I have a question.
Facebook probably uses ajax to notify user about a new message, is this correct?
If yes, would not this tax db to incredible levels?
I mean millions of users every second of being online requesting message status.
Or am I thinking about this in a wrong way?
You are asking about a technique called polling. And you are correct that it has scalability issues. In general not a good idea.
[rant]I have no idea what facebook does. I hate facebook. It's like a drunk whore who won't stop texting/emailing you and needs to be used.[/rant].
There are better alternatives to polling. One technique is called long polling, and then there is server side push.
See
How do I implement basic "Long Polling"?
and
https://stackoverflow.com/questions/6883540/http-server-to-client-push-technologies-standards-libraries.
In long polling, the client sends a request but does not expect a response immediately; the response could come immediately, in a second, or in an hour. The challenge is for the server to manage the outstanding requests in a non-resource intensive way.
With server side push, the server maintains a connection with clients and can broadcast messages to its connections when events occur.
Which alternative to use depends a bit on your technology stack. For example, node.js has something called socket.io (which I think is server side push using html5 websockets) which I hear good things about.
Related
I'm trying to understand whether the HTML5 Web Notifications API can help me out, but I'm falling short in understanding how it works.
I'd like user_a to be able to send user_b a message within my webapp.
I'd like user_b to receive a notification of this.
Can the web notifications API help here? Does it let me specifically target a user (rather than notify everyone the site has been updated_? I can't see how I would create an alert for one person.
Can anyone help me understand a little more?
The notifications API is client side, so it needs to get events from another client-side technology. Here, read THIS: http://nodejs.org/api/. Just kidding. Node.js+socket.io is probably the best way to go here, you can emit events to one or all clients (broadcast). That's a push scenario. Or each user could be pulling their notifications from the server.
HTML5 Web Notifications API gives you ability to display desktop notifications that your application has generated.
What you are trying to achieve is a different thing and web notification is just a part of your scenario.
Depending upon how you are managing your application, for chat and messaging purpose as humbolight mentioned, you should look into node.js. it will provide you the necessary back-end to manage sending and receiving messages between users.
To notify a user that (s)he has received a message, you can opt for ajax polling on client side.
Simply create a javascript that pings the server every x seconds and checks if there is any notification or new message available for this user.
If response is successful, then you can use HTML5 notification API to show a message to user that (s)he has a new message.
The main problem with long polling is server load, and bandwidth usage even when there are no messages, and if number of users are in thousands then you can expect your server always busy responding to poll calls.
An alternate is to use Server Sent Events API, where you send a request to server and then server PUSHES the notifications/messages to the client as soon as they are available.
This reduces the unnecessary client->server polling and seems much better option in your case.
To get started you can check a good tutorial at
HTML5Rocks
What you're looking for is WebSocket. It's the technology that allows a client (browser) to open a persistent connection to the server and receive data from it at the server's whim, rather than having to "poll" the server to see if there's anything new.
Other answers here have already mentioned node.js, but Node is simply one (though arguably the best) option for implementing websockets on your server. You might also be comfortable with Ratchet, which is a websocket server library for PHP, or Tornado which is in Python.
How you handle your real-time communication is up to you. Websockets are merely the underlying technology that you can use to pass data back and forth. The client side of this will be fairly easy, but on the server side, you'll need a mechanism for websocket handlers to get information from each other. Look at tools like ZeroMQ for handling queues, and Memcached or Redis to handle large swaths of data which don't need to be stored permanently.
Ian Hickson says:
I expect the iframe sandboxing feature
will be a big boon to developers if it
takes off. My own personal favorite
feature is probably the Web Sockets
API, which allows two-way
communication with a server so that
you can implement games, chatting,
remote controls, and so forth.
What can you get with web sockets that you can't get with AJAX? Is it just convenience, or is it somehow more efficient? Is it that the server can send data to the client, without having to wait for a message so it can respond?
