RE: [FileAPI] Deterministic release of Blob proposal

The feedback is implementation feedback that we have refined in the past few weeks as we've updated our implementation. 
We're happy with it to be treated as a LC comment, but we'd also give this feedback in CR too since in recent weeks we've found it to be a problem in apps which make extensive use of the APIs.

> -----Original Message-----
> From: Arthur Barstow [mailto:art.barstow@nokia.com]
> Sent: Monday, March 05, 2012 12:52 PM
> To: Feras Moussa; Arun Ranganathan; Jonas Sicking
> Cc: public-webapps@w3.org; Adrian Bateman
> Subject: Re: [FileAPI] Deterministic release of Blob proposal
> 
> Feras - this seems kinda' late, especially since the two-week pre-LC comment
> period for File API ended Feb 24.
> 
> Is this a feature that can be postponed to v.next?
> 
> On 3/2/12 7:54 PM, ext Feras Moussa wrote:
> >
> > At TPAC we discussed the ability to deterministically close blobs with
> > a few
> >
> > others.
> >
> > As we've discussed in the createObjectURL thread[1], a Blob may
> > represent
> >
> > an expensive resource (eg. expensive in terms of memory, battery, or
> > disk
> >
> > space). At present there is no way for an application to
> > deterministically
> >
> > release the resource backing the Blob. Instead, an application must
> > rely on
> >
> > the resource being cleaned up through a non-deterministic garbage
> > collector
> >
> > once all references have been released. We have found that not having
> > a way
> >
> > to deterministically release the resource causes a performance impact
> > for a
> >
> > certain class of applications, and is especially important for mobile
> > applications
> >
> > or devices with more limited resources.
> >
> > In particular, we've seen this become a problem for media intensive
> > applications
> >
> > which interact with a large number of expensive blobs. For example, a
> > gallery
> >
> > application may want to cycle through displaying many large images
> > downloaded
> >
> > through websockets, and without a deterministic way to immediately
> > release
> >
> > the reference to each image Blob, can easily begin to consume vast
> > amounts of
> >
> > resources before the garbage collector is executed.
> >
> > To address this issue, we propose that a close method be added to the
> > Blob
> >
> > interface.
> >
> > When called, the close method should release the underlying resource
> > of the
> >
> > Blob, and future operations on the Blob will return a new error, a
> > ClosedError.
> >
> > This allows an application to signal when it's finished using the Blob.
> >
> > To support this change, the following changes in the File API spec are
> > needed:
> >
> > * In section 6 (The Blob Interface)
> >
> > - Addition of a close method. When called, the close method releases
> > the
> >
> > underlying resource of the Blob. Close renders the blob invalid, and
> > further
> >
> > operations such as URL.createObjectURL or the FileReader read methods
> > on
> >
> > the closed blob will fail and return a ClosedError. If there are any
> > non-revoked
> >
> > URLs to the Blob, these URLs will continue to resolve until they have
> > been
> >
> > revoked.
> >
> > - For the slice method, state that the returned Blob is a new Blob
> > with its own
> >
> > lifetime semantics - calling close on the new Blob is independent of
> > calling close
> >
> > on the original Blob.
> >
> > *In section 8 (The FIleReader Interface)
> >
> > - State the FileReader reads directly over the given Blob, and not a
> > copy with
> >
> > an independent lifetime.
> >
> > * In section 10 (Errors and Exceptions)
> >
> > - Addition of a ClosedError. If the File or Blob has had the close
> > method called,
> >
> > then for asynchronous read methods the error attribute MUST return a
> >
> > "ClosedError" DOMError and synchronous read methods MUST throw a
> >
> > ClosedError exception.
> >
> > * In section 11.8 (Creating and Revoking a Blob URI)
> >
> > - For createObjectURL - If this method is called with a closed Blob
> > argument,
> >
> > then user agents must throw a ClosedError exception.
> >
> > Similarly to how slice() clones the initial Blob to return one with
> > its own
> >
> > independent lifetime, the same notion will be needed in other APIs
> > which
> >
> > conceptually clone the data - namely FormData, any place the
> > Structured Clone
> >
> > Algorithm is used, and BlobBuilder.
> >
> > Similarly to how FileReader must act directly on the Blob's data, the
> > same notion
> >
> > will be needed in other APIs which must act on the data - namely
> > XHR.send and
> >
> > WebSocket. These APIs will need to throw an error if called on a Blob
> > that was
> >
> > closed and the resources are released.
> >
> > We've recently implemented this in experimental builds and have seen
> > measurable
> >
> > performance improvements.
> >
> > The feedback we heard from our discussions with others at TPAC
> > regarding our
> >
> > proposal to add a close() method to the Blob interface was that
> > objects in the web
> >
> > platform potentially backed by expensive resources should have a
> > deterministic
> >
> > way to be released.
> >
> > Thanks,
> >
> > Feras
> >
> > [1]
> > http://lists.w3.org/Archives/Public/public-webapps/2011OctDec/1499.htm
> > l
> >

Received on Monday, 5 March 2012 23:20:27 UTC