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Re: Contacts API draft

From: Jonas Sicking <jonas@sicking.cc>
Date: Fri, 9 Nov 2012 12:41:36 -0800
Message-ID: <CA+c2ei_T4BbMqYRLcpmErcc=+iv0KNK2DTA3LimZLdqXUfnFcg@mail.gmail.com>
To: "Dumez, Christophe" <christophe.dumez@intel.com>, Tantek «elik <tantek@mozilla.com>
Cc: EDUARDO FULLEA CARRERA <efc@tid.es>, "public-sysapps@w3.org" <public-sysapps@w3.org>, Wayne Carr <wayne.carr@intel.com>, Sakari Poussa <sakari.poussa@intel.com>, JOSE MANUEL CANTERA FONSECA <jmcf@tid.es>
On Wed, Nov 7, 2012 at 1:45 AM, Dumez, Christophe
<christophe.dumez@intel.com> wrote:
> Hi Eduardo,
>
> Thanks for taking a look and highlighting the differences.
>
> On Wed, Nov 7, 2012 at 11:03 AM, EDUARDO FULLEA CARRERA <efc@tid.es> wrote:
>>
>> Hi Christophe, all,
>>
>>
>>
>> I have been comparing the two proposals on the table for Contacts API, and
>> they happen to have more similarities than differences what is good news in
>> order to reach a consolidated proposal. Let me comment on the main
>> differences:
>>
>>
>>
>> -Filtering: Intelís proposal allows applying virtually any type of filter
>> as it allows the definition of composite filters. Note though that in order
>> to have an appropriate performance the runtime will need to define indexes
>> for the different filtering operations. Having so flexible filtering implies
>> creating a virtually unlimited set of indexes. Christophe, what is the
>> performance you expect the implementation to have? and how do you propose to
>> solve that issue?
>
>
> Well, it is a good idea for the backend to define indexes to make the common
> filtering operations faster. I fail to see the relation between having a
> flexible filtering system and creating a virtually unlimited set of indexes.
> In my opinion, just because the backend cannot optimize for all possible
> queries does not mean that we should restrict which queries are supported by
> the API.

A concern with the current approach is that performance matters a
whole lot to developers.

For example when opening a particular part of an app which wants to
display a contact list to the user, the application will need to query
the contacts API and get the list of contacts before they can be
displayed. If the result takes too long to come back, the result can
easily be that the API simply doesn't fulfill the developers needs.
I.e. even if the API comes back with a "correct" result, if it takes
too long to get that result the API can be essentially non-functioning
from a developers point of view.

This is a very fuzzy area from a W3C API point if view. Most of the
specs that we've developed in the WebApps WG has been developed
without any forms of guarantees regarding performance. Though in
reality developers have come to expect certain performance
characteristics from the implementations anyway. I believe for example
that most implementations have made iterating a child-list of nodes a
O(n) operation both when iterated using Node.firstNode/.nextSibling or
when using Node.childNodes[n]. This requires quite a bit of
implementation complexity but it's something that all developers found
necessary to do.

This becomes especially important when designing database APIs, like
the contacts API. In a database, it's incredibly important if an
operation is O(n), O(log n) or O(1) since you can be dealing with many
more records than you normally deal with in a DOM.

This is especially important on mobile devices where CPU and IO
performance is often severely limited.

We ran into this recently in Firefox OS where some of our apps weren't
displaying UI to the user fast enough because the SMS and Contacts
APIs weren't returning results fast enough.

This was something that we had in mind when designing IndexedDB. It
was specifically designed such that all operations could be
implemented with predictable performance characteristics. I think this
is something that we need to continue doing for all APIs which are
essentially wrappers around databases, like Contacts API or SMS.

> If you consider SQL, you can query pretty much anything even if some of
> these queries may be slow. All we can do is define indexes in the database
> to optimize the most frequent/common queries.
> I think the same thing applies here. The addressbook is basically a layer on
> top of the database and the filtering system is merely a layer on top of
> whatever query language is being used by the backend (e.g. SQL).

SQL is a good example. While SQL does have the benefit that you can
easily get the result that you want, many times that's simply not good
enough for developers. People end up spending large amounts of effort
getting indexes and expressions to line up correctly such that the
execution engine in the database gives them the performance that they
want.

> Also note that if we don't provide flexible filtering then this means that
> the app will likely need to do post-filtering by itself on JavaScript side.
> I believe that even if the native backend is not optimized for all queries,
> those are still going to be faster compared to the alternative
> (post-filtering on JS side).

Why? Do you have data to back this up?

Javascript is really fast these days. Fast enough that we in Firefox
OS are moving towards implementing more and more of our APIs in
javascript. In fact, both our SMS API and our Contacts API is
implemented in javascript.

I would actually make the opposite argument. That if an implementation
can't implement a particular filter in such a way that it gets
inherent benefits, usually by using indexes, then I think its better
to rely on application logic written in javascript to accomplish the
same thing.

Providing more flexible filters as just a convenience function is
essentially just syntactic sugar. Syntactic sugar isn't inherently
bad, but is something that should be added with care, especially in
the first version of the API.

Another thing to keep in mind is that no matter how much syntax sugar
we add, we are unlikely to fully support all the things that people
need to do for even the most simple UIs. For example, the current API
doesn't seem to make it possible to render a list of letters which
there are user names that start with. Or make it possible to get the
total number of contacts as to render a scrollbar of the correct size.
Or implement a quick-search feature like "give me all the contacts
where one of the properties contain the string X" At least not without
doing very slow full table scans.

However, rather than just whining, let me make a counter proposal.

The first thing that I think we should do is to make it possible for
applications to keep their own database which is "caching" the
information that they need. To do this, we need to make it possible
for the app to find out about all modifications that are done to the
contacts database. I.e. it needs to be able to be informed about any
contact which is added, removed or modified.

There are many ways we can do this. A somewhat naive solution is to
have system-message-like callbacks for any time a modification is done
to the database. This callback could then provide information about
which contact was added/removed/modified and if modified, which fields
were changed. However the downside of this approach is that we'll be
constantly waking up applications in the background which will result
in a lot of battery usage.

A better solution might be to introduce the ability for an app to say
"give me a list of all changes that happened since the last time I
asked". That way the application can on first startup scan through the
contacts database and cache any information that it wants, it would
then say "please start recording changes for me". On next startup it
would simply get the list of changes since last run and update its
cache based on that. The API implementation would then queue any
modifications that are made and only drop those modifications once all
applications that have said "please record" has received the record.

To make it more concrete, it could look something like this:

interface ContactsManager {
  ...
  startTrackingChanges();
  ContactRequest getNewChanges();
  attribute EventHandler onchangesavailable;
  stopTrackingChanges(); // Not strictly needed, but seems like a good idea
  ...
};

We'd have to have some way of dealing with the situation that we end
up with a lot of changes getting queued up and the system wanting to
dump them. An easy solution would be to all getNewChanges to return
something indicating "i've lost track of your changes, please do a
full rescan". That is very likely quite good enough since it'll only
happen for apps that aren't run very often.

Another solution is to introduce a way for the system to send a system
message indicating "i'm overflowing with changes, please request the
change-list". This can be done in a backwards compatible way, so
something we can hold off on for now if we want to keep things simple.

Once we have something like this, and once we are fine with relying on
application-side javascript code to do filters that aren't using
special indexes, I think that greatly reduces the pressure to add a
ton of features to the API for advanced filters. It doesn't mean that
we shouldn't have *any* filters, but it means that we can get away
with much simpler ones without affecting the type of applications
people can build.

>> -ContactChangeEvent: Intel's proposal allows notifying changes in multiple
>> contacts simultaneously indicating which contacts have been deleted, created
>> and modified. Is the system expected to wait for some time before firing an
>> event with multiple changes or do these notifications of multiple changes
>> apply just to really simultaneous changes (e.g. sync with an external
>> address book).
>
> We were thinking about database transactions in the backend. For those, I
> think it is a good idea to return all the changes in the same event, this is
> better for performance that firing one event per change.
> The idea is also to have a generic ContactChange event which handles all
> possible Addressbook changes in one object.

I'm not sure I understand the answer here. The question is: In what
situations would the implementation be able to deliver a list of
changes, rather than deliver the changes individually?

It seems like your answer is that you'd deliver a list rather than
individual changes whenever you have a transaction in the backend that
changes multiple things at once. However then the question simply
becomes: In what situations would the implementation have a
transaction which change multiple things at once?

I.e. can this happen when another app is using the contacts API to
modify the database? Or can this only happen if someone is modifying
the database through means other than the contacts API?

I'm not necessarily opposed the ability to notify about several
changes at once, but I'd like to understand better how it works.

>> -getSimContacts(): Our proposal included this method to be able to
>> retrieve the contacts from the SIM card (e.g. to copy them to the device
>> address book). It is not the intention to deal in any other way with the
>> SIMís address book, so neither the rest of the methods (find, remove, etc)
>> nor the ContactChangeEvent apply in principle to SIM contacts.
>
> We removed this method because it seemed too specific. The device may not
> have a SIM card and I did not understand why we needed to handle SIM
> contacts any differently at API level.
> I don't have strong feeling about this one but I thought that the
> Contact.readonly flag would allow us to achieve the same thing (Contacts can
> be retrieved from SIM using find() but cannot be removed or saved because
> they are marked as 'readOnly').

We need this function *somewhere*. But I'm fine with it living
somewhere other than in the Contacts API. Especially for now.

>> -read-only contacts in Intelís proposal: Christophe, could you elaborate
>> what is the use case for that?
>
> In some cases, the backend is not able to edit specific contacts. It can be
> useful for SIM contacts for example. In case we are only able to read those,
> not edit them.
> It can also be useful for contacts coming from online services which may not
> support editing.

As a user, this seems extremely annoying. If something shows up in my
address book I'd expect to be able to modify it. I suspect that if we
add this, people will simply write apps that if a contact is read-only
the app simply copies the information over to a new, editable, contact
and deletes the read-only one.

I don't think the API as written could work with a sim-card backend
anyway so I'm not really worried about that use-case.

>> -Additional fields in Intel's proposal: relationship, phoneticGivenName;
>> phoneticFamilyName; favorite; ringtone
>
> Yes, we introduced those attributes because we had them in Tizen and they
> are common vCard extensions. They also have practical use cases for the
> AddressBook application.
>
>>
>> -Additional fields in our proposal: genderIdentity; carrier attribute in
>> tel field
>
> Yes,
> - GenderIdentity is part of vCard 4.0 but we could not find much use for it
> (we already have "gender" attribute).
> - carrier is not part of the vCard standard AFAIK and seemed very specific.
> we were not sure this should be part of the standard API. The application
> could easily handle this by itself if needed.

I'll defer to Tantek for contact field questions.

FWIW, the ringtone field seems just as application specific to me as
the carrier field.

This is also another argument for introducing the ability for
applications to keep their own app-local cache. That way they can in
that cache add any fields that they need, without having to convince
us or the vCard community that their field is important enough that it
should be added to the worlds contact formats.

/ Jonas
Received on Friday, 9 November 2012 20:42:37 GMT

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