Unstable search results - possible causes

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Unstable search results - possible causes

Hogan, Michael C
I'm using the built-in Mediawiki search engine. We just updated from 1.30.0 to 1.31.0. Since the update, search results are unstable. The same search term gives different results in different web browsers. We also see different results across browser sessions. Any advice on how I can troubleshoot this?

Thank you!

-Michael

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Re: Unstable search results - possible causes

Stas Malyshev
Hi!

> I'm using the built-in Mediawiki search engine. We just updated from 1.30.0 to 1.31.0. Since the update, search results are unstable. The same search term gives different results in different web browsers. We also see different results across browser sessions. Any advice on how I can troubleshoot this?

Could you provide more info - which search you're talking about (URL
might help, or description of the sequence of actions), which terms,
what you're getting in different cases? Some search results might depend
on your preferences or user language selected, so there might be
difference, e.g. for different logged-in users.

Thanks,
--
Stas Malyshev
[hidden email]

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Re: Unstable search results - possible causes

Hogan, Michael C
I'm running a self-hosted wiki, so unfortunately can't provide a URL. The search is performed on the standard [[Special:Search]] page. It may matter that we're running the wiki on two servers, using Percona MySQL for data replication between the servers. A load balancer sends users to whichever server has the fewest active connections.

-----Original Message-----
From: Stas Malyshev [mailto:[hidden email]]
Sent: Tuesday, March 05, 2019 5:39 PM
To: Wikimedia developers <[hidden email]>; Hogan (US), Michael C <[hidden email]>
Subject: Re: [Wikitech-l] Unstable search results - possible causes

Hi!

> I'm using the built-in Mediawiki search engine. We just updated from 1.30.0 to 1.31.0. Since the update, search results are unstable. The same search term gives different results in different web browsers. We also see different results across browser sessions. Any advice on how I can troubleshoot this?

Could you provide more info - which search you're talking about (URL might help, or description of the sequence of actions), which terms, what you're getting in different cases? Some search results might depend on your preferences or user language selected, so there might be difference, e.g. for different logged-in users.

Thanks,
--
Stas Malyshev
[hidden email]
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Re: Unstable search results - possible causes

Mark A. Hershberger-2
"Hogan (US), Michael C" <[hidden email]> writes:

> It may matter that we're running the wiki on two servers, using
> Percona MySQL for data replication between the servers.

I'm guessing that this is the cause.

If you're using the XtraDB Cluster, you have probably set PXC Strict
Mode to permissive or disabled it and, as a result, there is some
inconsistency between the two databases.

Mark.

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Re: Unstable search results - possible causes

Jaime Crespo
In reply to this post by Hogan, Michael C
> A load balancer sends users to whichever server has the fewest active connections.

That could be it- I don't think there has been a lot of focus on full
read-read stability on Mediawiki, so some stickiness may be needed for
some queries. Things that worry about that is: potential non
deterministic queries, which can return results differently depending
on the physical distribution of the rows. Those are mistakes and are
corrected when detected (I am trying to force strict sql mode into our
CI), but because not many people run mediawiki in large clusters AND
use the internal search engine it could have been missed. To verify it
is that- without any writes, rebuild the tables involved and they
would return results in different order.

Please share details about versions, url and queries, or contact us
privately in Phabricator if you cannot share those details publicly.

Percona Cluster/Galera is not officially supported as far as I know,
and while it should work with no problem because it has the same sql
interface, there are no specific controls for the distributed nature
of such cluster (unlike regular replication, which has specific
functionality). Enabling the chronology protector with heartbeat may
work for PXC, but I have not tested it. Another option is to use PXC
in a failover-only mode, being sticky to a primary server and leaving
the other nodes for failover, backups, etc.

On Wed, Mar 6, 2019 at 3:05 AM Hogan (US), Michael C
<[hidden email]> wrote:

>
> I'm running a self-hosted wiki, so unfortunately can't provide a URL. The search is performed on the standard [[Special:Search]] page. It may matter that we're running the wiki on two servers, using Percona MySQL for data replication between the servers. A load balancer sends users to whichever server has the fewest active connections.
>
> -----Original Message-----
> From: Stas Malyshev [mailto:[hidden email]]
> Sent: Tuesday, March 05, 2019 5:39 PM
> To: Wikimedia developers <[hidden email]>; Hogan (US), Michael C <[hidden email]>
> Subject: Re: [Wikitech-l] Unstable search results - possible causes
>
> Hi!
>
> > I'm using the built-in Mediawiki search engine. We just updated from 1.30.0 to 1.31.0. Since the update, search results are unstable. The same search term gives different results in different web browsers. We also see different results across browser sessions. Any advice on how I can troubleshoot this?
>
> Could you provide more info - which search you're talking about (URL might help, or description of the sequence of actions), which terms, what you're getting in different cases? Some search results might depend on your preferences or user language selected, so there might be difference, e.g. for different logged-in users.
>
> Thanks,
> --
> Stas Malyshev
> [hidden email]
> _______________________________________________
> Wikitech-l mailing list
> [hidden email]
> https://lists.wikimedia.org/mailman/listinfo/wikitech-l



--
Jaime Crespo
<http://wikimedia.org>

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