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The Client Communication Engine

The Client Communication Engine enbables the client side web control to send control signals to its server side. If the signal changes any web control, the server responds with one or more change objects. The Client Communication Engine updates the client UI accordingly.


This page describes the Client Communication Engine process.

See also the following page for background information:

  • The Web Document
    The web document is a node tree on the server. When rendered, it translates the document model into a rendered web document that can be visualized in a web browser.

The Process

The following flow chart below displays the Client Communication Engine process. Blue boxes indicate JavaScript functions. Red boxes correspond to web service calls:

Client-Server interaction
  1. serverCall
    A web control providing user interaction attaches this function as the event handler to an event, for instance a mouse click. It wraps the node id of the web control, an action type and some arguments as parameters into a request object. The request object is pushed to the request queue.
  2. request
    Sends the first request, if any, in the request queue to the server through the Request web service call.
  3. Request
    Provides an asynchronous web service call.
    Returns the server status.
  4. preReceiveCallback
    Forwards the status to the main receive callback function. At this point another request may be sent. The client feeds the server with requests as fast as possible, given that the requests must reach the server in the same order they are sent.
  5. receiveCallback
    If the status is Idle or Executing it forwards the received change objects to the document update function.
    If the status is either Executing or Loading the client will query for changes after some time, if querying is not already scheduled by an earlier callback.
  6. update
    Parses each change object and updates the document accordingly.
  7. sleep
    Schedules a query for changes in a dynamically determined number of milliseconds.
  8. Query for changes
    Provides an asynchronous web service call.
    Returns the server status and possibly a list of change objects.
  9. error
    Redirects the browser to an error page.
    This function is called not only when the server reports an error as indicated, but also upon failure in the update function or in any of the asynchronous server calls.
  10. Needs refresh
    Provides an asynchronous web service call.
    If the client has been inactive for some time it returns true whereby a refresh request is pushed to the request queue. This call also serves as a signal to tell the server not to remove the document.

Idle or Finished?

When the response status is Idle the Client Communication Engine updates the client document and goes to an idle state, unless another request has recently been pushed to the request queue.

If the request queue is empty and the server has returned Idle, it is still often the case that the last update altered one or more image sources. The interaction is not finished until all images have been downloaded from the server.

It is possible to register JavaScript functions that trigger on this condition. When the user clicks on the Close button it may have an additional function sending a close signal to the server in order to clean up resources.