|Re: [eclipse-incubator-e4-dev] [resources] EFS, ECF and asynchronous
Hi Martin,I agree with your examples below. RE: proper programming patterns...I think this is *the* hard thing in terms of API design. That is, completely valid assumptions in a local world (that a directory browse access won't block for multiple seconds and block the entire UI) are easily and frequently violated in the network world (e.g. because NFS blocks frequently and doesn't handle remote failure very robustly). This makes it extremely hard to define APIs that aren't based upon the 'worst case'.
'Well-behaved' programmers could protect every potentially blocking i/o method by using threads/jobs, but that would make it very cumbersome to use, and be wasted effort (and OS resources) for the common case (local disk access).
You are right that asynchronous APIs force the client to do processing and not wait (unless the programmer explicitly builds in such a wait). That frequently makes them harder to use (because when a result is required doing all that listening for callbacks and explicit waiting is a pain).
Scott Oberhuber, Martin wrote:
Hi Michael, to me, the difference between sync and async is not so much about speed or the number of Threads anymore - it's about enforcing proper programming patterns. That's something I actually learned during this discussion. Some examples: * Open a Directory Browse Dialog that happens to be initialized with the URI remote://foohost/bar/baz and foohost happens notto be online. All UI is blocked, you cannot even cancel the request.* This can even happen with a LOCAL file system, I've seen thisrepeatedly: My UNIX homedir is shared via SMB to my Windows machine. In my UNIX home I have some symbolic links that pointto other NFS-shared folders from machines that are offline. Just opening a directory browse dialog takes like forever (even on Windows Explorer!) * Dbl click large file foo.txt which is stored on a local SMBshared, to load into the editor. While loading the file, your network cable gets plugged off for some reason. Dependingon how the editor loading is implemented, all of Eclipse may hang. * How often have you seen an Eclipse Progress Monitor like "Waiting for Refresh Job to complete..." ? Is it really necessary that the Refresh Job locks the workspace for writing? Or could we allow more concurrency here? Yes, of course you can defer all synchronous queries into Jobswith Progress etc... but do we actually do that? Not always. And rightly so, because the hassle of creating a Job to makethe synchronous API happy is likely more than dealing with an async API right away. Asynchronous APIs just *force* the client to do something usefuluntil the response of the request comes in. Where "something useful" could be just as simple as allowing a user to press CANCEL.As an end user, I'm OK with waiting if I know I must wait. But I'd like to cancel operations that I believe won't return anyways,and I'd like to do other stuff in parallel until my request completes.Cheers, -- Martin Oberhuber, Senior Member of Technical Staff, Wind River Target Management Project Lead, DSDP PMC Member http://www.eclipse.org/dsdp/tm-----Original Message-----From: eclipse-incubator-e4-dev-bounces@xxxxxxxxxxx [mailto:eclipse-incubator-e4-dev-bounces@xxxxxxxxxxx] On Behalf Of Michael ScharfSent: Wednesday, October 22, 2008 9:14 AM To: E4 developer listSubject: Re: [eclipse-incubator-e4-dev] [resources] EFS, ECF and asynchronousWhen it comes to sync versus async at the EFS level, there is something I don't understand (probably because I don't know all the details of the APIs): I thought that IResource is a kind of snapshot of the underlying EFS structure. If I don't synchronize my workspace then IResource might show me a structure that is not consistent with the file system. Eclipse can deal with that. It happens often to me that I open a file that does not exist anymore because I forget to synchronize a directory that I have changed externally. The synchronization is already a process that can take long (and it does with some huge workspaces I have). So, where/when is the of fast (synchronous) access to EFS needed/used/expected? I think a user that deals with a remote workspace is able to understand that things cannot go as fast as on a local file system. She might understand that caching is involved. And that an update (of the cache) takes time. I would not hide this. So, what are the cases/workflows where asynchronous access to EFS is important if a local cache is involved? Michaelan API onHi Scott,2) Asynchronous access to files/resources is desirable and in some cases necessary (for some use cases)Could you cite a use case where async access is necessary?I think that (assuming all synchronous methods have progress monitors for cancellation, which is the case in EFS), the only difference between sync and async access is (1) the number of Threads in "wait" state,(2) locking