Re: Synchronous commit behavior during network outage

Hello Aleksander,

Thank you for the reaction. This was tested on version 13.2.

There are also other possible situations with the same setup and similar
issue:

-----------------
When the background process on server fails....

On postgresql1:
tecmint=# select * from a; --> LAN on sync replica is OK
 id
----
  1
(1 row)

tecmint=# insert into a values (2); ---> LAN on sync replica is DOWN and
insert is waiting. During this time kill the background process on the
PostgreSQL server for this session
WARNING:  canceling the wait for synchronous replication and terminating
connection due to administrator command
DETAIL:  The transaction has already committed locally, but might not
have been replicated to the standby.
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.
tecmint=# select * from a;
 id
----
  1
  2
(2 rows)

tecmint=# ---> LAN on sync replica is still DOWN

The potgres session will restore after the background process failed.
When you run select on master, it still looks OK. But data is still not
replicated on the sync replica. If we lost the master now, we would lost
this data as well.

**************
Another case
**************

Kill the client process.

tecmint=# select * from a;
 id
----
  1
  2
  3
(3 rows)
tecmint=#                --> Disconnect the sync replica now. LAN on
replica is DOWN
tecmint=# insert into a values (4); --> Kill the client process
Terminated
xzizka(at)service-vm:~$ psql -U postgres -h 192.168.122.6 -p 5432 -d tecmint
Password for user postgres:
psql (13.2 (Debian 13.2-1.pgdg100+1))
Type "help" for help.

tecmint=# select * from a;
 id
----
  1
  2
  3
(3 rows)

tecmint=# --> Number 4 is not there. Now switch the LAN on sync replica ON.

----------

Result from sync replica after the LAN is again UP:
tecmint=# select * from a;
 id
----
  1
  2
  3
  4
(4 rows)

In this situation, try to insert the number 4 again to the table.

tecmint=# select * from a;
 id
----
  1
  2
  3
(3 rows)

tecmint=# insert into a values (4);
ERROR:  duplicate key value violates unique constraint "a_pkey"
DETAIL:  Key (id)=(4) already exists.
tecmint=#

This is really strange... Application can be confused, It is not
possible to insert record, which is not there, but some systems which
use the sync node as a read replica maybe already read that record from
the sync replica database and done some steps which can cause issues and
can be hard to track.

If I say, that it would be hard to send the CTRL+C to the database from
the client, I need to say, that the 2 situations I described here can
happen in real.

What do you think?

Thank you and regards
Ondrej

On 20/04/2021 17:23, Aleksander Alekseev wrote:
> Hi Ondřej,
>
> Thanks for the report. It seems to be a clear violation of what is
> promised in the docs. Although it's unlikely that someone implemented
> an application which deals with important data and "pressed Ctr+C" as
> it's done in psql. So this might be not such a critical issue after
> all. BTW what version of PostgreSQL are you using?
>
>
> On Mon, Apr 19, 2021 at 10:13 PM Ondřej Žižka <ondrej(dot)zizka(at)stratox(dot)cz> wrote:
>> Hello all,
>> I would like to know your opinion on the following behaviour I see for PostgreSQL setup with synchronous replication.
>>
>> This behaviour happens in a special use case. In this use case, there are 2 synchronous replicas with the following config (truncated):
>>
>> - 2 nodes
>> - synchronous_standby_names='*'
>> - synchronous_commit=remote_apply
>>
>>
>> With this setup run the following steps (LAN down - LAN between master and replica):
>> -----------------
>> postgres=# truncate table a;
>> TRUNCATE TABLE
>> postgres=# insert into a values (1); -- LAN up, insert has been applied to replica.
>> INSERT 0 1
>> Vypnu LAN na serveru se standby:
>> postgres=# insert into a values (2); --LAN down, waiting for a confirmation from sync replica. In this situation cancel it (press CTRL+C)
>> ^CCancel request sent
>> WARNING: canceling wait for synchronous replication due to user request
>> DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
>> INSERT 0 1
>> There will be warning that commit was performed only locally:
>> 2021-04-12 19:55:53.063 CEST [26104] WARNING: canceling wait for synchronous replication due to user request
>> 2021-04-12 19:55:53.063 CEST [26104] DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
>>
>> postgres=# insert into a values (2); --LAN down, waiting for a confirmation from sync replica. In this situation cancel it (press CTRL+C)
>> ^CCancel request sent
>> WARNING: canceling wait for synchronous replication due to user request
>> DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
>> INSERT 0 1
>> postgres=# insert into a values (2); --LAN down, waiting for sync replica, second attempt, cancel it as well (CTRL+C)
>> ^CCancel request sent
>> WARNING: canceling wait for synchronous replication due to user request
>> DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
>> INSERT 0 1
>> postgres=# update a set n=3 where n=2; --LAN down, waiting for sync replica, cancel it (CTRL+C)
>> ^CCancel request sent
>> WARNING: canceling wait for synchronous replication due to user request
>> DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
>> UPDATE 2
>> postgres=# update a set n=3 where n=2; -- run the same update,because data from the previous attempt was commited on master, it is sucessfull, but no changes
>> UPDATE 0
>> postgres=# select * from a;
>> n
>> ---
>> 1
>> 3
>> 3
>> (3 rows)
>> postgres=#
>> ------------------------
>>
>> Now, there is only value 1 in the sync replica table (no other values), data is not in sync. This is expected, after the LAN restore, data will come sync again, but if the main/primary node will fail and we failover to replica before the LAN is back up or the storage for this node would be destroyed and data would not sync to replica before it, we will lose data even if the client received successful commit (with a warning).
>> From the synchronous_commit=remote_write level and "higher", I would expect, that when the remote application (doesn't matter if flush, write or apply) would not be applied I would not receive a confirmation about the commit (even with a warning). Something like, if there is no commit from sync replica, there is no commit on primary and if someone performs the steps above, the whole transaction will not send a confirmation.
>>
>> This can cause issues if the application receives a confirmation about the success and performs some follow-up steps e.g. create a user account and sends a request to the mail system to create an account or create a VPN account. If the scenario above happens, there can exist a VPN account that does not have any presence in the central database and can be a security issue.
>>
>> I hope I explained it sufficiently. :-)
>>
>> Do you think, that would be possible to implement a process that would solve this use case?
>>
>> Thank you
>> Ondrej
>
>