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Re: [IndexedDB] Current editor's draft

From: Nikunj Mehta <nikunj@o-micron.com>
Date: Sat, 10 Jul 2010 00:14:31 +0530
Cc: Andrei Popescu <andreip@google.com>, public-webapps <public-webapps@w3.org>
Message-Id: <8AE10C6C-30CE-469A-B187-296E44BE33AB@o-micron.com>
To: Jonas Sicking <jonas@sicking.cc>

On Jul 8, 2010, at 12:38 AM, Jonas Sicking wrote:

> On Wed, Jul 7, 2010 at 10:41 AM, Andrei Popescu <andreip@google.com> wrote:
>> 
>> 
>> On Wed, Jul 7, 2010 at 8:27 AM, Jonas Sicking <jonas@sicking.cc> wrote:
>>> 
>>> On Tue, Jul 6, 2010 at 6:31 PM, Nikunj Mehta <nikunj@o-micron.com> wrote:
>>>> On Wed, Jul 7, 2010 at 5:57 AM, Jonas Sicking <jonas@sicking.cc> wrote:
>>>>> 
>>>>> On Tue, Jul 6, 2010 at 9:36 AM, Nikunj Mehta <nikunj@o-micron.com>
>>>>> wrote:
>>>>>> Hi folks,
>>>>>> 
>>>>>> There are several unimplemented proposals on strengthening and
>>>>>> expanding IndexedDB. The reason I have not implemented them yet is
>>>>>> because I am not convinced they are necessary in toto. Here's my
>>>>>> attempt at explaining why. I apologize in advance for not responding
>>>>>> to individual proposals due to personal time constraints. I will
>>>>>> however respond in detail on individual bug reports, e.g., as I did
>>>>>> with 9975.
>>>>>> 
>>>>>> I used the current editor's draft asynchronous API to understand
>>>>>> where
>>>>>> some of the remaining programming difficulties remain. Based on this
>>>>>> attempt, I find several areas to strengthen, the most prominent of
>>>>>> which is how we use transactions. Another is to add the concept of a
>>>>>> catalog as a special kind of object store.
>>>>> 
>>>>> Hi Nikunj,
>>>>> 
>>>>> Thanks for replying! I'm very interested in getting this stuff sorted
>>>>> out pretty quickly as almost all other proposals in one way or another
>>>>> are affected by how this stuff develops.
>>>>> 
>>>>>> Here are the main areas I propose to address in the editor's spec:
>>>>>> 
>>>>>> 1. It is time to separate the dynamic and static scope transaction
>>>>>> creation so that they are asynchronous and synchronous respectively.
>>>>> 
>>>>> I don't really understand what this means. What are dynamic and static
>>>>> scope transaction creation? Can you elaborate?
>>>> 
>>>> This is the difference in the API in my email between openTransaction
>>>> and
>>>> transaction. Dynamic and static scope have been defined in the spec for
>>>> a
>>>> long time.
>>> 
>> 
>> In fact, dynamic transactions aren't explicitly specified anywhere. They are
>> just mentioned. You need some amount of guessing to find out what they are
>> or how to create one (i.e. pass an empty list of store names).
> 
> Yes, that has been a big problem for us too.
> 
>>> Ah, I think I'm following you now. I'm actually not sure that we
>>> should have dynamic scope at all in the spec, I know Jeremy has
>>> expressed similar concerns. However if we are going to have dynamic
>>> scope, I agree it is a good idea to have separate APIs for starting
>>> dynamic-scope transactions from static-scope transactions.
>>> 
>> 
>> I think it would simplify matters a lot if we were to drop dynamic
>> transactions altogether. And if we do that,  then we can also safely move
>> the 'mode' to parameter to the Transaction interface, since all the object
>> stores in a static transaction can be only be open in the same mode.
> 
> Agreed.
> 
>>>>>> 2. Provide a catalog object that can be used to atomically add/remove
>>>>>> object stores and indexes as well as modify version.
>>>>> 
>>>>> It seems to me that a catalog object doesn't really provide any
>>>>> functionality over the proposal in bug 10052? The advantage that I see
>>>>> with the syntax proposal in bug 10052 is that it is simpler.
>>>>> 
>>>>> http://www.w3.org/Bugs/Public/show_bug.cgi?id=10052
>>>>> 
>>>>> Can you elaborate on what the advantages are of catalog objects?
>>>> 
>>>> To begin with, 10052 shuts down the "users" of the database completely
>>>> when
>>>> only one is changing its structure, i.e., adding or removing an object
>>>> store.
>>> 
>>> This is not the case. Check the steps defined for setVersion in [1].
>>> At no point are databases shut down automatically. Only once all
>>> existing database connections are manually closed, either by calls to
>>> IDBDatabase.close() or by the user leaving the page, is the 'success'
>>> event from setVersion fired.
>>> 
>>> [1] http://www.w3.org/Bugs/Public/show_bug.cgi?id=10052#c0
>>> 
>>>> How can we make it less draconian?
>>> 
>>> The 'versionchange' event allows pages that are currently using the
>>> database to handle the change. The page can inspect the new version
>>> number supplied by the 'versionchange' event, and if it knows that it
>>> is compatible with a given upgrade, all it needs to do is to call
>>> db.close() and then immediately reopen the database using
>>> indexedDB.open(). The open call won't complete until the upgrade is
>>> finished.
>>> 
>> 
>> I had a question here: why does the page need to call 'close'? Any pending
>> transactions will run to completion and new ones should not be allowed to
>> start if a VERSION_CHANGE transaction is waiting to start. From the
>> description of what 'close' does in 10052, I am not entirely sure it is
>> needed.
> 
> The problem we're trying to solve is this:
> 
> Imagine an editor which stores documents in indexedDB. However in
> order to not overwrite the document using temporary changes, it only
> saves data when the user explicitly requests it, for example by
> pressing a 'save' button.
> 
> This means that there can be a bunch of potentially important data
> living outside of indexedDB, in other parts of the application, such
> as in textfields and javascript variables.
> 
> If we were to automatically close all other open IDBDatabase objects
> when IDBDatabase.setVersion is closed, that would mean that the
> application by default risks losing data. Without a 'versionchange'
> event, there is little the application can do to ensure that
> IDBDatabase object is closed under it, preventing it from saving data
> that lived outside the database. So while run-to-completion and
> allowing existing transactions to finish ensures that the database
> stays in a consistent state, it does not prevent dataloss on an
> application level.
> 
> But even with the 'versionchange' event, we're only solving part of
> the problem. First of all it requires that people listen to it and act
> appropriately by saving information in indexedDB. Second, it requires
> that the application is able to synchronously store all the data in
> indexedDB before returning from the 'versionchange' event handler.
> 
> By instead not closing databases automatically, we do the safe thing
> by default. And by adding the IDBDatabase.close() function, we allow
> pages to asynchronously interact with the user to ask the user if he
> wants to save the data. Or to perform other asynchronous actions as
> part of saving the data. Once all data has been saved, the application
> can call IDBDatabase.close().
> 
> Alternatively, and by default, the setVersion callback simply won't
> fire until all other IDBDatabase connections are closed by the user
> closing other tabs.

