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Re: [IndexedDB] Current editor's draft

From: Andrei Popescu <andreip@google.com>
Date: Wed, 7 Jul 2010 18:41:52 +0100
Message-ID: <AANLkTilY_jBJknEkU6_3e54WJuYdBj1IreUEcdHQ0I85@mail.gmail.com>
To: Jonas Sicking <jonas@sicking.cc>
Cc: Nikunj Mehta <nikunj@o-micron.com>, public-webapps <public-webapps@w3.org>
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).


> 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.


> >> > 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.


> > 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.



> > 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.


> 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?


>      part = db.createObjectStore("part", "number", false);
>      supplier = db.createObjectStore("supplier", "number", false);
>      shipment = db.createObjectStore("shipment", id);
>      part.createIndex("partName", "name");
>      part.createIndex("partColor", "color");
>      part.createIndex("partCity", "city");
>      supplier.createIndex("supplierStatus", "status");
>      supplier.createIndex("supplierCity", "city");
>      shipment.createIndex("partSupplier", "[part, supplier]");
>       var data = [{
>        store: part, values: parts}, {
>        store: supplier, values: suppliers}, {
>        store: shipment, values: shipments}];
>      for (var storeIndex = 0; storeIndex < data.length; ++storeIndex) {
>        var task = data[storeIndex];
>        for (var valueIndex = 0; valueIndex < task.values.length;
> ++valueIndex) {
>           task.store.add(task.values[index]); // Ignoring the
> possibility that there may be errors while adding
>        }
>      }
>     };
>  }
> };
>
> dbRequest.onerror = function(event) {
> if (event.code === IDBDatabaseException.UNKNOWN_ERR)
>  throw new Error("Could not open the database");
> }
>
> There are a few advantages. First of all all the operations can easily
> be done in a single transaction, including populating the
> objectStores. Second, no need for a separate catalog object. Third,
> the index creator functions can live directly on the objectStore
> interface. Fourth, there is less nesting of asynchronous callbacks.
>
> >> > ------- IDL --------
> >> >
> >> > [NoInterfaceObject]
> >> > interface IDBDatabase {
> >> >  readonly attribute DOMString     name;
> >> >  readonly attribute DOMString     description;
> >> >  readonly attribute DOMStringList objectStores;
> >> >  /*
> >> >  Open an object store in the specified transaction. The transaction
> can
> >> > be
> >> >  dynamic scoped, or the requested object store must be in the static
> >> > scope.
> >> >  Returns IDBRequest whose success event of IDBTransactionEvent type
> >> > contains
> >> >  a result with IDBObjectStore and transaction is an
> >> > IDBTransactionRequest.
> >> >  */
> >> >  IDBRequest openObjectStore(in DOMString name, in IDBTransaction txn,
> >> >  in optional unsigned short mode /* defaults to READ_WRITE */);
> >>
> >> I don't understand the advantage of this proposal over mozillas
> >> proposal.
> >
> > The above proposal allows opening multiple object stores in the same
> > transaction in dynamic scope, even without having explicitly identified
> each
> > one of them at the time of creating the transaction.
>
> We don't mention dynamic transactions much in our proposal, mostly
> because we're not convinced that it's something that we want to
> implement. However if we really want to keep them, I suggest that we
> create a separate transaction interface, like:
>
> IDBDynamicTransaction {
> interface IDBTransaction {
>  readonly attribute IDBDatabase db;
>
>  // Asynchronousy attempts to lock more objectStores.
>  IDBRequest openObjectStore(in DOMString name, mode);
>
>  void abort();
>  attribute Function oncomplete;
>   attribute Function onabort;
>  attribute Function ontimeout;
> }
>
> IDBDatabase {
>  ...
>  IDBDynamicTransaction openDynamicTransaction();
>  ...
> }
>
> This interface allows asynchronously requesting more objectStores to
> be locked. The author must take care whenever calling openObjectStores
> that the request might fail due to deadlocks.
>
> But as previously stated, I think this adds too much complexity and
> too much racyness to the API. And so I'd prefer to not add this.
>
>
Same here.



