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RE: [IndexedDB] Current editor's draft

From: Pablo Castro <Pablo.Castro@microsoft.com>
Date: Wed, 14 Jul 2010 02:52:39 +0000
To: Andrei Popescu <andreip@google.com>, Nikunj Mehta <nikunj@o-micron.com>
CC: Jonas Sicking <jonas@sicking.cc>, public-webapps <public-webapps@w3.org>
Message-ID: <F753B2C401114141B426DB383C8885E058EEE0D6@TK5EX14MBXC124.redmond.corp.microsoft.com>

From: public-webapps-request@w3.org [mailto:public-webapps-request@w3.org] On Behalf Of Andrei Popescu
Sent: Monday, July 12, 2010 5:23 AM

Sorry I disappeared for a while. Catching up with this discussion was an interesting exercise...there is no particular message in this thread I can respond to, so I thought I'd just reply to the last one. Overall I think the new proposal is shaping up well and is being effective in simplifying scenarios. I do have a few suggestions and questions for things I'm not sure I see all the way.

READ_ONLY vs READ_WRITE as defaults for transactions:
To be perfectly honest, I think this discussion went really deep over an issue that won't be a huge deal for most people. My perspective, trying to avoid performance or usage frequency speculation, is around what's easier to detect. Concurrency issues are hard to see. On the other hand, whenever we can throw an exception and give explicit guidance that unblocks people right away. For this case I suspect it's best to default to READ_ONLY, because if someone doesn't read or think about it and just uses the stuff and tries to change something they'll get a clear error message saying "if you want to change stuff, use READ_WRITE please". The error is not data- or context-dependent, so it'll fail on first try at most once per developer and once they fix it they'll know for all future cases.

Dynamic transactions:
I see that most folks would like to see these going away. While I like the predictability and simplifications that we're able to make by using static scopes for transactions, I worry that we'll close the door for two scenarios: background tasks and query processors. Background tasks such as synchronization and post-processing of content would seem to be almost impossible with the static scope approach, mostly due to the granularity of the scope specification (whole stores). Are we okay with saying that you can't for example sync something in the background (e.g. in a worker) while your app is still working? Am I missing something that would enable this class of scenarios? Query processors are also tricky because you usually take the query specification in some form after the transaction started (especially if you want to execute multiple queries with later queries depending on the outcome of the previous ones). The background tasks issue in particular looks pretty painful to me if we don't have a way to achieve it without freezing the application while it happens. 

Implicit commit:
Does this really work? I need to play with sample app code more, it may just be that I'm old-fashioned. For example, if I'm downloading a bunch of data form somewhere and pushing rows into the store within a transaction, wouldn't it be reasonable to do the whole thing in a transaction? In that case I'm likely to have to unwind while I wait for the next callback from XmlHttpRequest with the next chunk of data. I understand that avoiding it results in nicer patterns (e.g. db.objectStores("foo").get(123).onsuccess = ...), but in practice I'm not sure if that will hold given that you still need error callbacks and such.

Nested transactions:
Not sure why we're considering this an advanced scenario. To be clear about what the feature means to me: make it legal to start a transaction when one is already in progress, and the nested one is effectively a no-op, just refcounts the transaction, so you need equal amounts of commit()'s, implicit or explicit, and an abort() cancels all nested transactions. The purpose of this is to allow composition, where a piece of code that needs a transaction can start one locally, independently of whether the caller had already one going.

Schema versioning:
It's unfortunate that we need to have explicit elements in the page for the versioning protocol to work, but the fact that we can have a reliable mechanism for pages to coordinate a version bump is really nice. For folks that don't know about this the first time they build it, an explicit error message on the schema change timeout can explain where to start. I do think that there may be a need for non-breaking changes to the schema to happen without a "version dance". For example, query processors regularly create temporary tables during sorts and such. Those shouldn't require any coordination (maybe we allow non-versioned additions, or we just introduce temporary, unnamed tables that evaporate on commit() or database close()...).

Thanks
-pablo
Received on Wednesday, 14 July 2010 02:53:15 GMT

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