of resources while Threads synchronously wait, (3) potential for coalescing multiple requests to the same item in the case of asynchronous queries. In the asynchronous case, no Threads are waiting and resources *may* be unlocked until the callback returns, but this unlockingof resources needs to be carefully considered in each case. Does the system always remain in a consistent state? RESTful systems ensure this by placing all state info right into the request, which is a great idea but likely not always possible. It's not only a matter of the API being complex or not. The fact is that the concept of being asynchronous as such is more flexible,but also requires adopters to be more careful, or at least think along different lines.I also think that we should look into the need for being asynchronous or not separately for the kinds of requests:(A) Directory retrieval (aka childNames()) (B) Full file tree retrieval (C) Status/Attribute retrieval for an individual file (D) File contents retrievalFor (D) we already use Streams in EFS, which can be implemented in an asynchronous manner. What's currently missing in EFS is the ability to perform random access, like the JSR 203 SeekableByteChannel . Interestingly, nio2 has both a synchronous FileChannel  and AsynchronousFileChannel .For (A), (B), (C) I'm not sure how much we would win from an asynchronous variant, since I'd assume that not much work could be done (and not much resources freed) while asynchronously waiting for their result anyways. But perhaps I'm wrong?3) Using (e.g.) adapters it's not necessary to force suchHm... so, let's assume that client X wants to do something asynchronous. So it doesanyone (rather it can be available when needed)myFileStore.getAdapter(IAsyncFileStore.class); some file systems would provide that adapter, others not.What's the client's fallback strategy in case the async adapter is not available?I'm afraid that if we use such adapters, we end up with the same code in clients again and again, because they need somefallbacks strategy. It seems wiser to place the fallback strategy right into the EFS provider, since it is always possible to write a bridge between a synchronous and anasynchronous API in a single, generic way. Therefore, I'm more in favor of determining what APIs we want to be asynchronous, and just adding them to EFS. The adapter idea could be used for adding provisional API, but the final API should not need that.To that extent, let's start assuming that files are quickand local. Andlet's investigate how we could leverage ECF to support remote filesystems. If that doesn't meet our needs, we can always addasync later.I'm not sure if this is a good strategy. It seems to leadtowards more and more separation of local vs. remote -- which, I think, leads to either duplication of code in the end, or non-uniform workflows for end users.Let me draw some sceanrio of what the world could look like in 10 years: with the Internet getting more and more into our lives, you'd want to use an Eclipse based product to dive into some code base that you just found on the net.Without downloading everything in advance. Or you browse intosome mp3 music store. Add some remotely hosted Open Source Library to your UML drawing just by drag and drop.I think that users will more and more want to operate onremote networked resources just the same as on local resources. E4 gives us the chance to try and come up with models that support such workflows in a uniform way. Let's not throw away that chance prematurely.I agree that we need to start on concrete work items rather than endlessly discussing concepts. But as we start on these work items, let's keep the concept that things may be remote in our minds.Sounds reasonable. Just as an aside: I think there's a lot of potential to use asynchronous file transfer + replicationThat's a great approach, especially if it works on the file block level (such that random access to huge remote files can be cached). Again, one thing that's missing from EFSto do caching of remote resources.today is random access to files. Does ECF have it? yteChannel.html el.html ousFileChannel.html Cheers, -- Martin Oberhuber, Senior Member of Technical Staff, Wind River Target Management Project Lead, DSDP PMC Member http://www.eclipse.org/dsdp/tm _______________________________________________ eclipse-incubator-e4-dev mailing list eclipse-incubator-e4-dev@xxxxxxxxxxx https://dev.eclipse.org/mailman/listinfo/eclipse-incubator-e4-dev_______________________________________________ eclipse-incubator-e4-dev mailing list eclipse-incubator-e4-dev@xxxxxxxxxxx https://dev.eclipse.org/mailman/listinfo/eclipse-incubator-e4-dev_______________________________________________ eclipse-incubator-e4-dev mailing list eclipse-incubator-e4-dev@xxxxxxxxxxx https://dev.eclipse.org/mailman/listinfo/eclipse-incubator-e4-dev
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