[Development] Qt PIM object ids: the price of being generic

[Robin Burchell]

Hi Cristiano (& other interested PIM folks),

Some months ago now, I raised the problem that QString wasn't an
optimal data format for QContactLocalId in Qt 5
(http://lists.qt-project.org/pipermail/development/2011-November/000164.html).
There was some consensus at the time, but things sort of tapered off
until the recent change implementing one of the planned solutions:
doing away with QContactLocalId altogether, and having contact IDs be
engine-defined (http://codereview.qt-project.org/#change,13060).

This in itself is a pretty sensible change, I mostly agree with it,
and it matches what the Organizer API does, which is probably a Good
Thing. There is still, however, the problem of using QString in unique
identifiers for contacts (and for organizer items, presuming they use
the exact same approach), and I do think that this is probably a
mistake. After all, you really don't need unicode specifiers for what
is essentially opaque data only of importance to the engine.
QByteArray would probably do just as well.

Using a blob of data does give you somewhat more 'portability' between
backends, however, you may want to consider directly store a QUuid.
Most contacts backends I have seen often have some form of unique
identifier involved, and for those that don't, it wouldn't be
impossible to provide a map of native engine ID to QUuid, similar to
what was done with native engine ID <-> QContactLocalId (uint) back in
Qt 4 times.

I did some (admittedly overly simplistic, I was in a hurry)
benchmarking of QString vs QByteArray vs QUuid, both memory overhead
and performance (as in, time taken to copy). Code can be found at
http://qtl.me/nBG.txt.

Here's the results:

tst_QtContacts_bench::tst_QtContacts_bench() Memory overhead of
QList<1000 string UUIDs>:  108104
tst_QtContacts_bench::tst_QtContacts_bench() Memory overhead of
QList<1000 bytearray UUIDs>:  68152
tst_QtContacts_bench::tst_QtContacts_bench() Memory overhead of
QList<1000 RFC form UUIDs>:  44136
tst_QtContacts_bench::tst_QtContacts_bench() Memory overhead of
QList<1000 QUuid>:  28768

********* Start testing of tst_QtContacts_bench *********
Config: Using QTest library 5.0.0, Qt 5.0.0
PASS   : tst_QtContacts_bench::initTestCase()
RESULT : tst_QtContacts_bench::benchString():
     0.00064 msecs per iteration (total: 85, iterations: 131072)
PASS   : tst_QtContacts_bench::benchString()
RESULT : tst_QtContacts_bench::benchBytearray():
     0.00060 msecs per iteration (total: 79, iterations: 131072)
PASS   : tst_QtContacts_bench::benchBytearray()
RESULT : tst_QtContacts_bench::benchToRfc4122():
     0.00033 msecs per iteration (total: 87, iterations: 262144)
PASS   : tst_QtContacts_bench::benchToRfc4122()
RESULT : tst_QtContacts_bench::benchRawUuid():
     0.000091 msecs per iteration (total: 96, iterations: 1048576)
PASS   : tst_QtContacts_bench::benchRawUuid()
PASS   : tst_QtContacts_bench::cleanupTestCase()
Totals: 6 passed, 0 failed, 0 skipped
********* Finished testing of tst_QtContacts_bench *********

This demonstrates that a list of QUuid has around a 70% decrease from
using QString in required memory usage when storing lists of contacts
IDs (not at all unusual for things like this), and would also be
roughly an order of magnitude faster to copy individual IDs around.

I hope that this helps demonstrate the point I'm trying to make for
evaluating future work on PIM.

BR,

Robin