On Tue, Oct 4, 2011 at 7:15 PM, Mark Silberbauer <bowserm...@gmail.com>wrote:
> I would like to write a method which handles the flow of communication on > XMPP. The sequence of things I'd like to do is: > > 1. Send message. > 2. Wait for response. > 3. Process the response. > > Since we could be waiting longer than 30s for the response (step 2) I'll be > teeing up a task to take care of this. This task will need to send the > message and then wait for a response on the XMPP servlet handling the > incoming message. My question is: How do I wait in the task servlet thread > for the response to arrive in the XMPP Servlet? > You can't. Instead, you should use an asynchronous pattern: Send the message, and register a handler for incoming XMPP messages. That handler should match up the response to the corresponding request (stored in the datastore if necessary) and perform appropriate processing on it. > > I'd normally use a listener pattern where the listener would store the > message in a field in the Task object and then trigger a Semaphore to signal > that a message has arrived. Like this: > 1. Install listener in XMPP servlet in a static field. > 2. Send message. > 3. Wait for semaphore. ........ Meanwhile, in the XMPP servlet thread, a > response will arrive and it will call the listener's callback method which > stores the message and releases the semaphore. > 4. Get message from field and process. > > I tried this and it worked fine on the development server. However, when I > uploaded to the cloud I found that I'd install the listener on the XMPP > servlet (step 1) but then a new instance of the servlet would be > instantiated when the message came in and there would no longer be a > reference to the listener to call, event through the listener is a static > field. My conclusion is XMPPServlet is run in a completely different VM > meaning the static field is not shared between that servlet and the task > one. Is this correct? > An App Engine app can be run on any number of machines; synchronization primitives designed for communication between threads will not work. -Nick Johnson > > In general what is the best practice for communication between these > servlets? How to I share data (normally I would've stored it in an object's > field) and how do I signal from one to the other when events occur (normally > I would've used a semaphore)? > > Sorry about the long winded question. Tell me if it's not clear and I'll > refine it a bit. > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine for Java" group. > To view this discussion on the web visit > https://groups.google.com/d/msg/google-appengine-java/-/Ek_Xr5Etj20J. > To post to this group, send email to > google-appengine-java@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine-java+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > -- Nick Johnson, Developer Programs Engineer, App Engine -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to google-appengine-java@googlegroups.com. To unsubscribe from this group, send email to google-appengine-java+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.
