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Re: [eclipse-incubator-e4-dev] [resources] EFS, ECF and asynchronous 
Hi Martin,

Oberhuber, Martin wrote:

Hi Scott,

to me, Futures and Listeners don't need to be a contradiction.
Before my further comments...I don't believe they are in conflict either (that is, both can be used in some cases as described by Martin). I guess I sort of presented them as exclusive, but I didn't really mean to have it be so. 


What's more interesting to me, is how to deal with Progress.
When a Progress Monitor already exists for the client, then
using it makes a lot of sense even if the result is obtained
asynchronously:

final CBFuture<IFileStore[]> childrenF = myFileStore.list(myProgress);
childrenF.chain(new Callback() {
   public void onDone(IStatus result) {
      if (result.isOK()) {
         handleResult(childrenF.get());
      }
   };
});

I'm using class "CBFuture" as an "enhanced Future" that allows
registering Callbacks. Using a Callback style of handling things,
or CBFuture.waitFor() remains up to the client. Note that I'm using a "chain()" method to indicate that the Framework/Future could allow chaining multiple callbacks such that one is exeucuted after the other. Also note how the callback retrieves the result of computation from the Future, and not from the callback itself.
I agree that the general issue of how to handle progress monitors is tricky. Although I accept your ideas above as a possible solution, I'm not sure whether this is the 'right' mechanism or not for 'remote progress monitoring'. I've been thinking about this for some time, but still don't feel like I have a good general solution for supporting IProgressMonitor for remote procedures. 


The problems that I have seen with callbacks in our products
in the past are listed on http://wiki.eclipse.org/E4/Pervasive_Themes#Becoming_More_Asynchronous 

* Much boilerplate code - Closures would be nice to avoid explosion
  of anonymous inner classes, which could cause bloat
* Need clarification on what thread and in what context the callback will be called
* When debugging, it is very hard to trace back the flow of operation across multiple callback invocations. It can even make debuging close to impossible unless some Tracing functionality for the callbacks is built into the Framework 
  (we ended up doing this in our commercial product).

* Exception handling needs to be clarified. Java6 Future only
provides Future#isCanceled(), that's not enough since the result of an operation might also be an exception. I'm introducint "Istatus result" above but that's also not optimal. 

I agree these are other issues...thanks.
The synchronous variant needs more verbosity writing it than one would expect, because cancellation and errors (exceptions) need to be handled, wrapped and potentially re-wrapped with Futures: 

final CBFuture<IFileStore[]> childrenF = myFileStore.list(myProgress);
try {
   handleResult(childrenF.get());
} catch(CancellationException e) {
   throw new OperationCancelledException(e);
} catch(ExecutionExeption e) {
   throw new CoreException(new Status(/*.blabla*/));
}
although that could perhaps be simplified if we declared some Eclipse specific implementation of Future which throws the 
kinds of Exceptions that we already know (like CoreException
embedding an Istatus) instead of the JRE's ExecutionException
that's really alien to our current code.
Yes, I agree that these are issues. I also agree that it would be useful to have Equinox-specific impls of Future (which is really what the IAsyncResult interface was meant to be and can/will change to be if desired). Further, I've recently also realized that there also should probably be something like remote impls of IStatus/MultiStatus, as I've been doing some remote mgmt interfaces (i.e. accessing and managing a remote processes' OSGi framework, p2, etc)...and it's clear to me that it is going to be very helpful to support the usage of IStatus/Multistatus as return values, as well as exceptions in remote service access. I agree that Future/IAsyncResult as well as IStatus/Multistatus and exception types should be widely available (i.e. in Equinox rather than redone/available in many locations above Equinox). We (ECF) are willing to contribute (and modify as desired) what we've done in this area (e.g. IAsyncResult+impl, RemoteStatus/RemoteMultiStatus, exception types) as desired. 

Scott




Cheers,
--
Martin Oberhuber, Senior Member of Technical Staff, Wind River
Target Management Project Lead, DSDP PMC Member
http://www.eclipse.org/dsdp/tm
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