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From Erick Erickson <erickerick...@gmail.com>
Subject Re: Realtime get not always returning existing data
Date Wed, 10 Oct 2018 14:47:38 GMT
Well assigning a bogus version that generates a 409 error then
immediately doing an RTG on the doc doesn't fail for me either 18
million tries later. So I'm afraid I haven't a clue where to go from
here. Unless we can somehow find a way to generate this failure I'm
going to drop it for the foreseeable future.

Erick
On Tue, Oct 9, 2018 at 7:39 AM Erick Erickson <erickerickson@gmail.com> wrote:
>
> Hmmmm. I wonder if a version conflict or perhaps other failure can
> somehow cause this. It shouldn't be very hard to add that to my test
> setup, just randomly add n _version_ field value.
>
> Erick
> On Mon, Oct 1, 2018 at 8:20 AM Erick Erickson <erickerickson@gmail.com> wrote:
> >
> > Thanks. I'll be away for the rest of the week, so won't be able to try
> > anything more....
> > On Mon, Oct 1, 2018 at 5:10 AM Chris Ulicny <culicny@iq.media> wrote:
> > >
> > > In our case, we are heavily indexing in the collection while the /get
> > > requests are happening which is what we assumed was causing this very rare
> > > behavior. However, we have experienced the problem for a collection where
> > > the following happens in sequence with minutes in between them.
> > >
> > > 1. Document id=1 is indexed
> > > 2. Document successfully retrieved with /get?id=1
> > > 3. Document failed to be retrieved with /get?id=1
> > > 4. Document successfully retrieved with /get?id=1
> > >
> > > We've haven't looked at the issue in a while, so I don't have the exact
> > > timing of that sequence on hand right now. I'll try to find an actual
> > > example, although I'm relatively certain it was multiple minutes in between
> > > each of those requests. However our autocommit (and soft commit) times are
> > > 60s for both collections.
> > >
> > > I think the following two are probably the biggest differences for our
> > > setup, besides the version difference (v6.3.0):
> > >
> > > > index to this collection, perhaps not at a high rate
> > > > separate the machines running solr from the one doing any querying or
> > > indexing
> > >
> > > The clients are on 3 hosts separate from the solr instances. The total
> > > number of threads that are making updates and making /get requests is
> > > around 120-150. About 40-50 per host. Each of our two collections gets an
> > > average of 500 requests per second constantly for ~5 minutes, and then the
> > > number slowly tapers off to essentially 0 after ~15 minutes.
> > >
> > > Every thread attempts to make the same series of requests.
> > >
> > > -- Update with "_version_=-1". If successful, no other requests are made.
> > > -- On 409 Conflict failure, it makes a /get request for the id
> > > -- On doc:null failure, the client handles the error and moves on
> > >
> > > Combining this with the previous series of /get requests, we end up with
> > > situations where an update fails as expected, but the subsequent /get
> > > request fails to retrieve the existing document:
> > >
> > > 1. Thread 1 updates id=1 successfully
> > > 2. Thread 2 tries to update id=1, fails (409)
> > > 3. Thread 2 tries to get id=1 succeeds.
> > >
> > > ...Minutes later...
> > >
> > > 4. Thread 3 tries to update id=1, fails (409)
> > > 5. Thread 3 tries to get id=1, fails (doc:null)
> > >
> > > ...Minutes later...
> > >
> > > 6. Thread 4 tries to update id=1, fails (409)
> > > 7. Thread 4 tries to get id=1 succeeds.
> > >
> > > As Steven mentioned, it happens very, very rarely. We tried to recreate it
> > > in a more controlled environment, but ran into the same issue that you are,
> > > Erick. Every simplified situation we ran produced no problems. Since it's
> > > not a large issue for us and happens very rarely, we stopped trying to
> > > recreate it.
> > >
> > >
> > > On Sun, Sep 30, 2018 at 9:16 PM Erick Erickson <erickerickson@gmail.com>
> > > wrote:
> > >
> > > > 57 million queries later, with constant indexing going on and 9 dummy
> > > > collections in the mix and the main collection I'm querying having 2
> > > > shards, 2 replicas each, I have no errors.
> > > >
> > > > So unless the code doesn't look like it exercises any similar path,
> > > > I'm not sure what more I can test. "It works on my machine" ;)
> > > >
> > > > Here's my querying code, does it look like it what you're seeing?
