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Re: [IndexedDB] Implicit transactions

From: Jonas Sicking <jonas@sicking.cc>
Date: Fri, 6 Aug 2010 08:04:20 -0700
Message-ID: <AANLkTik2yXbJoUXFsXtkSMRc1u54PpAy4NVrxnon-DtM@mail.gmail.com>
To: Jeremy Orlow <jorlow@chromium.org>
Cc: Shawn Wilsher <sdwilsh@mozilla.com>, public-webapps WG <public-webapps@w3.org>
On Fri, Aug 6, 2010 at 6:52 AM, Jeremy Orlow <jorlow@chromium.org> wrote:
> On Fri, Aug 6, 2010 at 1:56 PM, Jeremy Orlow <jorlow@chromium.org> wrote:
>>
>> On Thu, Aug 5, 2010 at 8:56 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>>>
>>> Ok, I'm going to start by taking a step back here.
>>>
>>> There is no such thing as implicit transactions.
>>>
>>> db.objectStore("foo", mode)
>>>
>>> is just syntactic sugar for
>>>
>>> db.transaction(["foo"], mode).objectStore("foo")
>>>
>>> so it always starts a new transaction. I think for now, lets take
>>> db.objectStore(..) out of the discussion and focus on how we want
>>> db.transaction() to work. In the end we may or may not want to keep
>>> db.objectStore() if it causes too much confusion.
>>>
>>> One thing that we have to first realize is that every IDBObjectStore
>>> instance is tied to a specific transaction. This is required to avoid
>>> ambiguity in what transaction a request is made against. Consider the
>>> following code
>>>
>>> trans1 = db.transaction(["foo", "bar"], READ_WRITE);
>>> trans2 = db.transaction(["foo", "students"], READ_ONLY);
>>> os1 = trans1.objectStore("foo");
>>> os2 = trans2.objectStore("foo");
>>> alert(os1 === os2);
>>> os1.get("someKey").onsuccess = ...;
>>>
>>> In this code, the alert will always display "false". The os1 and os2
>>> are two distinct objects. They have to be, otherwise we wouldn't know
>>> which transaction to place the get() request against.
>>>
>>> Once a transaction has been committed or aborted, using any of the
>>> IDBObjectStore objects connected with it will throw an error. So the
>>> example mentioned earlier in the thread (i'll use different syntax
>>> than used previously in the thread):
>>>
>>> var gMyos = null;
>>> function fun1() {
>>>  gMyos = db.transaction(["foo"]).objectStore("foo");
>>>  gMyos.get("someKey").onsuccess = ...;
>>> }
>>> function fun2() {
>>>  gMyos.get("someOtherKey");
>>> }
>>>
>>> If we return to the main even loop between calling fun1 and fun2, the
>>> .get() call in fun2 will *always* throw. IMHO it's a good thing that
>>> this consistently throws. Consider also
>>>
>>> function fun3() {
>>>  var trans = db.transaction(["foo", "bar"], READ_WRITE);
>>>  trans.objectStore("bar").openCursor(...).onsuccess = ...;
>>> }
>>>
>>> It would IMHO be a bad thing if calling fun3 right before calling fun2
>>> all of a sudden made fun2 not throw and instead place a request
>>> against the transaction created in fun3.
>>>
>>> While I definitely think it can be confusing that there are several
>>> IDBObjectStore instances referring to the same underlying objectStore,
>>> I think this is ultimately a good thing as it reduces the risk of
>>> accidentally placing a request against the wrong transaction. It means
>>> that in order to place a request against a transaction, you must
>>> either have a reference to that transaction, or a reference to an
>>> objectStore retrieved from that transaction.
>>>
>>> Another way to think of it is this. You generally don't place requests
>>> against an objectStore or index. You place them against a transaction.
>>> By tying IDBObjectStores to a given transaction, it's always explicit
>>> which transaction you are using.
>>>
>>> On Thu, Aug 5, 2010 at 3:04 AM, Jeremy Orlow <jorlow@chromium.org> wrote:
>>> > On Wed, Aug 4, 2010 at 7:47 PM, Shawn Wilsher <sdwilsh@mozilla.com>
>>> > wrote:
>>> >>
>>> >>  On 8/4/2010 10:53 AM, Jeremy Orlow wrote:
>>> >>>
>>> >>>
>>> >>> Whoa....transaction() is synchronous?!?  Ok, so I guess the entire
>>> >>> premise
>>> >>> of my question was super confused.  :-)
>>> >>>
>>> >> It is certainly spec'd that way [1].  The locks do not get acquired
>>> >> until
>>> >> the first actual bit of work is done though.
>>> >
>>> > I fully understand how the trick works.  I just didn't comprehend the
>>> > fact
>>> > that the Mozilla proposal (what's now in the spec) was removing any way
>>> > to
>>> > get into an IDBTransactionEvent handler besides doing an initial data
>>> > access.  I wouldn't have agreed to the proposal had I realized this.
>>> > Lets say I had the following bit of initialization code in my program:
>>> > var myDB = ...
>>> > var myObjectStore = myDB.objectStore("someObjectStore");
>>> > var myIndex = myObjectStore.index("someIndex");
>>> > var anotherObjectStore = myDB.objectStore("anotherObjectStore");
>>>
>>> As described above, grabbing references like this is not what you want
