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How to make a server API tell a client API that an update is available

I have rest client API accessing data through /read endpoint from my server. However, I have an internal batch service that calls my server's /write endpoint which updates the data. So from time to time the data from the server gets updated. How will I tell the client that the data is updated and ask it to call the /read endpoint again to get the latest data?
Below is a highlevel diagram on the scenario.

Try learning webhooks? Basically, the client will "subscribe" to your webhook, or to make it easier, the client will provide an endpoint to the server. Whenever there's an update, your server just sends a request to the client. The client should simply call a service that, whenever it receives a request from the webhook, fetches read.
It basically goes like this
Client subscribes to server
The client gives the server an endpoint /updateAvailable
When there's an update, the server sends a request to client's endpoint /updateAvailable
'/updateAvailable' invokes a service that calls on '/read'
The '/updateAvailable' endpoint could invoke a service that updates content. Let's say the request sent has a parameter
{
"updateFound":true
}
So whenever the client's '/updateAvailable' is called and receives a request, you do something like this (pseudo code)
if (updateBody.updateFound.message=true)
then call read()
Edit
By creating an endpoint for client, you can also do automatic updates. So client has an /updateAvailable endpoint. The server sends the update to the /updateAvailable endpoint, which from the client side invokes whatever service is used for /read

There are two solutions for your question:
Using timer on restful client to request to restful server in every N seconds/minutes... However, it's really bad idea because it has to request to server many times even though there's no updated data.
Using third party publish-subscribe-based messaging protocol solutions to implement push notification feature whenever there is new data updated. These solutions are: Google FCM,MQTT,AMQP

One time pass a data to Soap service that remain available

We have a web application that is using client service model. We are using SOAP Client and Soap Service.
Technology and architecture of current application -
We have a WEBEAR it is the client.Request comes to WEBEAR.
2. We have ServiceEAR, it is service side Request goes from client to this service.
This is developed using SOAP webservice (JAX-WS).
Now at the time of login I am taking location value from the user and save it to the session, since session is available only in client side and I passing location in every service request. I want I pass location in service side at once at the time of login when session starts and it remains throughout the session I don't need to pass the location again and again with every service request.
Please suggest some solution/framework/architecture.

Communication between Client and RESTful web services

I have a RESTful web Service that provide function of returning some data whenever a client send GET requests to ask for it:
#GET
#Path("/{deviceId}")
#Produces(MediaType.TEXT_PLAIN)
public String getDataResource(#PathParam("deviceId") long id){
return dataService.getData(id);
}
And the flow for this case would be the client sends request -> the web service returns value. But I want to ask that is it possible that the web service will automatically send response to the client when ever it has new data change inside of it? That means it not need to wait for the client to send request to ask for it. Because I would like to establish a communication between a client and some services running on an Application Server so that the client can always receives the newest data from the Application Server, so I think RESTful web Service can be a solution for it. And in oder to be ensure that the newest data will be transfered to the client side, so the server has to send to the client, not wait for the client to ask for it. Is RESTful web service provides any function like this?
Thank you all!

Is RESTful web service provides any function like this?
No. Not in the context you're asking for.
As answered before, the client could periodically poll for updates on the server. This is usually common option.
Another option - the original server posting updated on the "client". The client then becomes server itself. Viable, if you can expose services on the "client" side.
Maybe what are you looking for are web sockets. It is a long-lasting connection from client, where the server could keep returning data as they come.
There are some books around but you could search the net for more resources depending on the framework you use

You can implement notification system(observer pattern), so that client will poll the server in certain interval and any state change, it can get the result.

You may use the Schedulers to push the data to the client in a certain intervals.

How to return a session id from a http server

I have come across many examples of implementing a simple http server in Java. This one fits my needs: http://www.rgagnon.com/javadetails/java-have-a-simple-http-server.html
However, I can't find an example of how to generate, return, and maintain a session id from such a simple http server.
Is that even possible? Is there a way to modify the sample code referred above to incorporate this functionality?
Thanks.

HTTP does not have session support on it self once it is a stateless application protocol. So you need implement it by your self.
For example, on servlet containers like Tomcat there is a cookie called JSESSIONID that is generated and stored on the browser. The client sends back the cookie to the server on each request. Once each client has a different cookie the server can identify the client session.
When cookies are not allowed the parameter JSESSIONID is added to the URL for each request. This technique is called URL Rewriting.
There is a question, not specific for Java HTTP servers, that has implementation details for this problem.
HTTP Session Tracking

JAX-WS session management

I have been trying to maintain user session in a standalone JAX-WS client. The server code is as explained in https://weblogs.java.net/blog/ramapulavarthi/archive/2006/06/maintaining_ses.html.
My problem is with the client. The server does send the JSESSIONID with the response. But it looks like that the client's calls do not send the JSESSIONID with the request(I have tested this on TCPMon). Because of that the server creates a new session every time a request comes.
After some googling I found that I had to set BindingProvider.SESSION_MAINTAIN_PROPERTY = true in request conext in order to maintain user session in client. But sadly this also is NOT working for me.
I have tried generating the stub using AXIS and the session here is being properly maintained.
The web service has been deployed in Tomcat.
Thanks.