I'm currently workin' on a simulation environment build with spring (webflow) and jsf (and primefaces). Within my environment messages are generated when some events are triggered by the server (for instance the "cash Position" of a player has changed). I want this messages to be displayed in my webapplication every time they are triggered.
I think I'll have to use someting like reverse AJAX, but I wasn't able to make it run within my application. Now I wanted to know if you see any other possibilities respectively techniques to acheive my goal.
You can do a constant AJAX poll using the components shown here http://www.primefaces.org/showcase/ui/ajaxPollHome.jsf.
In a typical Server/Client model a TCP socket connection would be the best approach because the connection is typically left open allowing the server and client to send messages back and forth to each other bidirectionally.
AJAX communication is essentially HTTP protocol and is by default a stateless Client Request/Server Response unidirectional model.
A custom polling AJAX component like the one linked from micfra is a good example of how to address this problem. A custom polling component can be built with AJAX and Javascript as well. Essentially the client needs to ask the server for status updates after so many intervals. If the server has a status update or new information, then using Javascript events one can update the information on the client as well as the server.
Related
I am developing a servlet based application. One situation is that a client requests some data from a database which is sent back in the form of html. The client will modify this data and then sent it back to the server. Now the twist starts. There is not a single client. So multiple clients can request the same data. So what I am doing is that when the first client makes a request, this request is stored somewhere so that when the next user makes the same request he is denied the data.
Now suppose the first user gets the data and 2nd is denied. Now while the first user is on the html page which allows him to modify the data. I want to send continuous javascript async post requests at a fixed interval to inform the server that the client is active.
At the server side I need a thread or something which can keep waiting in a loop for the javascript async requests and if the request is not received within the fixed time then the thread removes the saved request so that future requests to the data will be accepted.
I have searched the entire day and looked at things like async servlets, ServletContext listener and scheduledExecutorservice. I dont want to use scheduledExecutorService as it is invoked at app startUp which I dont want to do since this specific situation is a minor part of the code and to handle it I dont want something running all the time. I need some background service which keeps running even after the server has returned requested data.
Servlets won't fulfill your requirements, therefore you should use WebSockets.
As per my understanding, you are trying to push data from the server, therefore you need to a push architecture instead of pull architecture (Servlets are based upon pull architecture).
Java has native support of WebSockets
You can find several tutorials on how to use WebSockets in a Java Web Application.
Here is a link to a basic WebSockets Tutorial.
Hope this helps
I have a client in AngularJS where I consume multiple SSE (Server-Sent-Events) in Java from the Server Side (there are multiple endpoints in different web servers in the backend).
Note: I have to use SSE.
I currently register a listener to each type of event coming from each SSE connection, such as:
source.addEventListener('alpha', function(e) {
doSomething();
}, false);
The purpose is to show a notification based on these events, and with this I have a few questions:
How can the client know if the information has changed in the backend?
How to organise and filter these events? For example, by receiving simultaneously multiple events from multiple connections, how can I manage them in order to show the client an specific notification regarding an specific event?
Note: I'm not only talking only about organising an event per type, but I also need to have in mind if an event is more important than another.
So far I only think of receiving all the events, and save them in a list that I could order and filter. Is there a problem if two SSE events are fired at the same time? Do you know of an example of this?
Is it a good idea to make the logic for organising the events on the client side?
Should I create a database for these events?
Thank you,
In the client side you need to make ajax calls periodically to know if there is change data in the server (SSE). In angularjs there is $http and $q service for server side call. you can use $scope.$watch as well.
In JavaScript there is not way to handle the way android handles push notification.
You can also see the web socket option if its matches your requirement.
I'm new in this area and a little bit confused about my current thoughts on a web-service implementation for my application. I want to synchronize all connected clients with the current data on a MySQL DB. I came up to use web-services as for coordination purposes, but I got stuck.
The way I tried to implement it is...
The web-service is running a deamon to check/listen on CRUD operations affecting DB data. Once an CRUD operation has been occured it undetakes to notify all the connected clients and each client in turn refreshes the JTable with current DB data and informs the user with the change that took place on the DB.
This is where I'm stuck...
The way I'm thinking it's (maybe?) wrong, since conceptually web-services are awaiting for client requests and then send him the corresponding response. But here, there is no client requests. The web-service just sends a response (noification), which must multicast since it's unaware of a "calling" client -there's no request.
