Re: Support for N synchronous standby servers - take 2

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: sawada(dot)mshk(at)gmail(dot)com
Cc: masao(dot)fujii(at)gmail(dot)com, memissemerson(at)gmail(dot)com, josh(at)agliodbs(dot)com, robertmhaas(at)gmail(dot)com, amit(dot)kapila16(at)gmail(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Support for N synchronous standby servers - take 2
Date: 2015-12-14 09:27:38
Message-ID: 20151214.182738.130827803.horiguchi.kyotaro@lab.ntt.co.jp
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Thank you for the new patch.

At Wed, 9 Dec 2015 20:59:20 +0530, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote in <CAD21AoDcn1fToCcYRqpU6fMY1xnpDdAKDTcbhW1R9M1mPM0kZg(at)mail(dot)gmail(dot)com>
> On Wed, Nov 18, 2015 at 2:06 PM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > I agree with #3 way and the s_s_name format you suggested.
> > I think that It's extensible and is tolerable for future changes.
> > I'm going to implement the patch based on this idea if other hackers
> > agree with this design.
>
> Please find the attached draft patch which supports multi sync replication.
> This patch adds a GUC parameter synchronous_replication_method, which
> represent the method of synchronous replication.
>
> [Design of replication method]
> synchronous_replication_method has two values; 'priority' and
> '1-priority' for now.
> We can expand the kind of its value (e.g, 'quorum', 'json' etc) in the future.
>
> * s_r_method = '1-priority'
> This method is for backward compatibility, so the syntax of s_s_names
> is same as today.
> The behavior is same as well.
>
> * s_r_method = 'priority'
> This method is for multiple synchronous replication using priority method.
> The syntax of s_s_names is,
> <number of sync standbys>, <standby name> [, ...]

Is there anyone opposed to this?

> For example, s_r_method = 'priority' and s_s_names = '2, node1, node2,
> node3' means that the master waits for acknowledge from at least 2
> lowest priority servers.
> If 4 standbys(node1 - node4) are available, the master server waits
> acknowledge from 'node1' and 'node2.
> The each status of wal senders are;
>
> =# select application_name, sync_state from pg_stat_replication order
> by application_name;
> application_name | sync_state
> ------------------+------------
> node1 | sync
> node2 | sync
> node3 | potential
> node4 | async
> (4 rows)
>
> After 'node2' crashed, the master will wait for acknowledge from
> 'node1' and 'node3'.
> The each status of wal senders are;
>
> =# select application_name, sync_state from pg_stat_replication order
> by application_name;
> application_name | sync_state
> ------------------+------------
> node1 | sync
> node3 | sync
> node4 | async
> (3 rows)
>
> [Changing replication method]
> When we want to change the replication method, we have to change the
> s_r_method at first, and then do pg_reload_conf().
> After changing replication method, we can change the s_s_names.

Mmm. I should be able to be changed at once, because s_r_method
and s_s_names contradict each other during the intermediate
state.

> [Expanding replication method]
> If we want to expand new replication method additionally, we need to
> implement two functions for each replication method:
> * int SyncRepGetSynchronousStandbysXXX(int *sync_standbys)
> This function obtains the list of standbys considered as synchronous
> at that time, and return its length.
> * bool SyncRepGetSyncLsnXXX(XLogRecPtr *write_pos, XLogRecPtr *flush_pos)
> This function obtains LSNs(write, flush) considered as synced.
>
> Also, this patch debug code is remain yet, you can debug this behavior
> using by enable DEBUG_REPLICATION macro.
>
> Please give me feedbacks.

I haven't looked into this fully (sorry) but I'm concerned about
several points.

- I feel that some function names looks too long. For example
SyncRepGetSynchronousStandbysOnePriority occupies more than the
half of a line. (However, the replication code alrady has many
long function names..)

- The comment below of SyncRepGetSynchronousStandbyOnePriority,
> /* Find lowest priority standby */

The code where the comment is for is doing the correct
thing. Howerver, the comment is confusing. A lower priority
*value* means a higher priority.

- SyncRepGetSynchronousStandbys checks all if()s even when the
first one matches. Use switch or "else if" there if you they
are exclusive each other.

- Do you intende the DEBUG_REPLICATION code in
SyncRepGetSynchronousStandbys*() to be the final shape? The
same code blocks which can work for both method should be in
their common caller but SyncRepGetSyncLsns*() are
headache. Although it might need more refactoring, I'm sorry
but I don't see a desirable shape for now.

By the way, palloc(20)/free() in such short term looks
ineffective.

- SyncRepGetSyncLsnsPriority

For the comment "/* Find lowest XLogRecPtr of both write and
flush from sync_nodes */", LSN is compared as early or late so
the comment would be better to be something like "Keep/Collect
the earliest write and flush LSNs among prioritized standbys".

And what is more important, this block handles write and flush
LSN jumbled and it reults in missing the earliest(= most
delayed) LSN for certain cases. The following is an example.

Standby 1: write LSN = 10, flush LSN = 5
Standby 2: write LSN = 8 , flush LSN = 6

For this case, finally we get tmp_write = 10 and tmp_flush = 5
from the current code, where tmp_write has wrong value since
LSN = 10 has *not* been written yet on standby 2. (the names
"tmp_*" don't seem appropriate here)

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center

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