML5 websokets, wich could be an interesting solution to your problem (not very compatible with current browser, anyway)
Check out Node.js and nowjs for node.js. Node.js helps you build very efficient servers using server side JavaScript and nowjs is a library that allows you to build real time web apps. There is even a example screen cast that puts together a basic chat application in 12 lines of code.
You could also checkout Socket.io which is another node library thats helps you build real time apps by abstracting away different transport mechanisms and giving you a unified interface to code against (supports WebSockets, Flash Sockets, AJAX long polling, JSONP Polling and Forever IFrames).
I realize you tagged your question PHP but if you are seriously considering writing a scalable system with the least amount of effort (relatively speaking) then learning Node.js is worth your time (and the learning curve is not thats steep since you probably already know JS).
I need to implement real-time page data update with php and jquery.
(I found www.ape-project.org/ but it seems site is down)
Is any other solutions?
Very TNX!
You might want to check out Comet:
Comet is a web application model in
which a long-held HTTP request allows
a web server to push data to a
browser, without the browser
explicitly requesting it.[1][2] Comet
is an umbrella term, encompassing
multiple techniques for achieving this
interaction. All these methods rely on
features included by default in
browsers, such as JavaScript, rather
than on non-default plugins. The Comet
approach differs from the original
model of the web, in which a browser
requests a complete web page at a
time.
http://en.wikipedia.org/wiki/Comet_%28programming%29
If you want to do streaming (sending multiple messages over a single long lived, low latency connection), you probably need a comet server. Check out http://cometdaily.com/maturity.html for details on a variety of server implementations (I am the maintainer of one of them - Meteor).
If you are happy to reconnect after each message is received, you can do without complicated servers and transports and just use long polling - where you make an ajax request and the server simply sleeps until it has something to send back. But you will end up with LOTS of connections hanging off your web server, so if you're using a conventional web server like Apache, make sure it's configured to handle that. By default Apache doesn't like having more than a few hundred concurrent connections.
There is lots of solutions to do this...
Depending on what is your data, how your data are organized and stored (mysql ?).
Your question is too open to have a real answer.