> > Secondly, where static
> > scope is desired, in order to run to completion without being aborted due
> to
> > unavailable objects, this proposal ensures that locks are obtained at the
> > time of creating the transaction.
> >
> > My examples illustrates this approach. It uses both dynamic and static
> > scoped transactions. The dynamic scope transactions are synchronously
> > created since no locks have to be obtained when creating the transaction.
>
> There might be some misunderstanding about how our proposal works. The
> idea is that while you can synchronously create a IDBTransaction
> object, no locks are acquired synchronously. Instead, the object
> asynchronously requests the required locks to be acquired. It also
> remembers all requests being placed against it. Not until all required
> locks have been acquired will it create a database transaction and
> start making requests into the database and firing 'success' events
> against the previously returned IDBRequest objects.
>
> This makes it possible to only have one level of asynchronous
> callbacks exposed to the author. The implementation takes care of the
> multiple asynchronous actions required in order to honor the authors
> request.
>
> This significantly simplifies the API as demonstrated in [2] vs [3]
>
> [2]
> http://docs.google.com/document/pub?id=1I__XnwvvSwyjvxi-FAAE0ecnUDhk5DF7L2GI6O31o18
> [3]
> http://docs.google.com/document/pub?id=1KKMAg_oHLeBvFUWND5km6FJtKi4jWxwKR0paKfZc8vU
>
> >> One of our main points was to make getting objectStore
> >> objects a synchronous operation as to avoid having to nest multiple
> >> levels of asynchronous calls. Compare
> >>
> >> var req = db.openObjectStore("foo", trans);
> >> req.onerror = errorHandler;
> >> req.onsuccess = function(e) {
> >>  var fooStore = e.result;
> >>  var req = fooStore.get(12);
> >>  req.onerror = errorHandler;
> >>  req.onsuccess = resultHandler;
> >> }
> >>
> >> to
> >>
> >> var fooStore = db.openObjectStore("foo", trans);
> >> var req = fooStore.get(12);
> >> req.onerror = errorHandler;
> >> req.onsuccess = resultHandler;
> >>
> >>
> >> I also don't understand the advantage of having the transaction as an
> >> argument to openObjectStore rather than having openObjectStore live on
> >> transaction. Compare
> >>
> >> db.openObjectStore("foo", trans);
> >>
> >> to
> >>
> >> trans.openObjectStore("foo");
> >>
> >> I also don't understand the meaning of specifying a mode when a
> >> objectStore is opened, rather than specifying the mode when the
> >> transaction is created.
> >
> > Have you reviewed the examples? Different object stores in a transaction
> are
> > used in different modes, and that requires us to identify the mode when
> > opening the object store. This also increases concurrency. This is
> > particularly useful for dynamic transactions.
>
> I'm following you better now. I do see how this can work for dynamic
> transactions where locks are not acquired upon creation of the
> transaction. But I don't see how this makes sense for static
> transactions. And it indeed seems like you are not using this feature
> for static transactions.
>
>
I don't think it's even possible with the current API since
openTransaction() takes a list of objectStore names but a single mode.


> If it is the case that specifying a mode when opening an objectStore
> only makes sense on dynamic transactions, then I think we should only
> expose that argument on dynamic transactions.
>
> Now that I understand your proposal better, I don't understand how
> IDBTransaction.objectStore works for dynamically scoped transactions
> in your proposal. It seems to require synchronously grabbing a lock
> which I thought we wanted to avoid at all cost.
>
>
This is rather confusing: is IDBTransaction::objectStore() creating an
object store, now? If yes, then it must lock it synchronously. If it just
returns an object store that was previously added to the transaction, what
is the 'mode' parameter for?