Yes, it's all about the server being able to push data to the client. Currently, simulating bi-directional communication without Flash/Silverlight/Java/ActiveX takes the form of one of two workarounds:
Traditional polling: Clients make small requests to the server frequently, checking for updates. Even if no update has occurred, the client doesn't know that and must continuously poll for updates. Though each request may be lightweight, constant polling by many clients can add up quickly.
Long polling: Clients make periodic requests for updates, like regular polling, but if there are no updates yet available then the server does not respond immediately and holds the connection open. When an update is finally available, the server pushes that down to the client, which acts on it and then repeats that process. Long polling offers push-like update resolution, but is basically a self-inflicted DDoS attack and can be very resource intensive for many types of web servers.
With WebSockets, you get all of the responsiveness advantages of long polling, with dramatically less server-side overhead.
WebSockets are more efficient (and "more real-time") than AJAX calls because you keep connection open and don't send extra protocol headers and other stuff after each request and response. Look at this article:
During making connection with
WebSocket, client and server exchange
data per frame which is 2 bytes each,
compared to 8 kilo bytes of http
header when you do continuous polling.
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?
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.
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
Personally, I think that websockets will be used more and more in web applications instead of AJAX. They are not well suited to web sites where caching and SEO are of greater concern, but they will do wonders for webapps.
Projects such as DNode and socketstream help to remove the complexity and enable simple RPC-style coding. This means your client code just calls a function on the server, passing whatever data to that function it wants. And the server can call a function on the client and pass it data as well. You don't need to concern yourself with the nitty gritties of TCP.
Furthermore, there is a lot of overhead with AJAX calls. For instance, a connection needs to be established and HTTP headers (cookies, etc.) are passed with every request. Websockets eliminate much of that. Some say that websockets are more wasteful, and perhaps they are right. But I'm not convinced that the difference is really that substantial.
I answered another related question in detail, including many links to related resources. You might check it out:
websocket api to replace rest api?
I think that sooner or later websocket based frameworks will start to popup not just for writing real-time chat like parts of web apps, but also as standalone web frameworks. Once permanent connection is created it can be used for receiving all kinds of stuff including UI parts of web application which are now served for example through AJAX requests. This approach may hurt SEO in some way although it can reduce amount of traffic and load generated by asynchronous requests which includes redundant HTTP headers.
However I doubt that websockets will replace or endanger AJAX because there are numerous scenarios where permanent connections are unnecessary or unwanted. For example mashup applications which are using (one time) single purpose REST based services that doesn't need to be permanently connected with clients.
There's nothing "wrong" about it.
The only difference is mostly readability. The main advantage of Ajax is that it allows you fast development because most of the functionality is written for you.
There's a great advantage in not having to re-invent the wheel every time you want to open a socket.
WS:// connections have far less overhead than "AJAX" requests.
As other people said, keeping the connection open can be overkill in some scenarios where you don't need server to client notifications, or client to server request happens with low frecuency.
But another disadvantage is that websockets is a low level protocol, not offering additional features to TCP once the initial handshake is performed. So when implementing a request-response paradigm over websockets, you will probably miss features that HTTP (a very mature and extense protocol family) offers, like caching (client and shared caches), validation (conditional requests), safety and idempotence (with implications on how the agent behaves), range requests, content types, status codes, ...
That is, you reduce message sizes at a cost.
So my choice is AJAX for request-response, websockets for server pushing and high frequency low latency messaging
If you want the connection to server open and if continuous polling to the server will be there then go for sockets else you are good to go with ajax.
Simple Analogy :
Ajax asks questions(requests) to server and server gives answers(responses) to these questions. Now if you want to ask continuous questions then ajax wont work, it has a large overhead which will require resources at both the ends.
Will web sockets when used in all web browsers make ajax obsolete?
Cause if I could use web sockets to fetch data and update data in realtime, why would I need ajax? Even if I use ajax to just fetch data once when the application started I still might want to see if this data has changed after a while.
And will web sockets be possible in cross-domains or only to the same origin?
