[whatwg] RFC: Alternatives to storage mutex for cookies andlocalStorage

Chris Jones cjones at mozilla.com
Fri Sep 4 13:31:31 PDT 2009

Mike Wilson wrote:
> Interesting. I've been following this discussion as my
> experience is that it is *extremely* hard to make an
> invisible locking mechanism that is to provide both 
> consistency and performance (no lockouts). 
> So far it seems a silver bullet hasn't been found.
> Your suggestion is in line with what I would expect from a 
> solution that makes a "best effort" compromise between the
> multi-tab browsing experience and the burden put on 
> application authors.
> What if cookies are accessed between beginTransaction() and
> commitTransaction(), would it make sense to throw for 
> updates with side-effects here as well? (Even though this
> would not be the case if done outside the transaction.)
> In some cases it may be helpful to get this "side-effect
> notification" for cookies as well...

I would prefer that cookies and localStorage not interact in this way. 
It seems confusing to have cookies be "sometimes transactional, 
sometimes not," although your proposal is certainly feasible.

The side-effect ("stomp") notification for cookies seems like a 
separate, and good, idea, irrespective of localStorage.


> Best regards
> Mike Wilson
> Chris Jones wrote:
>> I'd like to propose that HTML5 specify different schemes than a 
>> conceptual global storage mutex to provide consistency guarantees for 
>> localStorage and cookies.
>> Cookies would be protected according to Benjamin Smedberg's 
>> post in the 
>> "[whatwg] Storage mutex and cookies can lead to browser deadlock" 
>> thread.  Roughly, this proposal would give scripts a 
>> consistent view of 
>> document.cookie until they completed.  AIUI this is stronger 
>> consistency 
>> than Google Chrome provides today, and anecdotal evidence 
>> suggests even 
>> their weaker consistency hasn't "broken the web."
>> localStorage would be changed in a non-backwards-compatible way.  I 
>> believe that web apps can be partitioned into two classes: those that 
>> have planned for running concurrently (single-event-loop or not) in 
>> multiple "browsing contexts", and those that haven't.  I 
>> further posit 
>> that the second class would break when run concurrently in multiple 
>> contexts regardless of multiple event loops, and thus 
>> regardless of the 
>> storage mutex.  Even in the single-event-loop world, sites 
>> not prepared 
>> to be loaded in multiple tabs can stomp each other's data even though 
>> script execution is atomic.  (I wouldn't dare use my bank's 
>> website in 
>> two tabs at the same time in a single-event-loop browser.)  In other 
>> words, storage mutex can't help the second class of sites.
>> (I also believe that there's a very large, third class of pages that 
>> work "accidentally" when run concurrently in multiple contexts, even 
>> though they don't plan for that.  This is likely because they 
>> don't keep 
>> quasi-persistent data on the client side.)
>> Based on that, I believe localStorage should be designed with 
>> the first 
>> class of web apps (those that have considered data consistency across 
>> multiple concurrent contexts) in mind, rather than the second 
>> class.  Is 
>> a conceptual global storage mutex the best way for, say, gmail to 
>> guarantee consistency of its e-mail/contacts database?  I 
>> don't believe 
>> so: I think that a transactional localStorage is preferable. 
>> Transactional localStorage is easier for browser vendors to implement 
>> and should result in better performance for web apps in multi-process 
>> UAs.  It's more of a burden on web app authors than the 
>> hidden storage 
>> mutex, but I think the benefits outweigh the cost.
>> I propose adding the functions
>>    window.localStorage.beginTransaction()
>>    window.localStorage.commitTransaction()
>> or
>>    window.beginTransaction()
>>    window.commitTransaction()
>> (The latter might be preferable if we later decide to add 
>> more resources 
>> with transactional semantics.)
>> localStorage.getItem(),. setItem(), .removeItem(), and .clear() would 
>> remain specified as they are today.  beginTransaction() would do just 
>> that, open a transaction.  Calling localStorage.*() outside 
>> of an open 
>> transaction would cause a script exception to be thrown; this would 
>> unfortunately break all current clients of localStorage.  
>> There might be 
>> cleverer ways to mitigate this breakage by a UA pretending not to 
>> support localStorage until a script called beginTransaction().
>> yieldForStorageUpdates() would no longer be meaningful and should be 
>> removed.
>> A transaction would successfully "commit", atomically applying its 
>> modifications to localStorage, if localStorage was not 
>> modified between 
>> beginTransaction() and commitTransaction().  Note that a transaction 
>> consisting entirely of getItem() could fail just as those actually 
>> modifying localStorage.  If a transaction failed, the UA 
>> would throw a 
>> TransactionFailed exception to script.  The UA would be 
>> allowed to throw 
>> this exception at any time between beginTransaction() and 
>> commitTransaction().
>> There are numerous ways to implement transactional semantics. 
>> Single-event-loop UAs could implement beginTransaction() and 
>> commitTransaction() as no-ops.  Multi-event-loop UAs could reuse the 
>> global storage mutex if they had already implemented that 
>> (beginTransaction() == lock, commitTransaction() == unlock).
>> Some edge cases:
>>   * calling commitTransaction() without beginTransaction() 
>> would throw 
>> an exception
>>   * transactions would not be allowed to be nested, even on different 
>> localStorage DBs.  E.g. if site A's script begins a transaction on 
>> A.localStorage, and calls into site B's script embedded in an iframe 
>> which begins a transaction on B.localStorage, an exception 
>> would be thrown.
>>   * transactions *could* be spread across script executions, alert() 
>> dialogs, sync XHR, or anywhere else the current HTML5 spec 
>> requires the 
>> storage mutex be released.  Note that UAs wishing to forbid that 
>> behavior could simply throw a TransactionFailed exception where the 
>> storage mutex would have been released in the current spec.  Or this 
>> could be made illegal by the spec.
>>   * it's not clear to me how to handle async XHRs and Worker messages 
>> sent from within a failed transaction.  They could be specified to be 
>> sent or not and either behavior implemented easily.  My gut tells me 
>> that they *should* be sent regardless.
>> Feedback very much desired.
>> Cheers,
>> Chris
>> Addendum: I think that a past argument against a 
>> transactional approach 
>> was that scripts can cause side effects during transactions 
>> that can't 
>> be (easily, performantly) rolled back.  This is true, and 
>> troubling in 
>> that it deviates from SQL semantics, but because this proposal is 
>> designed for the first class of web apps I don't believe it's a 
>> compelling argument.  Further, a script can only corrupt its 
>> browsing-context-local state by mishandling failed 
>> transactions.  Using 
>> gmail as a convenient example, if a transaction failed but 
>> gmail wasn't 
>> prepared to handle the failure, that particular gmail instance would 
>> just break.  No e-mails or contacts would be corrupted, and the user 
>> could reload gmail and regain full functionality.  Servers should 
>> already be prepared to deal with clients behaving unpredictably.

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