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From Michael Dürig <mdue...@apache.org>
Subject Re: Observation
Date Mon, 11 Nov 2013 11:10:47 GMT

On 8.11.13 10:37 , Alexander Klimetschek wrote:
> Regarding journaled observation: retrieving a journal with
> ObservationManager#getEventJournal can allow access to events that
> happend earlier, right?

The spec. doesn't say much here. I'd interpret it as getEventJournal() 
returns the journal from the point where that call was made rather then 
from the beginning of times.

So the goal of supporting filtering as early
> as possible wouldn't quite work here, as you still have to keep
> access to all events for anyone asking for them later. OTOH, the spec
> seems to leave journal implementations a lot of freedom on how many
> events are still accessible at all (which makes me wonder how much
> you can make application code rely on it) [0].

The same actually holds true for asynchronous observation: 

> I assume due to the MVCC principle in oak it is easy to implement the
> journal and it is cheap to "keep all events". How far can you go
> back? I assume that depends on the implementation, e.g. when the last
> tar optimization happened?

Right, just diff old revisions as needed. The retention time for old 
revisions is currently an implementation detail. But see OAK-114 for an 
ongoing effort on this.


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