>> Unless we're planning on making all
> >> transactions dynamic (I hope not), locks have to be grabbed when the
> >> transaction is created, right? If a transaction is holding a READ_ONLY
> >> lock for a given objectStore, then attempting to open that objectStore
> >> as READ_WRITE should obviously fail. Consecutively, if a transaction
> >> is holding a READ_WRITE lock for a given objectStore, then opening
> >> that objectStore as READ_ONLY doesn't seem to have any benefit over
> >> opening it as READ_WRITE. In short, I can't see any situation when
> >> you'd want to open an objectStore in a different mode than what was
> >> used when the transaction was created.
> >>
> >> Finally, I would stronly prefer to have READ_ONLY be the default
> >> transaction type if none is specified by the author. It is better to
> >> default to what results in higher performance, and have people notice
> >> when they need to switch to the slower mode. This is because people
> >> will very quickly notice if they use READ_ONLY when they need to use
> >> READ_WRITE, since the code will never work. However if people use
> >> READ_WRITE when all they need is READ_ONLY, then the only effect is
> >> likely to be an application that runs somewhat slower, when they will
> >> unlikely detect.
> >
> > This approach is also likely to cause exceptions upon put, remove, and
> add.
> > I would prefer to not cause exceptions as the default behavior.
>
> If we use READ_WRITE as default behavior then it's extremely likely
> that people will use the wrong lock type and not realize. The downside
> will be that sites will run less concurrently, and thus slower, than
> they could. Another downside is that authors should specify lock-type
> more often, for optimal performance, if we think that READ_ONLY is
> more common.
>
> If we are using READ_ONLY as default behavior, then it's extremely
> likely that people will use the wrong lock type, notice that their
> code isn't working, and fix it. The downside is that people will have
> to fix their bugs. Another downside is that authors will have to
> specify lock-type more often if we think that READ_WRITE is more
> common.
>
> To me the downsides of using READ_WRITE as a default are much worse
> than the downsides of using READ_ONLY.
>
>
>> >  /*
> >> >  Open the database catalog in the specified transaction for exclusive
> >> > access.
> >> >  Returns IDBRequest whose success event of IDBTransactionEvent type
> >> > contains
> >> >  a result with IDBCatalog and transaction is an IDBTransactionRequest.
> >> >  */
> >> >  IDBRequest openCatalog(in IDBTransaction txn);
> >>
> >> See above regarding catalog objects.
> >>
> >> >  /*
> >> >  Create a new static scoped transaction asynchronously.
> >> >  Returns IDBRequest whose success event of IDBSuccessEvent type
> contains
> >> > a
> >> >  result with IDBTransactionRequest.
> >> >  */
> >> >  IDBRequest openTransaction (in optional DOMStringList storeNames /*
> >> > defaults to mean all object stores */,
> >> >  in optional unsigned short mode /* defaults to READ_WRITE */,
> >> >  in optional IDBTransaction parent /* defaults to null */,
> >> >  in optional unsigned long timeout /* defaults to no timeout*/);
> >> >  /*
> >> >  Create a new dynamic scoped transaction. This returns a transaction
> >> > handle
> >> >  synchronously.
> >> >  */
> >> >  IDBTransaction transaction (in optional IDBTransaction parent /*
> >> > defaults to null */,
> >> >  in optional unsigned long timeout /* defaults to no timeout*/);
> >>
> >> I don't understand this. When would you ever want to use the
> >> asynchronous version?
> >
> > See my examples.
>
> I understand now.
>
> >> Why can't you specify a list of objectStores in the synchronous version?
> >
> > Because I don't see what is the benefit of doing that if you can't obtain
> > locks, which synchronous behavior cannot.
>
> See my example above how our proposals allow you to specify which
> objectStores should be locked, while still allowing the locks to be
> acquired asynchronously.
>
> >> Do we really need nested transactions? I do see that you added them in
> >> latest revision of the editor drafts, but it seems like a fairly
> >> advanced feature that would be nice to leave for a v2. If everyone
> >> else wants them, I can check how much work it would be for us to
> >> implement them, but my vote is to leave it for a later version.
> >
> > I added them because they are useful in advanced scenarios. I am fine
> with
> > leaving them to a later date.
>
> I would prefer to leave them out for now.
>
>
+1. Same for dynamic transactions, at least for v1.

Thanks,
Andrei
Received on Wednesday, 7 July 2010 17:42:26 GMT

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