WebSockets will not make AJAX entirely obsolete and WebSockets can do cross-domain.
AJAX
AJAX mechanisms can be used with plain web servers. At its most basic level, AJAX is just a way for a web page to make an HTTP request. WebSockets is a much lower level protocol and requires a WebSockets server (either built into the webserver, standalone, or proxied from the webserver to a standalone server).
With WebSockets, the framing and payload is determined by the application. You could send HTML/XML/JSON back and forth between client and server, but you aren't forced to. AJAX is HTTP. WebSockets has a HTTP friendly handshake, but WebSockets is not HTTP. WebSockets is a bi-directional protocol that is closer to raw sockets (intentionally so) than it is to HTTP. The WebSockets payload data is UTF-8 encoded in the current version of the standard but this is likely to be changed/extended in future versions.
So there will probably always be a place for AJAX type requests even in a world where all clients support WebSockets natively. WebSockets is trying to solve situations where AJAX is not capable or marginally capable (because WebSockets its bi-directional and much lower overhead). But WebSockets does not replace everything AJAX is used for.
Cross-Domain
Yes, WebSockets supports cross-domain. The initial handshake to setup the connection communicates origin policy information. The wikipedia page shows an example of a typical handshake: http://en.wikipedia.org/wiki/WebSockets
I'll try to break this down into questions:
Will web sockets when used in all web browsers make ajax obsolete?
Absolutely not. WebSockets are raw socket connections to the server. This comes with it's own security concerns. AJAX calls are simply async. HTTP requests that can follow the same validation procedures as the rest of the pages.
Cause if I could use web sockets to fetch data and update data in realtime, why would I need ajax?
You would use AJAX for simpler more manageable tasks. Not everyone wants to have the overhead of securing a socket connection to simply allow async requests. That can be handled simply enough.
Even if I use ajax to just fetch data once when the application started I still might want to see if this data has changed after a while.
Sure, if that data is changing. You may not have the data changing or constantly refreshing. Again, this is code overhead that you have to account for.
And will web sockets be possible in cross-domains or only to the same origin?
You can have cross domain WebSockets but you have to code your WS server to accept them. You have access to the domain (host) header which you can then use to accept / deny requests. This can, however, be spoofed by something as simple as nc. In order to truly secure the connection you will need to authenticate the connection by other means.
Websockets have a couple of big downsides in terms of scalability that ajax avoids. Since ajax sends a request/response and closes the connection (..or shortly after) if someone stays on the web page it doesn't use server resources when idling. Websockets are meant to stream data back to the browser, and they tie up server resources to do so. Servers have a limit in how many simultaneous connections they can keep open at one time. Not to mention depending on your server side technology, they may tie up a thread to handle the socket. So websockets have more resource intensive requirements for both sides per connection. You could easily exhaust all of your threads servicing clients and then no new clients could come in if lots of users are just sitting on the page. This is where nodejs, vertx, netty can really help out, but even those have upper limits as well.
Also there is the issue of state of the underlying socket, and writing the code on both sides that carry on the stateful conversation which isn't something you have to do with ajax style because it's stateless. Websockets require you create a low level protocol which is solved for you with ajax. Things like heart beating, closing idle connections, reconnection on errors, etc are vitally important now. These are things you didn't have to solve when using AJAX because it was stateless. State is very important to the stability of your app and more importantly the health of your server. It's not trivial. Pre-HTTP we built a lot of stateful TCP protocols (FTP, telnet, SSH), and then HTTP happened. And no one did that stuff much anymore because even with its limitations HTTP was surprisingly easier and more robust. Websockets bring back the good and the bad of stateful protocols. You'll learn soon enough if you didn't get a dose of that last go around.
If you need streaming of realtime data this extra overhead is warranted because polling the server to get streamed data is worse, but if all you are doing is user interaction->request->response->update UI, then ajax is easier and will use less resources because once the response is sent the conversation is over and no additional server resources are used. So I think it's a tradeoff and the architect has to decide which tool fits their problem. AJAX has its place, and websockets have their place.