Therefore, in order a client to send a request it must "cron" all the time the web-service if a change on the DB has been occured. Then the web-service responts as true or false with the CRUD details (CRUDEventFired(event_type, event_details)). But, this is not optimal as it will create big and (much of time) useless traffic especially when there is no CRUD operation occured. Also, there will be two different deamons (one in web-service and the other in client-app) that check/listen for the same type of event and that's redundant.
Also, with that implementation it seems that there's no need for a web-service at all and the code to check the DB for changes could be implemented in the client. On the other hand setting the client responsible for the DB changes would require to code the same functions in different platforms (desktop, ios, android, etc), while setting the web-service as the coordinator for data synchronization between clients would require to code only once the appropriate function and each client simply implement its own NotificationListener().
Is it wrong the way I'm thinking using web-services? How it should be changed? Do I even need to use web-services?
I attach my diagram for your convinience.
I think I'm just missing a little detail that is preventing me from seeing the whole picture.
I have a web application which use ajax request every x time to update client with new information or tasks.
I also have a long running process on the server which is a java computation engine. I would like this engine to send update to the client.
I am wondering how to migrate my web app to using websocket. Probably phpwebsocket or similar. Can my server 'decide' to send information to a specific client? It seems possible looking at the php-websocket.
Can my java backend long process use the websocket server to send notification to a specific client. How? well I can say that my java app could use a class that could send over websocket instead of http.
But how the websocket server knows to which client to send the 'info'. I am puzzle by all this. Any document that explain this in more details? It seems that the websocket could create an instance of my web application.
Thanks
Your server, which will have an arbitrary number of active client sockets, decides which ones to write to (possibly in response to input from the user).
phpwebsocket (which is still very rough around the edges) has a User class with $id, $socket (this is the underlying TCP socket), and $handshake fields. You could extend that class with additional metadata about the User (e.g. a computation identifier). Or you could use an array mapping from computation id to User.
Perhaps when Java computation n finishes, you can look up the socket associated with that computation, and write to its socket.
I have a school project in which I have to implement a chat application, whose server will be a java web service.
The problem is that I've always thought of a web service as a way of calling remote functions, and I have no idea how to keep a "session" active on the web service, nor how to keep track of all the people currently in chat, rooms etc.
To the best of my knowledge, a chat server is supposed to know its clients after an initial connection, and send every client message to all clients. This definitely calls for some sort of session maintenance. I think the right way to do this is as follows:
Client calls web service 'handshake' and provides some minimal identification details.
Server returns an acknowledgment that includes a unique client identifier.
Client calls web service 'message' and sends a new message, together with its identifier.
Server identifies client by the identifier, distributes message to all clients.
I'm not really sure how the message distribution should work, as web services are essentially a pull-service and not push. Perhaps the client should expose its own web service for the server to call.
Hope this helps,
Yuval =8-)
You could consider implementing a COMET solution. This will effectively give you push communication, thus eliminating latency, a VERY nice feature for a chat application.
If you want to go for the gold, consider implementing more advanced features:
spell check
URLs/email addresses converted to links automatically
separate chat rooms
moderator functions (terminate chat, kick user)
event info like "User is typing..."
statuses (available, busy, away...)
avatars
...
I don't know Java so this answer will be language agnostic.
In my opinion the simplest way to do this without running a process on the server would be to store all your data in a database.
Here is a short list of the basic things that will need to be done:
Need a table with a list of users and passwords for authentication
Need a table for the currently logged in uses
A. needs a time stamp field of the last contact
When a users does something update the last contact field to the current time
If the user' last contact time is > current time + 2 minutes then they are logged out
client side application will need to send periodic messages to the server to say "Im still here"
You'll need to find a way to determine when a message has been sent and when to update the client's display that a message has been received, this I will leave to you.
If you still need some help here is an AJAX/ASP.Net chat app that should (I didn't look at its source) work much the same way.
I wrote a chat engine which had a service in the background and everything stored in a database, an input form frame and an output frame which received the html stream.
If you want to skip the service part and only implement via a web service, you need to implement at least two operations: Post for inputs, and GetLatestChanges to receive the chat's output, which translates into HTML using some Javascript magic.
Of course you need to keep track of rooms, users, messages, which user receives which texts etc, as sketched by Unknwntech.