[pgpool-general: 9403] Re: Question about backend_application_name

[Tatsuo Ishii]

> On Wed, Mar 26, 2025 at 7:56 PM Tatsuo Ishii <ishii at postgresql.org> wrote:
> 
>> > PgPool 4.5.4
>> >
>> > Three node pgpool cluster:
>> > Node0 = Postgresql primary
>> > Node1 = Postgresql streaming replica
>> > Node2 = PgPool leader, where the VIP sits.
>> >
>> >
>> https://www.pgpool.net/docs/45/en/html/runtime-config-backend-settings.html
>> >
>> > The docs consistently refer to setting backend_application_nameX to
>> > "serverX", so that's what I did:
>>
>> "serverX" is just an example. You can use arbitrary name for
>> backend_application_nameX as long as:
>>
>> a) It matches with the application name set in the primary_conninfo in
>> postgresql.conf.
>>
> 
> On the replica (aka Node 1), there is no primary_coninfo in either
> postgresql.conf or postgresql.auto.conf.

Maybe you are using older version of PostgreSQL (11 or earlier)? Then
you will have recovery.conf instead.

> pg_stat_wal_receiver.conninfo has this section:
> fallback_application_name="walreceiver".
> 
> (I initiate replication using "pg_basebackup --wal-method=stream
> --create-slot --slot=foo.)
> 
> 
>>
>> b) No same application names are used in all primary_conninfo.
>>
>> > But should it really be set to "walreceiver"?  Should both
>> > backend_application_name0 and backend_application_name1 be set to
>> > "walreceiver" even though it's only running on Node1?
>>
>> I never tried it but probably yes. However I recommend in general to
>> set unique application names for each
>> backend_application_nameX. Someday you might want to add more replica.
>>
> 
> How does the second replica know to set it's name different from the first?

I do not understand your question. Maybe I should have explained the
background:

The application name uniqueness is necessary requirement for streaming
replication check process to show pool_status's replication_state and
replication_sync_state columns. The process sends "SELECT
pg_stat_replication" to the primary PostgreSQL to know the synchronous
state. In this query the application name is used to distinguish each
standby's information. So if same application names are used, the
states will not be correctly shown.

>> > cdsmultip=# pgpool show backend_application_name2;
>> >  backend_application_name2
>> > ---------------------------
>> >
>> > (1 row)
>>
>> What's this? You said you only have one replica.
>>
>>
> You can successfully run "pgpool show backend_application_name2;" even
> though only _name0 and _name1 are defined in pgpool.conf.
> 
> I ran the command just for completeness.

Oh, I didn't know that! "pgpool show" should errors out in this case.

Best reagards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp