My setup consists of a Weblogic server that runs various REST services that accepts an HTTP request from a client(an android app), reads data from a MySql database and sends this data as a response to the client.
My requirement is that the client should be notified as soon as the database is updated. I have implemented a "polling" mechanism. The client constantly sends requests and checks for any changes made in the database. However this is not a very efficient mechanism.
Can you please suggest any efficient ways of achieving this?
Thanks in advance!
One of the way to use writing a CRON job for observation of Database change.
also, SO has already discussed this topic. refer below link,
Oracle database to send notification / Invoke a java method on reaching Time specified in a column
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 currently am developing a semi-simple chat app. I want the user to be able to communicate with one other person in a private chat. Just plain text will be sent. Currently my system calls php scripts from a webpage, passes in parameters and then parses the data returned. I have it so that the client sends the message, which calls a send message script on my webserver, the script then makes a message file on the webserver and then returns a success or failure back to the client. Then for the client to view this message, it would have to call a script that checks the server for a message file with a message for him. If it finds one, it sends the message back, if not, it sends a response about not having messages.
This technique works perfectly besides the fact that the client either would have to manually refresh to check to see if he had messages, or a background thread would have to refresh every few seconds. That would be fine, however that would use data if the user was on a mobile network. Not to mention the kind of resources a background loop would pull if it was refreshing at a speed that would be convenient.
So, I decided on a second idea, this would be a server programmed in Java, which would communicate over sockets. The user would send the message as a packet over the socket and the server would check to see who it was meant to go to. If the person is online, it passes the message along to that user. However this method requires a constant connection between the client and the server and that is unreliable because what if the user is in the car and data cuts out. Or some other situation where the connection gets severed. Then it would throw errors and have to reconnect.
Anyhow, my question is which technique is better. Or are they both terrible? If so, what is the correct way of doing this? Thanks in advance.
AngularJs and Ajax will be the perfect solution for you , try to learn
for actually real time messaging Use AngularJs
If the amount of data is very less ..say 20-25 messages per day...you can REST APIs on your server to transfer actual text messages and Google Cloud messaging for pushing notifications..Recently I followed this approach to develop private chat for one of my friend.
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 have several PC's on each of them I set small swing application that get data with JSON request to one web server. Can I receive the data from web server without to send request to the web server, with other words can the Web server send the data without the Java application to ask for this?
If you have enough server resources
you can consider usage of websockets.
Every PC can open a socket to the server.
When you open the socket you need to send to the server, the pc's unique ID.
Then you need to store this ID in some database or file that will contain all online pc's and sockets .
Then the Server will be aware which pc's are online and which socket to use to communicate with this pc. After this you can send whatever information you need to this PC depending on your application.
This can be implemented in several ways. One common way would be to open a connection and do blocking read in the client application. On receiving something it will look like push from the server. Then you process the push and do another blocking read.
Another option would be doing regular checks if there is something for you on the web server. You set the retry interval frequent enough so it will look like real time push from your app point of view.
If you use HTTP i think the smartest way is to drop the realtime requirement and use a thread that polls the server every 5 seconds. Keeping a HTTP Connection open all time is expensive as it blocks a request processor thread and limits the amount of clients you can have.
You might also consider moving to something like a registration mechanism if you really need near-realtime updates which is often not the case. You would have to open a Server on the clients and have the server push the updates after clients registered their Address with the server.
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.