[pgpool-general: 6938] Re: Inconsistency between DB nodes with native replication
Tatsuo Ishii
ishii at sraoss.co.jp
Tue Mar 17 11:15:04 JST 2020
Hi Vladimír,
Thank you for the feedback.
> However, I don't remember ever seeing the line:
>
> DETAIL: postmaster on DB node 1 was shutdown by administrative command
>
> In the log, when my test script failed. I try to watch the log this specifically for this in the coming days and let you know, what I fin
I'm looking forward to hearing from you.
In the mean time I have pushed a fix for this (see attached emal) in
that Pgpool-II will do failover if shutdown event is detected. This
should prevent data inconsistency caused by the situation.
> Hello,
>
> I tried to replicated the procedure and pretty much got the same results:
>
> 1) Connect to the database via pgpool:
>
> psql -h aisgpgpvip.cca.cz -p 9999 -d aisgdvyv -U aisg
>
> 2) Run show pool_nodes command, result was as follows:
>
> aisgdvyv=> show pool_nodes;
> node_id | hostname | port | status | lb_weight | role | select_cnt | load_balance_node | replication_delay | replication_state | replication_sync_state | last_status_change
> ---------+------------------+------+--------+-----------+--------+------------+-------------------+-------------------+-------------------+------------------------+---------------------
> 0 | aisgdbd01.cca.cz | 5432 | up | 0.500000 | master | 1562942 | true | 0 | | | 2020-03-11 14:19:06
> 1 | aisgdbd02.cca.cz | 5432 | up | 0.500000 | slave | 567641 | false | 0 | | | 2020-03-16 12:19:08
>
> 3) Create the test table t2:
>
> aisgdvyv=> create table t2(i int);
> CREATE TABLE
>
> Mar 16 12:26:07 AISGPGP01 pgpool[21402]: [3103-1] 2020-03-16 12:26:07 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 0 backend pid: 17021 statement: BEGIN
> Mar 16 12:26:07 AISGPGP01 pgpool[21402]: [3104-1] 2020-03-16 12:26:07 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 1 backend pid: 19389 statement: BEGIN
> Mar 16 12:26:07 AISGPGP01 pgpool[21402]: [3105-1] 2020-03-16 12:26:07 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 0 backend pid: 17021 statement: create table t2(i int);
> Mar 16 12:26:07 AISGPGP01 pgpool[21402]: [3106-1] 2020-03-16 12:26:07 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 1 backend pid: 19389 statement: create table t2(i int);
> Mar 16 12:26:07 AISGPGP01 pgpool[21402]: [3107-1] 2020-03-16 12:26:07 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 1 backend pid: 19389 statement: COMMIT
> Mar 16 12:26:07 AISGPGP01 pgpool[21402]: [3108-1] 2020-03-16 12:26:07 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 0 backend pid: 17021 statement: COMMIT
>
> 4) Terminate the session with backend PID 19389 from different session:
>
> aisgdvyv=> SELECT pg_terminate_backend(19389);
> pg_terminate_backend
> ----------------------
> t
> (1 row)
>
> Mar 16 12:30:38 AISGPGP01 pgpool[21414]: [3108-1] 2020-03-16 12:30:38 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: found the pg_terminate_backend request for backend pid:19389 on backend node:1
> Mar 16 12:30:38 AISGPGP01 pgpool[21414]: [3108-2] 2020-03-16 12:30:38 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |DETAIL: setting the connection flag
> Mar 16 12:30:38 AISGPGP01 pgpool[21414]: [3109-1] 2020-03-16 12:30:38 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 1 backend pid: 19715 statement: SELECT pg_terminate_backend(19389);
> Mar 16 12:30:38 AISGPGP01 pgpool[21402]: [3109-1] 2020-03-16 12:30:38 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: reading and processing packets
