[whatwg] LocalStorage in workers

[Drew Wilson]

Jeremy, what's the use case here - do developers want workers to have access
to shared local storage with pages? Or do they just want workers to have
access to their own non-shared local storage?
Because we could just give workers their own separate WorkerLocalStorage and
let them have at it. A worker could block all the other accesses to
WorkerLocalStorage within that domain, but so be it - it wouldn't affect
page access, and we already had that issue with the (now removed?)
synchronous SQL API.

I think a much better case can be made for WorkerLocalStorage than for "give
workers access to page LocalStorage", and the design issues are much
simpler.

-atw

On Tue, Sep 15, 2009 at 8:27 PM, Jonas Sicking <jonas at sicking.cc> wrote:

> On Tue, Sep 15, 2009 at 6:56 PM, Jeremy Orlow <jorlow at chromium.org> wrote:
> > One possible solution is to add an asynchronous callback interface for
> > LocalStorage into workers.  For example:
> > function myCallback(localStorage) {
> >   localStorage.accountBalance = localStorage.accountBalance + 100;
> > }
> > executeLocalStorageCallback(myCallback);  // TODO: Make this name better
> >  :-)
> > The interface is simple.  You can only access localStorage via a
> callback.
> >  Any use outside of the callback is illegal and would raise an exception.
> >  The callback would acquire the storage mutex during execution, but the
> > worker's execution would not block during this time.  Of course, it's
> still
> > possible for a poorly behaving worker to do large amounts
> of computation in
> > the callback, but hopefully the fact they're executing in a callback
> makes
> > the developer more aware of the problem.
>
> First off, I agree that not having localStorage in workers is a big
> problem that we need to address.
>
> If I were designing the localStorage interface today I would use the
> above interface that you suggest. Grabbing localStorage can only be
> done asynchronously, and while you're using it, no one else can get a
> reference to it. This way there are no race conditions, but also no
> way for anyone to have to lock.
>
> So one solution is to do that in parallel to the current localStorage
> interface. Let's say we introduce a 'clientStorage' object. You can
> only get a reference to it using a 'getClientStorage' function. This
> function is available both to workers and windows. The storage is
> separate from localStorage so no need to worry about the 'storage
> mutex'.
>
> There is of course a risk that a worker grabs on to the clientStorage
> and holds it indefinitely. This would result in the main window (or
> another worker) never getting a reference to it. However it doesn't
> affect responsiveness of that window, it's just that the callback will
> never happen. While that's not ideal, it seems like a smaller problem
> than any other solution that I can think of. And the WebDatabase
> interfaces are suffering from the same problem if I understand things
> correctly.
>
> There's a couple of other interesting things we could expose on top of
> this:
>
> First, a synchronous API for workers. We could allow workers to
> synchronously get a reference to clientStorage. If someone is
> currently using clientStorage then the worker blocks until the storage
> becomes available. We could either use a callback as the above, which
> blocks until the clientStorage is acquired and only holds the storage
> until the callback exists. Or we could expose clientStorage as a
> property which holds the storage until control is returned to the
> worker eventloop, or until some explicit release API is called. The
> latter would be how localStorage is now defined, with the important
> difference that localStorage exposes the synchronous API to windows.
>
> Second, allow several named storage areas. We could add an API like
> getNamedClientStorage(name, callback). This would allow two different
> workers to simultaneously store things in a storage areas, as long as
> they don't need to use the *same* storage area. It would also allow a
> worker and the main window to simultaneously use separate storage
> areas.
>
> However we need to be careful if we add both above features. We can't
> allow a worker to grab multiple storage areas at the same time since
> that could cause deadlocks. However with proper APIs I believe we can
> avoid that.
>
> / Jonas
>
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