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Re: [IndexedDB] Lots of small nits and clarifying questions

From: Nikunj Mehta <nikunj@o-micron.com>
Date: Sat, 13 Mar 2010 01:02:26 -0800
Cc: public-webapps WG <public-webapps@w3.org>
Message-Id: <578877F5-AA87-47C3-98A7-D07487D32AF2@o-micron.com>
To: Jeremy Orlow <jorlow@google.com>
Thanks for your patience. Most questions below don't seem to need new  
spec text.

On Feb 18, 2010, at 9:08 AM, Jeremy Orlow wrote:

> I'm sorry that I let so much IndexedDB feedback get backlogged.  In  
> the future, I'll try to trickle things out slower.
>
>
> Indexes:
>
> 1) Creation of indexes really needs to be made more clear.

Did you mean the algorithm for this process or the API documentation?

> For example, does creation of the index block everything until it's  
> complete or does the database get created in the background?

If the index is created using the synchronous call, then the calling  
worker will block until the index is created. If the async API is  
used, the rest of the database can continue to work and the worker  
should be able to continue to other calls without waiting for the  
index creation to get over.

>  What if I have 1gb of my mail stored in IndexedDB and then a  
> database migration adds an index?

Depends on your choice of workers and sync/async API.

>  Is my app completely unusable during that time?

It doesn't have to be in a well-designed app.

>  What if the browser is exited half way through building (you can't  
> just delete it)?

Regardless of whether the sync or the async API, if you choose to  
perform DDL (not just index creation, but also database creation) in a  
transaction, then it could be done atomically.

>  What happens if you query the database while it's building in the  
> background-building case (should it emulate it via entity-store- 
> scans)?

The index doesn't exist until it is created. If you use an index  
before it exists (even if it is in the process of being created), then  
the outcome depends on whether the index is created in a transaction.  
If yes, the call will fail because the index will not exist. If no,  
then the call may use partial index results.

>  These are all very important questions whose answers should be  
> standardized.

This feels like good non-normative text to me.

>
> 2) Why are Indexes in some database-global namespace rather than  
> some entity-store-global namespace?

This approach is fine by me.

>  I know in SQL, most people use the table name as a prefix for their  
> index names to make sure they're unique.  Why inherit such silliness  
> into IndexedDB?  Why not connect every index to a particular entity- 
> store?

Indexes in IndexDB are already dependent on an entity store.

>
> 3) What happens when unique constraints are violated?

See 3.2.2 step 7 and 3.2.3 step 3. Is there something else you have in  
mind?

>
> 4) I don't remember anything explicitly stating that when a value  
> changes that an index has a keypath referring to, that index should  
> be updated.

See http://www.w3.org/TR/IndexedDB/#dfn-index-maintenance-conditions

>
> 5) It definitely would be nice to be able to index more than just  
> longs and strings.

I am planning to add long long. Beyond that would need to wait for now.

>
> 6) The specific ordering of elements should probably be specced  
> including a mix of types.

Can you propose spec text for this? What do you think about the text  
in http://www.w3.org/TR/IndexedDB/#key-construct?

>
>
> Key ranges / cursors:
>
> 1) Is an open or closed key range the default?

Not sure what you mean by default here?

>
> 2) What happens when data mutates while you're iterating via a cursor?

This is covered by http://www.w3.org/TR/IndexedDB/#dfn-mode

>
> 3) In the spec, get and getObject seem to assume that only one  
> element can be returned...but that's only true if unique is true.   
> What do you do if there are multiple?

ObjectStore doesn't allow storage of multiple values with a given key.  
See http://www.w3.org/TR/IndexedDB/#object-store-concept

Multiples only make sense if you are retrieving from an Index. The  
following spec text could be added to 3.2.5 and 3.2.6 to return the  
first value in the sorted order of their keys.

3.2.5

3. Let /value/ be the value of the record first in the sorted order of  
their keys corresponding to the key /key/ in /index/

3.2.6

3. Return the value for the record first in the sorted order of their  
keys corresponding to the key /key/ in /index/

>
> 4) Why can the cursor only travel in one direction?

Many things are possible. This is a scope limitation. You can open  
multiple cursors each going in different directions.