>>> to do. If we were to allow this I think we would run a severe risk of
>>> making it very hard to understand which transaction you are placing
>>> requests against.
>>>
>>> > And then I wanted to start a transaction that'd access some key and
>>> > then
>>> > presumably do some other work.  As currently specced, here's what I'd
>>> > need
>>> > to do:
>>> >
>>> > myDB.transaction().objectStore("someObjectStore").index("someIndex").get("someKey").onsuccess(function()
>>> > {
>>> >     anotherObjectStore.get("someOtherKey").onsuccess(...);
>>> > });
>>> > vs doing something like this:
>>> > myDB.asyncTransaction().onsuccess(function() {
>>> >     myIndex.get("someKey").onsuccess(function() {
>>> >         anotherObjectStore.get("someOtherKey").onsuccess(...);
>>> >     });
>>> > });
>>> > With the former, we actually have more typing and the code is harder to
>>> > read.  Sure, when I'm writing short code snipits, the synchronous form
>>> > can
>>> > be more convenient and readable, but forcing this upon every situation
>>> > is
>>> > going to be a hinderance.
>>> > Please, lets add back in a transaction method that returns an
>>> > IDBRequest.
>>>
>>> The current design of the spec is that which IDBObjectStore you place
>>> a request against determines which transaction you are using.
>>>
>>> It sounds to me like you want a design where each IDBObjectStore
>>> instance represents just a objectStore name, and depending on *when*
>>> you place a request against it determines which transaction is used.
>>>
>>> To me the latter is a much more subtle and risky. Especially if we
>>> *also* want to have the ability to synchronously start transactions.
>>> Which transaction would the following code snippets use for the .get()
>>> request?
>>>
>>> Example 1:
>>> trans1 = db.transaction(["foo", "bar"], READ_WRITE);
>>> trans2 = db.transaction(["foo", "fish"]);
>>> myStashedFooObjectStoreReference.get("someKey").onsuccess = ...;
>>>
>>> Example 2:
>>> trans = db.transaction(["foo"], READ_WRITE);
>>> var fooStore = trans.objectStore("foo");
>>> fooStore.add("someKey", someValue).onsuccess = function(e) {
>>>  trans2 = db.transaction(["foo", "bar"]);
>>>  fooStore.get("someOtherKey");
>>>  fooStore = trans2.objectStore("foo");
>>>  fooStore.get("aThirdKey");
>>> }
>>>
>>> Basically if we want to make *when* you place a request determine
>>> which transaction is used, rather than *which* IDBObjectStore you use,
>>> then we can't allow transactions to be synchronously created. That
>>> reason alone is enough to make me think that we should stick with the
>>> current model in the spec.
>>>
>>> Here is what your example above would look like:
>>> trans = myDB.transaction(...);
>>>
>>> trans.objectStore("someObjectStore").index("someIndex").get("someKey").onsuccess
>>> = ...;
>>> trans.objectStore("anotherObjectStore").get("someOtherKey").onsuccess =
>>> ...;
>>>
>>> Which I think is simpler than either of your examples. Yes, it's a bit
>>> more typing, but it's much less asynchronous nesting which is IMHO a
>>> bigger advantage.
>>
>> Ok, I think I now understand how your transaction model was intended to
>> work.  It scares me some that neither I nor Andrei understood the model.
>>  I'm curious to know whether it was just us or Pablo and Nikunj were also
>> confused by it.
>> Personally, I think the model of ObjectStores, Indexes, etc not being tied
>> to just one transaction and instead using knowledge of which
>> IDBTransactionEvent is currently firing to know which transaction an action
>> is tied to is more elegant.  But I agree such a model is kind of at odds
>> with your model and that just doing one or the other is probably for the
>> best.  I worry that your model will only increase the pressure for people to
>> wrap IndexedDB for syntactic sugar.  Yet I also think more thrashing of the
>> spec is a bad idea and clearly you guys feel pretty strongly about this.  So
>> I guess we should continue down our current path.
>> I'll file a bug for clarification of what's currently there.
>
> Hmm.  One more question.  Does the following fail or work?
> trans = myDB.transaction(...);
> setTimeout(function() {
>   trans.objectStore("someObjectStore").index("someIndex").get("someKey").onsuccess =
> ...;
> }, 0);
> In other words, will the transaction commit because we leave JavaScript and
> nothing more is queued up in the transaction or will such semantics only
> happen after we see the first request be queued up within a transaction?

It will not work. The transaction is automatically committed as soon
as control returns to the event loop if there are no requests placed
against it.

I guess technically we could make it work. However I think it would be
more inconsistent and confusing to make it so.

/ Jonas
Received on Friday, 6 August 2010 15:05:13 GMT

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