> > > >
> > > >       while (Main.allStop.get() == false) {
> > > >         try (SolrClient client = new HttpSolrClient.Builder()
> > > > //("http://my-solr-server:8981/solr/eoe_shard1_replica_n4")) {
> > > >             .withBaseSolrUrl("http://localhost:8981/solr/eoe").build())
{
> > > >
> > > >           //SolrQuery query = new SolrQuery();
> > > >           String lower = Integer.toString(rand.nextInt(1_000_000));
> > > >           SolrDocument rsp = client.getById(lower);
> > > >           if (rsp == null) {
> > > >             System.out.println("Got a null response!");
> > > >             Main.allStop.set(true);
> > > >           }
> > > >
> > > >           rsp = client.getById(lower);
> > > >
> > > >           if (rsp.get("id").equals(lower) == false) {
> > > >             System.out.println("Got an invalid response, looking for "
> > > > + lower + " got: " + rsp.get("id"));
> > > >             Main.allStop.set(true);
> > > >           }
> > > >           long queries = Main.eoeCounter.incrementAndGet();
> > > >           if ((queries % 100_000) == 0) {
> > > >             long seconds = (System.currentTimeMillis() - Main.start) /
> > > > 1000;
> > > >             System.out.println("Query count: " +
> > > > numFormatter.format(queries) + ", rate is " +
> > > > numFormatter.format(queries / seconds) + " QPS");
> > > >           }
> > > >         } catch (Exception cle) {
> > > >           cle.printStackTrace();
> > > >           Main.allStop.set(true);
> > > >         }
> > > >       }
> > > >   }On Sat, Sep 29, 2018 at 12:46 PM Erick Erickson
> > > > <erickerickson@gmail.com> wrote:
> > > > >
> > > > > Steve:
> > > > >
> > > > > bq.  Basically, one core had data in it that should belong to another
> > > > > core. Here's my question about this: Is it possible that two request
to
> > > > the
> > > > > /get API coming in at the same time would get confused and either
both
> > > > get
> > > > > the same result or result get inverted?
> > > > >
> > > > > Well, that shouldn't be happening, these are all supposed to be
> > > > thread-safe
> > > > > calls.... All things are possible of course ;)
> > > > >
> > > > > If two replicas of the same shard have different documents, that
could
> > > > account
> > > > > for what you're seeing, meanwhile begging the question of why that
is
> > > > the case
> > > > > since it should never be true for a quiescent index. Technically
there
> > > > _are_
> > > > > conditions where this is true on a very temporary basis, commits
on the
> > > > leader
> > > > > and follower can trigger at different wall-clock times. Say your
soft
> > > > commit
> > > > > (or hard-commit-with-opensearcher-true) is 10 seconds. It should
never
> > > > be the
> > > > > case that s1r1 and s1r2 are out of sync 10 seconds after the last
update
> > > > was
> > > > > sent. This doesn't seem likely from what you've described though...
> > > > >
> > > > > Hmmmm. I guess that one other thing I can set up is to have a bunch
of
> > > > dummy
> > > > > collections laying around. Currently I have only the active one,
and
> > > > > if there's some
> > > > > code path whereby the RTG request goes to a replica of a different
> > > > > collection, my
> > > > > test setup wouldn't reproduce it.
> > > > >
> > > > > Currently, I'm running a 2-shard, 1 replica setup, so if there's
some
> > > > > way that the replicas
> > > > > get out of sync that wouldn't show either.
> > > > >
> > > > > So I'm starting another run with these changes:
> > > > > > opening a new connection each query
> > > > > > switched so the collection I'm querying is 2x2
> > > > > > added some dummy collections that are empty
> > > > >
> > > > > One nit, while "core" is exactly correct. When we talk about a core
> > > > > that's part of a collection, we try to use "replica" to be clear
we're
> > > > > talking about
> > > > > a core with some added characteristics, i.e. we're in SolrCloud-land.
> > > > > No big deal
> > > > > of course....
> > > > >
> > > > > Best,
> > > > > Erick
> > > > > On Sat, Sep 29, 2018 at 8:28 AM Shawn Heisey <apache@elyograg.org>
> > > > wrote:
> > > > > >
> > > > > > On 9/28/2018 8:11 PM, sgaron cse wrote:
> > > > > > > @Shawn
> > > > > > > We're running two instance on one machine for two reason:
> > > > > > > 1. The box has plenty of resources (48 cores / 256GB ram)
and since
> > > > I was
> > > > > > > reading that it's not recommended to use more than 31GB
of heap in
> > > > SOLR we
> > > > > > > figured 96 GB for keeping index data in OS cache + 31 GB
of heap per
> > > > > > > instance was a good idea.
