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From Christopher Shannon <christopher.l.shan...@gmail.com>
Subject Re: mKahaDB and durable subscriptions
Date Mon, 28 Dec 2015 17:34:14 GMT
Multi-KahaDB already separates destinations (if configured that way) into
their own KahaDB store, so I'm not really sure what you mean by separate
durable subscriptions.  If a topic has a bunch of offline durables and that
topic is configured to have its own KahaDB instance then it would not
affect other destinations and their messages and those other destinations
would continue to GC properly.

On Fri, Dec 25, 2015 at 10:20 AM, Tim Bain <tbain@alumni.duke.edu> wrote:

> 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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