>
> 5) What if you modify a value that then implicitly (via the key- 
> path) changes the index that your cursor is currently iterating over?

It would be perfectly fine to remove or update a value while using a  
cursor. See http://dev.w3.org/2006/webapi/WebSimpleDB/#widl-IDBCursorSync-value 
  for errors that may occur upon set.

Specifically, if you remove the item at cursor's current position, the  
cursor stay in its position but accessing the cursor value will return  
null. You can't change the value at a cursor's current location, if  
the cursor is based on an auto-populated index. For other cursors, if  
you change the value at a cursor's current location, it will show up  
at its new position in the cursor.

Concurrency mode of an object store on which the cursor is used limits  
concurrent changes.

>
>
> Transactions:
>
> 1) We feel strongly that nested transactions should be allowed.   
> Closed nested transactions should be simple to implement and will  
> make it much easier for multiple layers of abstraction to use  
> IndexedDB without knowledge of each other.

Can you elaborate/provide evidence for this? We have avoided nested  
transactions to keep the footprint of this API small.

>
> 2) In the spec, dynamic transactions and the difference between  
> static and dynamic are not very well explained.

Can you propose spec text?

>
> 3) I'm not sure that I like how the spec talks about commits being  
> durable but then later says "Applications must not assume that  
> committing the transaction produces an instantaneously durable  
> result. The user agent may delay flushing data to durable storage  
> until an appropriate time."

This is also called - as durable as the underlying platform is.  
Nothing one can do here.

>  It seems like the language should be made more consistient.

We have had discussion in the past about this and the group agreed at  
the time that this language is both valid and desirable.

> Also, it seems like there should be some way to ensure it is durable  
> on disk for when it's absolutely necessary.  (But maybe with a note  
> that UAs are free to rate limit this.)

Implementations may not be able to support this due to platform  
reasons and there seems little benefit in offering this optional  
feature.

>
>
> Misc:
>
> 1) Structured clone is going to change over time.  And,  
> realistically, UAs won't support every type right away anyway.  What  
> do we do when a value is inserted that we do not support?

We will evolve the text as and when the same evolves in WebStorage.

>
> 2) It seems that you can only be connected to one database at a time?

Why does it so seem? A single worker (background or foreground) could  
be connected to several databases or be simultaneously connected  
through multiple connections to a single database. See http://dev.w3.org/2006/webapi/WebSimpleDB/#dfn-connection

>  If so, why?
>
> 3) Do we have enough distinct error codes?  For example, there are  
> multiple ways to get a NON_TRANSIENT_ERR when creating a  
> transaction.  Error strings can help with debugging, but they can  
> differ between UAs.  It seems as though all errors should be  
> diagnosable via the error codes.

More error codes are harder to justify. Nevertheless,  
NON_TRANSIENT_ERR in the case of transaction creation indicates bugs.

>
> 4) In 3.3.2, openCursor takes in an optional IDBKeyRange and then an  
> optional direction.  But what if you don't want a range but you do  
> want a particular direction?  Are implementations expected to handle  
> this by looking at whether the first parameter is a IDBKeyRange or  
> not?  Same goes for IDBIndexSync.

I am fine with whatever mechanism passes muster with IDL lawyers.

>
> 5) Similarly, put takes 2 optionals.  Depending on the object store  
> it may or may not make sense for there to be a key param.  I guess  
> the javascript bindings will need to have knowledge of whether a key  
> is expected and/or disallow boolean keys?  It'd probably be better  
> to avoid this from a bindings point of view.

Agreed. I am open to suggestions for improving this.

>
> 3.2.2.4 - why would you skip the next step?

Can you be more precise? There is no skipping in 3.2.2 step 4.

> 3.2.2.6 - should be preform one or the other, right?

Can you be more precise? There is no skipping in 3.2.2.6


> 3.2.2.6.1 - should be "if it has a key generator" right?

Can you be more precise? There is no skipping in 3.2.2.6.1

>
> 3.3.2 - if createObjectStore converts a null name to the empty  
> string, why woudln't openObjectStore, create/open index, and  
> removeObjectStore/removeIndex?

Perhaps it is best to remove [Null=Empty] annotations from 3.3.2  
completely. All the affected parameters are already non-optional.
Received on Saturday, 13 March 2010 09:04:23 GMT

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