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From Tim Bain <tb...@alumni.duke.edu>
Subject mKahaDB and durable subscriptions
Date Fri, 25 Dec 2015 15:20:18 GMT
Is it currently possible to configure multi-KahaDB such that all offline
durable subscriptions (for all destinations) go into one KahaDB instance
while all other messages go into a second instance?

Durable subscription messages seem to break the assumption implicit in
KahaDB's design decision to never compact data files, which is that as long
as consumers are keeping up, all messages in a journal file will quickly
become unneeded and the file can always be deleted in lieu of compacting
it.  And as has been discussed previously on this mailing list, it's
possible for a single old data file to keep alive every data file after it
if message-ack pairs span the data files, so a single durable subscription
can theoretically prevent KahaDB from ever deleting another data file
(which would be a bug, since correct operation of KahaDB is for data files
to be deleted when they contain no unconsumed messages).

Being able to push them into a separate KahaDB instance would make the
assumption valid for the remaining messages in the store, but if that's not
currently possible then one of the two features (mKahaDB for durable
subscriptions, or compaction) needs to be implemented in 5.14.0.

Tim

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