Update
So the architecture of your server is what matters when we are talking about threads. If you are using a traditionally multi-threaded server (or processes) where a each socket connection gets its own thread to respond to requests then websockets matter a lot to you. So for each connection we have a socket, and eventually the OS will fall over if you have too many of these, and the same goes for threads (more so for processes). Threads are heavier than sockets (in terms of resources) so we try and conserve how many threads we have running simultaneously. That means creating a thread pool which is just a fixed number of threads that is shared among all sockets. But once a socket is opened the thread is used for the entire conversation. The length of those conversations govern how quickly you can repurpose those threads for new sockets coming in. The length of your conversation governs how much you can scale. However if you are streaming this model doesn't work well for scaling. You have to break the thread/socket design.
HTTP's request/response model makes it very efficient in turning over threads for new sockets. If you are just going to use request/response use HTTP its already built and much easier than reimplementing something like that in websockets.
Since websockets don't have to be request/response as HTTP and can stream data if your server has a fixed number of threads in its thread pool and you have the same number of websockets tying up all of your threads with active conversations, you can't service new clients coming in! You've reached your maximum capacity. That's where protocol design is important too with websockets and threads. Your protocol might allow you to loosen the thread per socket per conversation model that way people just sitting there don't use a thread on your server.
That's where asynchronous single thread servers come in. In Java we often call this NIO for non-blocking IO. That means it's a different API for sockets where sending and receiving data doesn't block the thread performing the call.
So traditional in blocking sockets when you call socket.read() or socket.write() they wait until the data is received or sent before returning control to your program. That means your program is stuck waiting for the socket data to come in or go out until you can do anything else. That's why we have threads so we can do work concurrently (at the same time). Send this data to client X while I wait on data from client Y. Concurrencies is the name of the game when we talk about servers.
In a NIO server we use a single thread to handle all clients and register callbacks to be notified when data arrives. For example
socket.read( function( data ) {
// data is here! Now you can process it very quickly without waiting!
});
The socket.read() call will return immediately without reading any data, but our function we provided will be called when it comes in. This design radically changes how you build and architect your code because if you get hung up waiting on something you can't receive any new clients. You have a single thread you can't really do two things at once! You have to keep that one thread moving.
NIO, Asynchronous IO, Event based program as this is all known as, is a much more complicated system design, and I wouldn't suggest you try and write this if you are starting out. Even very Senior programmers find it very hard to build a robust systems. Since you are asynchronous you can't call APIs that block. Like reading data from the DB or sending messages to other servers have to be performed asynchronously. Even reading/writing from the file system can slow your single thread down lowering your scalability. Once you go asynchronous it's all asynchronous all the time if you want to keep the single thread moving. That's where it gets challenging because eventually you'll run into an API, like DBs, that is not asynchronous and you have to adopt more threads at some level. So a hybrid approaches are common even in the asynchronous world.
The good news is there are other solutions that use this lower level API already built that you can use. NodeJS, Vertx, Netty, Apache Mina, Play Framework, Twisted Python, Stackless Python, etc. There might be some obscure library for C++, but honestly I wouldn't bother. Server technology doesn't require the very fastest languages because it's IO bound more than CPU bound. If you are a die hard performance nut use Java. It has a huge community of code to pull from and it's speed is very close (and sometimes better) than C++. If you just hate it go with Node or Python.
Yes, yes it does. :D
The earlier answers lack imagination. I see no more reason to use AJAX if websockets are available to you.
I'm working on a web application that submits tasks to a master/worker system that farms out the tasks to any of a series of worker instances. The work queue master runs as a separate process (on a separate machine altogether) and tasks are submitted to the master via HTTP/REST requests. Once tasks are submitted to the work queue, client applications can submit another HTTP request to get status information about tasks.
