Date: Thu, 05 Mar 2026 13:32:14 +0900 (JST)
Message-Id: <20260305.133214.526232552934046503.ishii@postgresql.org>
To: Zdenek.Koktan@cdt.cz
Cc: pgpool-general@lists.postgresql.org
Subject: Re: pgpool-general@lists.postgresql.org
From: Tatsuo Ishii <ishii@postgresql.org>
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Hi,

From: Zdenek.Koktan@cdt.cz
Subject: pgpool-general@lists.postgresql.org
Date: Tue, 3 Mar 2026 11:02:03 +0100
Message-ID: <OFD7B76D63.D5F7C882-ONC1258DAF.00367DA1-C1258DAF.00371EE0@cdt.cz>

> Hello,
> i  have a problem with error which occurs on my pgpool cluster. The log 
> messages i collected for this error :
> 
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: ERROR:  unable 
> to write data to frontend
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: DETAIL: 
> pool_flush failed
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> pool_stream.c:539
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOG:  frontend 
> disconnection: session time: 0:00:32.989 user=network_app_prod_adm 
> database=network_app_prod host=cdis000phaux173.cdis.cz port=63675
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> child.c:2110
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOG:  DB node 
> id: 0 backend pid: 3935771 statement: DISCARD ALL
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> pool_proto_modules.c:3802
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOG:  DB node 
> id: 2 backend pid: 199834 statement: DISCARD ALL
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> pool_proto_modules.c:3802
> 2026-03-02 15:30:21.768: PostgreSQL JDBC Driver pid 197272: WARNING: 
> packet kind of backend 2 ['Z'] does not match with main/majority nodes 
> packet kind ['D']
> 2026-03-02 15:30:21.768: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> pool_process_query.c:3713
> 2026-03-02 15:30:21.768: PostgreSQL JDBC Driver pid 197272: FATAL:  failed 
> to read kind from backend
> 2026-03-02 15:30:21.768: PostgreSQL JDBC Driver pid 197272: DETAIL:  kind 
> mismatch among backends. Possible last query was: "DISCARD ALL" kind 
> details are: 0[D] 2[Z]
> 2026-03-02 15:30:21.768: PostgreSQL JDBC Driver pid 197272: HINT:  check 
> data consistency among db nodes
> 2026-03-02 15:30:21.768: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> pool_process_query.c:3810
> 
> The customer complaints that the statement he is trying to send to 
> database (INSERT) is basicaly not entered into the database. This error 
> happenes several times a day, 
> My version of pgpool and database : 
> 
> postgres@postgre-cdis-clu-pha-db3:/var/lib/pgsql/data/log> pgpool -version
> pgpool-II version 4.5.10 (hotooriboshi)
> 
> postgres@postgre-cdis-clu-pha-db3:/var/lib/pgsql/data/log> psql -version
> psql (16.11)
> Type "help" for help.
> 
> The configuration of cluster:
> 
> postgres@postgre-cdis-clu-pha-db3:/var/lib/pgsql/data/log> 
> pcp_watchdog_info -h 10.31.8.28 -p 9898 -U pgpool
> 3 3 YES postgre-cdis-clu-pha-db3:9999 Linux postgre-cdis-clu-pha-db3 
> postgre-cdis-clu-pha-db3
> 
> postgre-cdis-clu-pha-db3:9999 Linux postgre-cdis-clu-pha-db3 
> postgre-cdis-clu-pha-db3 9999 9000 4 LEADER 0 MEMBER
> postgre-cdis-clu-pha-db1:9999 Linux postgre-cdis-clu-pha-db1 
> postgre-cdis-clu-pha-db1 9999 9000 7 STANDBY 0 MEMBER
> postgre-cdis-clu-pha-db2:9999 Linux postgre-cdis-clu-pha-db2 
> postgre-cdis-clu-pha-db2 9999 9000 7 STANDBY 0 MEMBER
> 
> postgres@postgre-cdis-clu-pha-db3:/var/lib/pgsql/data/log> psql -h 
> 10.31.8.28 -p 9999 -U pgpool postgres -c "show pool_nodes"
>  node_id |         hostname         | port | status | pg_status | 
> lb_weight |  role   | pg_role | select_cnt | load_balance_node | 
> replication_delay | replication_state | replicati
> on_sync_state | last_status_change
> ---------+--------------------------+------+--------+-----------+-----------+---------+---------+------------+-------------------+-------------------+-------------------+----------
> --------------+---------------------
>  0       | postgre-cdis-clu-pha-db1 | 5432 | up     | up        | 0.333333 
>  | standby | standby | 14069796   | false             | 0 | streaming    | 
> async
>               | 2026-03-02 15:28:50
>  1       | postgre-cdis-clu-pha-db2 | 5432 | up     | up        | 0.333333 
>  | standby | standby | 13284878   | true              | 0 | streaming    | 
> async
>               | 2026-03-02 15:28:50
>  2       | postgre-cdis-clu-pha-db3 | 5432 | up     | up        | 0.333333 
>  | primary | primary | 15253310   | false             | 0 |    |
>               | 2026-03-02 15:28:50
> (3 rows)
> 
> There are no errors at database level in the database log.
> Any advice how to solve this ?

It seems your java disconnects to pgpool. I suggest to inspect the
application log and check why the application disconnected,

> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: ERROR:  unable 
> to write data to frontend
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: DETAIL: 
> pool_flush failed
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> pool_stream.c:539
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOG:  frontend 
> disconnection: session time: 0:00:32.989 user=network_app_prod_adm 
> database=network_app_prod host=cdis000phaux173.cdis.cz port=63675
> 2026-03-02 15:30:21.767: PostgreSQL JDBC Driver pid 197272: LOCATION: 
> child.c:2110

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