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From Erick Erickson <erickerick...@gmail.com>
Subject Re: Soft commit and new replica types
Date Mon, 10 Dec 2018 16:40:39 GMT
bq. but not every poll attempt they fetch new segment from the leader

Ah, right. Ignore my comment. Commit will only occur on the followers
when there are new segments to pull down, so your'e right, roughly
every second poll would commit find things to bring down and open a
new searcher.........
On Sun, Dec 9, 2018 at 4:14 PM Edward Ribeiro <edward.ribeiro@gmail.com> wrote:
>
> Hi Vadim,
>
> There is no commit on TLOG/PULL  follower replicas, only on the leader.
> Followers fetch the segments and **reload the core** every 150 seconds (if
> there were new segments, I suppose). Yeah, followers don't pay the CPU
> price of indexing, but there are still cache invalidation, autowarming,
> etc, in addition to network and IO demand. Is that ritht, Erick?
>
> Besides that, Erick is pointing out that under a heavy indexing workload
> you could either have:
>
> 1. Very large transaction logs;
>
> 2. Very large numbers of segments. If that is the case, you could have the
> following scenario numerous times:
>    2.1. follower replica downloads segment A and B from leader;
>    2.2 leader merges segments A + B into C;
>    2.3. follower replicas discard A and B and download C on next poll;
>
> Under the second condition followers needlessly downloaded segments that
> would eventually be merged.
>
> IMO, you should carefully evaluate if the use of TLOG/PULL is really
> recommended for your cluster setup, plus indexing and querying workload.
> You can very much stay with a NRT setup if it suits you better. The videos
> below provide a nice set of hints for when to choose between NRT or some
> combination of TLOG and PULL.
>
> https://youtu.be/XIb8X3MwVKc
>
> https://youtu.be/dkWy2ykzAv0
>
> https://youtu.be/XqfTjd9KDWU
>
> Regards,
> Edward
>
> Em dom, 9 de dez de 2018 16:56, <vadim.ivanov@spb.ntk-intourist.ru escreveu:
>
> >
> >  If hard commit max time is 300 sec then commit happens every 300 sec on
> > tlog leader. And new segments pop up on the leader every 300 sec, during
> > indexing. Polling interval on other replicas 150 sec, but not every poll
> > attempt they fetch new segment from the leader, afaiu. Erick, do you mean
> > that on all other  tlog replicas(not leaders) commit occurs every poll?
> > воскресенье, 09 декабря 2018г., 19:21 +03:00 от Erick Erickson
> > erickerickson@gmail.com :
> >
> > >Not quite, 600000. The polling interval is half the commit interval....
> > >
> > >This has always bothered me a little bit, I wonder at the utility of a
> > >config param. We already have old-style replication with a
> > >configurable polling interval. Under very heavy indexing loads, it
> > >seems to me that either the tlogs will grow quite large or we'll be
> > >pulling a lot of unnecessary segments across the wire, segments
> > >that'll soon be merged away and the merged segment re-pulled.
> > >
> > >Apparently, though, nobody's seen this "in the wild", so it's
> > >theoretical at this point.
> > >On Sun, Dec 9, 2018 at 1:48 AM Vadim Ivanov
> > < vadim.ivanov@spb.ntk-intourist.ru> wrote:
> > >
> > > Thanks, Edward, for clues.
> > > What bothers me is newSearcher start, warming, cache clear... all that
> > CPU consuming stuff in my heavy-indexing scenario.
> > > With NRT I had autoSoftCommit:  300000 .
> > > So I had new Searcher no more than  every 5 min on every replica.
> > > To have more or less  the same effect with TLOG - PULL collection,
> > > I suppose, I have to have  :  300000
> > > (yes, I understand that newSearchers start asynchronously on leader and
> > replicas)
> > > Am I right?
> > > --
> > > Vadim
> > >
> > >
> > >> -----Original Message-----
> > >> From: Edward Ribeiro [mailto:edward.ribeiro@gmail.com]
> > >> Sent: Sunday, December 09, 2018 12:42 AM
> > >> To:  solr-user@lucene.apache.org
> > >> Subject: Re: Soft commit and new replica types
> > >>
> > >> Some insights in the new replica types below:
> > >>
> > >> On Sat, December 8, 2018 08:42, Vadim Ivanov <
> > >> vadim.ivanov@spb.ntk-intourist.ru wrote:
> > >>
> > >>>
> > >>> From Ref guide we have:
> > >>> " NRT is the only type of replica that supports soft-commits..."
> > >>> "If TLOG replica does become a leader, it will behave the same as if
it
> > >>> was a NRT type of replica."
> > >>> Does it mean, that if we do not have NRT replicas in the cluster then
> > >>> autoSoftCommit section in solconfig.xml Ignored completely (even on
> > TLOG
> > >>> leader)?
> > >>>
> > >>
> > >> No, not completely. Both TLOG and PULL nodes will periodically poll the
> > >> leader for changes in index segments' files and download those segments
> > >> from the leader. If hard commit max time is defined in solrconfig.xml
> > the
> > >> polling interval of each replica will be half that value. Or else if the
> > >> soft commit max time is defined then the replicas will use half the soft
> > >> commit max time as the interval. If neither are defined then the poll
> > >> interval will be 3 seconds (hard coded). See here:
> > >> https://github.com/apache/lucene-
> > >> solr/blob/75b183196798232aa6f2dcaaaab117f309119053/solr/core/src/java/o
> > >> rg/apache/solr/cloud/ReplicateFromLeader.java#L68-L77
> > >>
> > >> If the TLOG is the leader it will index locally and append the doc to
> > >> transaction log as a NRT node would do as well as it will synchronously
> > >> replicate the data to other TLOG replicas' transaction logs (PULL nodes
> > >> don't have transaction logs). But TLOG/PULL replicas doesn't support
> > soft
> > >> commits nor real time gets, afaik.
> > >>
> > >>>
> > >>
> > >>>
> > >>> 60000
> > >>>
> > >>>
> > >>> Should we say that in autoCommit section openSearcher is always true
in
> > >>> that case?
> > >>
> > >>
> > >>
> > >> 10000
> > >> 30000
> > >> 512m
> > >> false
> > >>
> > >>
> > >> Does it mean that new Searcher always starts on all replicas when hard
> > >> commit happens on leader?
> > >>
> > >>
> > >> Nope. Or at least, the searcher is not synchronously created. Each non
> > >> leader replica will periodically fetch the index changes from the leader
> > >> and open a new searcher to reflect those changes as seen here:
> > >> https://github.com/apache/lucene-
> > >> solr/blob/75b183196798232aa6f2dcaaaab117f309119053/solr/core/src/java/o
> > >> rg/apache/solr/handler/IndexFetcher.java#L653
> > >> But it's important to note that the potential delay between the leader's
> > >> hard commit and the other replicas fetching those changes from the
> > leader
> > >> and opening a new searcher to reflect latest changes.
> > >>
> > >> PS: I am still digging these new replica types so I can have
> > misunderstood
> > >> or missed some aspect of it.
> > >>
> > >> Regards,
> > >> Edward
> > >
> >

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