> Mar 16 12:30:38 AISGPGP01 pgpool[21402]: [3109-2] 2020-03-16 12:30:38 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |DETAIL: postmaster on DB node 1 was shutdown by administrative command
>
> 5) INSERT a row into the table t2:
>
> aisgdvyv=> insert into t2 values(1);
> INSERT 0 1
>
> Mar 16 12:33:42 AISGPGP01 pgpool[21402]: [3110-1] 2020-03-16 12:33:42 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 0 backend pid: 17021 statement: SELECT attname, pg_get_expr(d.adbin, d.adrelid), coalesce((pg_get_expr(d.adbin, d.adrelid) LIKE '%now()%' OR pg_get_expr(d.adbin, d.adrelid) LIKE '%''now''::text%' OR pg_get_expr(d.adbin, d.adrelid) LIKE '%CURRENT_TIMESTAMP%' OR pg_get_expr(d.adbin, d.adrelid) LIKE '%CURRENT_TIME%' OR pg_get_expr(d.adbin, d.adrelid) LIKE '%CURRENT_DATE%' OR pg_get_expr(d.adbin, d.adrelid) LIKE '%LOCALTIME%' OR pg_get_expr(d.adbin, d.adrelid) LIKE '%LOCALTIMESTAMP%') AND (a.atttypid = 'timestamp'::regtype::oid OR a.atttypid = 'timestamp with time zone'::regtype::oid OR a.atttypid = 'date'::regtype::oid OR a.atttypid = 'time'::regtype::oid OR a.atttypid = 'time with time zone'::regtype::oid) , false) FROM pg_catalog.pg_class c, pg_catalog.pg_attribute a LEFT JOIN
> Mar 16 12:33:42 AISGPGP01 pgpool[21402]: [3110-2] pg_catalog.pg_attrdef d ON (a.attrelid = d.adrelid AND a.attnum = d.adnum) WHERE c.oid = a.attrelid AND a.attnum >= 1 AND a.attisdropped = 'f' AND c.oid = to_regclass('"t2"') ORDER BY a.attnum
> Mar 16 12:33:42 AISGPGP01 pgpool[21402]: [3111-1] 2020-03-16 12:33:42 ::: APP - psql :A: DB - aisgdvyv :D: USER - aisg :U: |LOG: DB node id: 0 backend pid: 17021 statement: insert into t2 values(1);
>
> 6) Compare content of the table between nodes:
>
> psql -h aisgdbd01.cca.cz -p 5432 -d aisgdvyv -U aisg
>
> aisgdvyv=> select count(*) from t2;
> count
> -------
> 1
> (1 row)
>
> psql -h aisgdbd02.cca.cz -p 5432 -d aisgdvyv -U aisg
>
> aisgdvyv=> select count(*) from t2;
> count
> -------
> 0
> (1 row)
>
>
> However, I don't remember ever seeing the line:
>
> DETAIL: postmaster on DB node 1 was shutdown by administrative command
>
> In the log, when my test script failed. I try to watch the log this specifically for this in the coming days and let you know, what I find.
>
> Vladimír Láznička
>
> -----Original Message-----
> From: Tatsuo Ishii <ishii at sraoss.co.jp>
> Sent: Wednesday, March 11, 2020 11:26 PM
> To: Láznička Vladimír <Vladimir.Laznicka at cca.cz>
> Cc: pengbo at sraoss.co.jp; pgpool-general at pgpool.net
> Subject: Re: [pgpool-general: 6896] Re: Inconsistency between DB nodes with native replication
>
>> Hello,
>>
>>> So my question is, did you use pg_terminate_backend() or kill the postgres backend process on node 1 by using kill command?
>>
>> Definitely not in the case of my test script. I am trying to replicate the process which you described below, but I don't have the load balance node on node 0:
>>
>> aisgdvyv=> show pool_nodes;
>> node_id | hostname | port | status | lb_weight | role | select_cnt | load_balance_node | replication_delay | replication_state | replication_sync_state | last_status_change
>> ---------+------------------+------+--------+-----------+--------+------------+-------------------+-------------------+-------------------+------------------------+---------------------
>> 0 | aisgdbd01.cca.cz | 5432 | up | 0.500000 | master | 340 | false | 0 | | | 2020-03-11 14:19:06
>> 1 | aisgdbd02.cca.cz | 5432 | up | 0.500000 | slave | 749 | true | 0 | | | 2020-03-11 14:19:06
>>
>> Can I force it or change some settings so I have the load balance node on the node 0?
