[oslc-core] TRS truncation of change log

[Joe Ross]

Actually, I wasn't thinking straight....  The cutoffEvent is not at all
related to lastEvent property I'm suggesting. The lastEvent property is
needed whenever a client is polling periodically, so that it can know if it
is useful to poll the log again or get a new snapshot. Clients need to
remember the last event they processed, and that is what they need to
compare with the lastEvent property in each page of the log to ensure that
they don't need to get another base snapshot. The cutoffEvent property is
only used to when polling the change log the first time.

================================================
Joe Ross/Austin/IBM, joeross at us.ibm.com
Tivoli Autonomic Computing & Component Technologies
512-286-8311, T/L 363-8311



From:	Joe Ross/Austin/IBM
To:	oslc-core at open-services.net,
Date:	05/02/2013 01:08 PM
Subject:	TRS truncation of change log


The Tracked Resource Set specification, states the following:

   "To ensure that a new Client can always get started, the Change Log MUST
   contain the base cutoff event of the corresponding Base, and all Change
   Events more recent than it. Thus the Server is only allowed to truncate
   Change Events older than the base cutoff event. "

However, since processing of the change log would happen some time after
the base was read, it is possible that truncation happens between the time
that a client reads the base and the time that it starts processing the
change log. Truncation could also happen while a client is paging through
the change log. In that case, it would be useful for a client to know about
the truncation event before processing the entire change log, so that it
can switch gears and obtain a new base snapshot instead. It seems that it
might be useful if each page of the change log also included a lastEvent
property. If at some point during change log processing, lastEvent becomes
more recent than the cutoffEvent value that client knows, the client could
then abandon change log processing and obtain a new base snapshot instead.

Of course, we can add this property in our implementation, but seems it
might be useful as an addition to the spec.


================================================
Joe Ross/Austin/IBM, joeross at us.ibm.com
Tivoli Autonomic Computing & Component Technologies
512-286-8311, T/L 363-8311
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