> > > > > >
> > > > > > Do you know that these Solr instances actually DO need 31 GB
of heap,
> > > > or
> > > > > > are you following advice from somewhere, saying "use one quarter
of
> > > > your
> > > > > > memory as the heap size"?  That advice is not in the Solr
> > > > documentation,
> > > > > > and never will be.  Figuring out the right heap size requires
> > > > > > experimentation.
> > > > > >
> > > > > >
> > > > https://wiki.apache.org/solr/SolrPerformanceProblems#How_much_heap_space_do_I_need.3F
> > > > > >
> > > > > > How big (on disk) are each of these nine cores, and how many
documents
> > > > > > are in each one?  Which of them is in each Solr instance?  With
that
> > > > > > information, we can make a *guess* about how big your heap should
be.
> > > > > > Figuring out whether the guess is correct generally requires
careful
> > > > > > analysis of a GC log.
> > > > > >
> > > > > > > 2. We're in testing phase so we wanted a SOLR cloud configuration,
> > > > we will
> > > > > > > most likely have a much bigger deployment once going to
production.
> > > > In prod
> > > > > > > right now, we currently to run a six machines Riak cluster.
Riak is a
> > > > > > > key/value document store an has SOLR built-in for search,
but we are
> > > > trying
> > > > > > > to push the key/value aspect of Riak inside SOLR. That
way we would
> > > > have
> > > > > > > one less piece to worry about in our system.
> > > > > >
> > > > > > Solr is not a database.  It is not intended to be a data repository.
> > > > > > All of its optimizations (most of which are actually in Lucene)
are
> > > > > > geared towards search.  While technically it can be a key-value
store,
> > > > > > that is not what it was MADE for.  Software actually designed
for that
> > > > > > role is going to be much better than Solr as a key-value store.
> > > > > >
> > > > > > > When I say null document, I mean the /get API returns:
{doc: null}
> > > > > > >
> > > > > > > The problem is definitely not always there. We also have
large
> > > > period of
> > > > > > > time (few hours) were we have no problems. I'm just extremely
> > > > hesitant on
> > > > > > > retrying when I get a null document because in some case,
getting a
> > > > null
> > > > > > > document is a valid outcome. Our caching layer heavily
rely on this
> > > > for
> > > > > > > example. If I was to retry every nulls I'd pay a big penalty
in
> > > > > > > performance.
> > > > > >
> > > > > > I've just done a little test with the 7.5.0 techproducts example.
 It
> > > > > > looks like returning doc:null actually is how the RTG handler
says it
> > > > > > didn't find the document.  This seems very wrong to me, but
I didn't
> > > > > > design it, and that response needs SOME kind of format.
> > > > > >
> > > > > > Have you done any testing to see whether the standard searching
handler
> > > > > > (typically /select, but many other URL paths are possible) returns
> > > > > > results when RTG doesn't?  Do you know for these failures whether
the
> > > > > > document has been committed or not?
> > > > > >
> > > > > > > As for your last comment, part of our testing phase is
also testing
> > > > the
> > > > > > > limits. Our framework has auto-scaling built-in so if we
have a
> > > > burst of
> > > > > > > request, the system will automatically spin up more clients.
We're
> > > > pushing
> > > > > > > 10% of our production system to that Test server to see
how it will
> > > > handle
> > > > > > > it.
> > > > > >
> > > > > > To spin up another replica, Solr must copy all its index data
from the
> > > > > > leader replica.  Not only can this take a long time if the index
is
> > > > big,
> > > > > > but it will put a lot of extra I/O load on the machine(s) with
the
> > > > > > leader roles.  So performance will actually be WORSE before
it gets
> > > > > > better when you spin up another replica, and if the index is
big, that
> > > > > > condition will persist for quite a while.  Copying the index
data will
> > > > > > be constrained by the speed of your network and by the speed
of your
> > > > > > disks.  Often the disks are slower than the network, but that
is not
> > > > > > always the case.
> > > > > >
> > > > > > Thanks,
> > > > > > Shawn
> > > > > >
> > > >

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