Would every page need to understand the versionchange protocol in addition to understanding transactions, so as not to starve itself? IOW, every page has to be prepared to listen for versionchange event in order to allow some page to upgrade the database. In the event there is a non-cooperating page, upgrades would fail. It seems like the only safe thing a page can do to respond to versionchange event is cease whatever it is doing, save the application state, and wait to be notified to continue. In short an approach that requires far greater coordination than we have evidence to suggest would be acceptable to programmers of different levels of sophistication.

It feels much "simpler" to just have another kind of object, e.g., catalog, and the same concepts of transactions to avoid stepping on each other's toes. I have already explained why adding an object store or index should not require anyone to discontinue using the database. Removing an object store or index only needs one to wait to stop using the affected object store or index. 

Moreover, the catalog object proposal is more concurrent, if that brings more attractiveness to the approach.

>>>> Secondly, I don't see how that
>>>> approach can produce atomic changes to the database.
>>> 
>>> When the transaction created in step 4 of setVersion defined in [1] is
>>> created, only one IDBDatabase object to the database is open. As long
>>> as that transaction is running, no requests returned from
>>> IDBFactory.open will receive a 'success' event. Only once the
>>> transaction is committed, or aborted, will those requests succeed.
>>> This guarantees that no other IDBDatabase object can see a partial
>>> update.
>>> 
>>> Further, only once the transaction created by setVersion is committed,
>>> are the requested objectStores and indexes created/removed. This
>>> guarantees that the database is never left with a partial update.
>>> 
>>> That means that the changes are atomic, right?
>>> 
>> 
>> Sounds atomic IMHO.
> 
> One thing that I forgot to mention. If the setVersion transaction is
> aborted for some reason, for example due to disk errors or due to
> explicit calls to .abort(), all changes are rolled back, including all
> calls to createObjectStore/createIndex/removeObjectStore/removeIndex.
> 
>>>> Thirdly, we shouldn't
>>>> need to change version in order to perform database changes.
>>> 
>>> First off, note that if the upgrade is compatible, you can just pass
>>> the existing database version to setVersion. So no version *change* is
>>> actually needed.
>>> 
>>> Second, I don't think there is much difference between
>>> 
>>> var txn = db.transaction();
>>> db.openCatalog(txn).onsuccess = ...
>>> 
>>> vs
>>> 
>>> db.setVersion("5").onsuccess = ...
>>> 
>>> I don't see that telling people that they have to use the former is a big
>>> win.
>>> 
>>> 
>>> The problem that I see with the catalog proposal, if I understand it
>>> correctly, is that it means that a page that has a IDBDatabase object
>>> open has to always be prepared for calls to
>>> openObjectStore/openTransaction failing. I.e. the page can't ever know
>>> that another page was opened which at any point created a catalog and
>>> removed an objectStore. This forces pages to at every single call
>>> either check that the version is still the same, or that each and
>>> every call to openObjectStore/openTransaction succeeds. This seems
>>> very error prone to me.
>>> 
>> 
>> I guess you could pass an event ("schemachange") as soon as the catalog
>> transaction is committed? This somewhat similar to the "versionchange" event
>> fired by setVersion(). At the same time, if setVersion() can be used to
>> provide atomic schema changes, I don't see what the catalog buys us.
> 
> Agreed. And even a "schemachange" event requires that people listen to
> it. I.e. it's unsafe by default, and only becomes safe once pages take
> explicit action.
> 
>>> Looking at your example, it also seems like it contains a race
>>> condition. There is a risk that when someone opens a database, the
>>> first transaction, which uses a catalog to create the necessary
>>> objectStores and indexes, is committed, but the second transaction,
>>> which populates the objectStores with data, has not yet started.
>>> 
>>>> Finally, I am
>>>> not sure why you consider the syntax proposal simpler. Note that I am
>>>> not
>>>> averse to the version change event notification.
>>> 
>>> Compare to how your code would look like with the proposals in bugs
>>> 9975 and 10052:
>>> 
>>> var db;
>>> var dbRequest = indexedDB.open("parts", 'Part database');
>>> dbRequest.onsuccess = function(event) {
>>>  db = event.result;
>>>  if (db.version != "1") {
>>>    versionRequest = db.setVersion("1");
>>>    versionRequest.ontimeout = function(event) {
>>>      throw new Error("timeout while waiting for other DBs to close");
>>>    };
>>>    versionRequest.onsuccess = function(event) {
>>>      event.transaction.onerror = function(event) {
>>>        throw new Error("Failed to set up database due to error: " +
>>> event.message);
>>>      };
>>>      db.setVersion("1");
>> 
>> 
>> Do you need to call setVersion again?
> 
> Oops, my bad, please remove that line.
Received on Friday, 9 July 2010 19:08:58 GMT

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