I am starting to use gwt and I am having some problems to identify the clients state.
I am using GWTEventService to push some data to the client. But this data depends on which tab the client is. But how to know the state of the client (eg. which is the radio button selected)without using cookies. As I am sending events each 5 seconds, check the client state using cookies to each event that I want to send would make the application become very slow.
Can anyone help me?
Thank you,
Maurício
I would suggest a different design approach. Instead of pushing data to the client, have the client pull the data from the server. I have used GWT-RPC extensively to do this: http://code.google.com/webtoolkit/doc/latest/tutorial/RPC.html
If that approach doesn't work for you, the next best approach, whenever the state of the client changes, notify the server, so it always knows the state. You could then to client tracking with sessions.
Related
I want to develop an application where I want to push the messages (or data) to UI from backend Spring boot application.
I have the following requirement -
Consider there is a REST service that accepts the data from other applications using the POST method.
This data will be pushed to UI.
OR
Consider that there is a background process running which generate events and we want to push these events to UI.
For this, I came across about the WebSocket component that we can use in the Spring Boot application.
However, is there any other settings required to make it possible to push the incoming data to the UI?
Any help is appreciated.
Thanks,
Avinash Deshmukh
The backend cannot magically push updates to a client UI. The backend will have no way of knowing where the UI exists (i.e. what the UI's ip address is) and even if it did, it may not have access to establish a connection (due to firewalls or a NAT).
For this reason a client UI has to request updates. One way this could be done would be to have a timer in the UI application that polls for updates via REST. But this is essentially what websockets do - with much less overhead.
This is how common applications that you use everyday work all the time. So I'm not sure why you do not want to go down the websockets route.
...
Starting with Spring 5.0.5.RELEASE, it isn’t necessary to do any customization because of the improvement of #SendToUser annotation, that allows us to send a message to a user destination via “/user/{sessionId}/…” rather than “/user/{user}/…“.
That means the annotation works relying on the session id of the input message, effectively sending a reply to destination private to the session:
...
There is a good example over here:
https://www.baeldung.com/spring-websockets-sendtouser
I'm using socket io, and I was wondering what was better.
Emiting an event every X seconds to keep always updated with the database or emit the event after e.g a POST event, so it's more efficient.
I believe updating X seconds should be easier, and maybe has better scalability, but don't know if that's the correct way.
EDIT-1: To give more context. The application is for an accounting team. They basically want their excel sheets converted to a app. They have a lot of data, so I don't know if emitting an event every X seconds is a good idea.
Thanks.
There is no "correct" way. It depends entirely upon the needs of your client and the capabilities of your server. If the client needs to be kept more instantly up-to-date, then send data from your server to the client whenever the server has new data. If the client only needs to be updated every once-in-a-while, then only send it data every once-in-a-while. There is no "correct" way. It depends upon your application.
It is always more efficient to only send data to the client when the data has actually changed and when the client actually cares that something has changed. So, it would be foolish to send a client update every few seconds if the data isn't actually changing that often. If you have a means of knowing when the data changes on the server, then use that event to know when to send data to the client and even then, don't send it more often than the client actually cares to know.
It is always more efficient to have the server do no more work than is actually required by the client. Things like caching and keeping track of what each client was last sent can sometimes save lots of work for the server too.
Any further advice on this matter would need to know a lot more about the needs of your application and how this particular data fits into that and how often the data in question actually changes.
A summary on this topic:
Send data to the client no more often than it needs it
Sending data to the client that has not changed since the last time you changed it is inefficient for the server and consumes bandwidth.
Only you can decide how often your client needs updates (it depends upon your application)
Only you can test the impact on scalability of sending data to every client every time the data changes.
Server-side caching and keeping track of what client already has what data can help you avoid sending data to a client that it already has.
Server-side scalability probably has a lot to do with how many simultaneous clients are connected and how frequently there is changed data to send them.
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.
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.
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)