From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Alfred Perlstein <alfred(at)freebsd(dot)org> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Stephen Frost <sfrost(at)snowman(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Geoff Winkless <pgsqladmin(at)geoff(dot)dj>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Why we lost Uber as a user |
Date: | 2016-08-03 02:33:15 |
Message-ID: | 20160803023315.GG32575@momjian.us |
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Lists: | pgsql-hackers |
On Tue, Aug 2, 2016 at 07:30:22PM -0700, Alfred Perlstein wrote:
> So for instance, let's say there is a bug in the master's write to disk.
> The logical replication acts as a barrier from that bad write going to the
> slaves. With bad writes going to slaves then any corruption experienced on
> the master will quickly reach the slaves and they too will be corrupted.
>
> With logical replication a bug may be stopped at the replication layer. At
> that point you can resync the slave from the master.
>
> Now in the case of physical replication all your base are belong to zuul and
> you are in a very bad state.
>
> That said with logical replication, who's to say that if the statement is
> replicated to a slave that the slave won't experience the same bug and also
> corrupt itself.
>
> We may be saying the same thing, but still there is something to be said for
> logical replication... also, didnt they show that logical replication was
> faster for some use cases at Uber?
I saw from the Uber article that they weren't going to per-row logical
replication but _statement_ replication, which is very hard to do
because typical SQL doesn't record what concurrent transactions
committed before a new statement's transaction snapshot is taken, and
doesn't record lock order for row updates blocked by concurrent activity
--- both of which affect the final result from the query.
So, for statement replication, it is not a question of whether the code
has bugs, but that the replay is not 100% possible in all cases, unless
you switch to some statement-row-lock hybrid ability.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
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