For my web application, I'd like it to provide some sort of progress bar view that gives the user some indication of how far along task processing has come. The obvious way to implement this would be an AJAX progress meter widget that periodically polls the work queue for status on the tasks that have been submitted. My question is, is there a better way to accomplish this without the frequent polling?
I've considered having the client web application open up a server socket on which it could listen for notifications from the work master. Another similar thought I've had is to use XMPP or a similar protocol for the status notifications. (Of course, the master/worker system would need to be updated to provide notifications either way but I own the code for that so can make any necessary updates myself.)
Any thoughts on the best way to set up a notification system like this? Is the extra effort involved worth it, or is the simple polling solution the way to go?
Polling
The client keeps polling the server to get the status of the response.
Pros
Being really RESTful means cacheable and scaleable.
Cons
Not the best responsiveness if you do not want to poll your server too much.
Persistent connection
The server does not close its HTTP connection with the client until the response is complete. The server can send intermediate status through this connection using HTTP multiparts.
Comet is the most famous framework to implement this behaviour.
Pros
Best responsiveness, almost real-time notifications from the server.
Cons
Connection limit is limited on a web server, keeping a connection open for too long might, at best load your server, at worst open the server to Denial of Service attacks.
Client as a server
Make the server post status updates and the response to the client as if it were another RESTful application.
Pros
Best of every worlds, no resources are wasted waiting for the response, either on the server or on the client side.
Cons
You need a full HTTP server and web application stack on the client
Firewalls and routers with their default "no incoming connections at all" will get in the way.
Feel free to edit to add your thoughts or a new method!
I guess it depends on a few factors
How accurate the feedback can be (1 percent, 5 percent, 50 percent) Accurate feedback makes it worth pursuing some kind of progress bar and comet style push. If you can only say "Busy... hold on... almost there... done" then a simple ajax "are we there yet" poll is certainly easier to code.
How timely the Done message has to be seen by the client
How long each task takes (1 second, 10 seconds, 10 minutes)
1 second makes it a bit moot. 10 seconds makes it worth it. 10 minutes means you're better off suggesting the user goes for a coffee break :-)
How many concurrent requests there will be
Unless you've got a "special" server, live push style systems tend to eat connections and you'll be maxed out pretty quickly. Having to throw more webservers in for a fancy progress bar might hurt the budget.
I've got some sample code on 871184 that shows a hand rolled "forever frame" which seems to work out well. The project I developed that for isn't hammered all that hard though, the operations take a few seconds and we can give pretty accurate percent. The code uses asp.net and jquery, but the general techniques will work with any server and javascript framework.
edit As John points out, status reporting probably isn't the job of the RESTful service. But there's nothing that says you can't open an iframe on the client that hooks to a page on the server that polls the service. Theory says the server and the service will at least be closer to one another :-)
Look into Comet. You make a single request to the server and the server blocks and holds the connection open until an update in status occurs. Once that happens the response is sent and committed. The browser receives this response, handles it and immediately re-requests the same URL. The effect is that of events being pushed to the browser. There are pros and cons and it might not be appropriate for all use cases but would provide the most timely status updates.
My opinion is to stick with the polling solution, but you might be interested in this Wikipedia article on HTTP Push technologies.
REST depends on HTTP, which is a request/response protocol. I don't think you're going to get a pure HTTP server calling the client back with status.
Besides, status reporting isn't the job of the service. It's up to the client to decide when, or if, it wants status reported.
One approach I have used is:
When the job is posted to the server, the server responds back a pubnub-channel id (one could alternatively use Google's PUB-SUB kind of service).
The client on browser subscribes to that channel and starts listening for messages.
The worker/task server publishes status on that pubnub channel to update the progress.
On receiving messages on the subscribed pubnub-channel, the client updates the web UI.
You could also use self-refreshing iframe, but AJAX call is much better. I don't think there is any other way.
PS: If you would open a socket from client, that wouldn't change much - PHP browser would show the page as still "loading", which is not very user-friendly. (assuming you would push or flush buffer to have other things displayed before)