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

When 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

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?


Hi 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 unlocking
of 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 retrieval

For (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 [1]. Interestingly, nio2 has both a synchronous FileChannel [2] and AsynchronousFileChannel [3].

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 such an API on anyone (rather it can be available when needed)

Hm... so, let's assume that client X wants to do something asynchronous. So it does
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 some
fallbacks 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 an
asynchronous 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 quick
and local. And
let's investigate how we could leverage ECF to support remote file
systems. If that doesn't meet our needs, we can always add
async later.

I'm not sure if this is a good strategy. It seems to lead
towards 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 into
some 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 on
remote 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 + replication
to do caching of remote resources.

That'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 EFS
today is random access to files. Does ECF have it?


Martin Oberhuber, Senior Member of Technical Staff, Wind River
Target Management Project Lead, DSDP PMC Member
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