[Kde-pim] Akonadi batch notifications review

[Daniel Vrátil]

Heya,

back in Berlin I talked with Volker about implementing batch notifications in 
Akonadi. The main idea is that some resources might perform better when they 
are given the entire batch of items instead of serving the items one by one. 
Good example is the IMAP resource. Instead of having to SELECT folder and 
STORE flags for every item separately, it would be better if it could SELECT 
the folder once and then STORE the new flags for the whole set of items - 
making "Mark All As Read" a single operation (also making it faster and saving 
potentially a lot of bandwidth).

The code is available in batch-notifications branch in Akonadi and akonadi-
batch-notifications branch in kdepimlibs. Port of the IMAP resource to batch 
notifications is in akonadi-batch-notifications branch in kdepim-runtime. I 
will port additional agents and resources (where it makes sense), but I want 
to make sure every one is happy with the API changes.

The code is backwards compatible, so you can run Akonadi server and kdepimlibs 
with batch-notifications, but kdepim-runtime from master without any problems.

I've been running the batch-notification branches on my production setup for 
over a week now without any troubles, so I think it's time to get it reviewed 
:-)

A bit of elaboration on the changes I've done.

Akonadi Server
Akonadi Server now uses and emits NotificationMessageV2. Unlike the "V1", it 
can represent a single operation on multiple entities. Akonadi server emits 
the NotificationMessageV2 via notifyV2() signal, and when there's at least one 
subscriber that does not support the new version (support for V2 is announced 
via client capabilities), it splits the V2 notification into multiple V1 
notifications and emits them "the old way", via notify(). 
A completely new operation called "ModifyFlags" has been added. For this 
operation, the NotificationMessageV2 has two additional attributes - 
addedFlags and removedFlags. When Akonadi::AgentBase::ObserverV3 is used 
itemModified() will no longer notify about flag changes (see below).


Akonadi::Monitor
The set of signals that Monitor can emit has been expanded by itemsMoved, 
itemsRemoved, itemsLinked, itemsUnlinked and itemsFlagsChanged.  Arguably,  
batch operations for collections could be added, but I don't see any gain here 
because such use case is extremely rare, if any.
Before emitting the signal, Monitor checks whether there's anyone listening to 
the old single-item signals (itemMoved, etc) and in such case, it splits the 
notification into many per-item notifications and emits them the old way. If 
there someone listening to the batch signals as well, the original batch 
notification is emitted too.

Akonadi::AgentBase::ObserverV3
A new ObserverV3 has been added, with virtual method for every new batch 
operation. Very important thing here is that if you use ObserverV3, you will 
never be delivered the non-batch versions of notification from V1 or V2. So 
even when ObserverV3::itemsRemoved() is not reimplemented in an 
agent/resource, ObserverV2::itemRemoved() will simply not be called. This is 
because of design limitation, that would require the Observer telling back to 
Monitor to re-queue, split and re-emit the batch notification as a set of per-
item notifications. 

IMAP resource
IMAP resource has been updated to support itemsMoved and itemsRemoved and 
itemsFlagsChanged.


Thanks

Dan

-- 
Daniel Vrátil
Associate Software Engineer, KDE Desktop Team
Red Hat, Inc

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