>
> The "load_balance_node" column in show pool_nodes command just shows the load balance node in the local session. So next time you start a session "load_balance_node" may show different value because the load balance node is calculated at the start up of each session.
>
> To force the load balance node on 0, you can set backend_weight1 to 0.
>
>> Thank you.
>>
>> Vladimír Láznička
>>
>> -----Original Message-----
>> From: Tatsuo Ishii <ishii at sraoss.co.jp>
>> Sent: Tuesday, March 10, 2020 1:18 AM
>> To: Láznička Vladimír <Vladimir.Laznicka at cca.cz>
>> Cc: pengbo at sraoss.co.jp; pgpool-general at pgpool.net
>> Subject: Re: [pgpool-general: 6896] Re: Inconsistency between DB nodes
>> with native replication
>>
>> I have found a case which could explain your problem. See following Pgpool-II log.
>>
>> 1. Connect to Pgpool-II using psql. Make sure that load balance node is node 0.
>>
>> test=# show pool_nodes;
>> node_id | hostname | port | status | lb_weight | role | select_cnt
>> | load_balance_node | replication_delay | replication_state |
>> replication_sync_state | last_status_change
>> ---------+----------+-------+--------+-----------+--------+------------+-------------------+-------------------+-------------------+------------------------+---------------------
>> 0 | /tmp | 11002 | up | 1.000000 | master | 1 | true | 0 | | | 2020-03-10 09:02:16
>> 1 | /tmp | 11003 | up | 0.000000 | slave | 0 | false | 0 | | | 2020-03-10 09:02:16
>> (2 rows)
>>
>> 2. CREATE a table "t2".
>> 2020-03-10 09:07:07: pid 12361: LOG: DB node id: 0 backend pid: 12459
>> statement: BEGIN
>> 2020-03-10 09:07:07: pid 12361: LOG: DB node id: 1 backend pid: 12460
>> statement: BEGIN
>> 2020-03-10 09:07:07: pid 12361: LOG: DB node id: 0 backend pid: 12459
>> statement: create table t2(i int);
>> 2020-03-10 09:07:07: pid 12361: LOG: DB node id: 1 backend pid: 12460
>> statement: create table t2(i int);
>> 2020-03-10 09:07:07: pid 12361: LOG: DB node id: 1 backend pid: 12460
>> statement: COMMIT
>> 2020-03-10 09:07:07: pid 12361: LOG: DB node id: 0 backend pid: 12459
>> statement: COMMIT
>>
>> 3. Issue pg_terminate_backend(12460) to kill the PostgreSQL process on node 1 from different terminal.
>>
>> 4. Pgpool-II detects it and set the local node 1 status down.
>>
>> 2020-03-10 09:08:20: pid 12361: LOG: reading and processing packets
>> 2020-03-10 09:08:20: pid 12361: DETAIL: postmaster on DB node 1 was
>> shutdown by administrative command
>>
>> 5. Issue INSERT to see it is forwarded to only node 0 on the same session as 1 and 2.
>>
>> 2020-03-10 09:08:36: pid 12361: LOG: DB node id: 0 backend pid: 12459
>> statement: SELECT attname, pg_get_expr(d.adbin, d.adrelid),
>> coalesce((pg_get_expr(d.adbin, d.adrelid) LIKE '%now()%' OR
>> pg_get_expr(d.adbin, d.adrelid) LIKE '%''now''::text%' OR
>> pg_get_expr(d.adbin, d.adrelid) LIKE '%CURRENT_TIMESTAMP%' OR
>> pg_get_expr(d.adbin, d.adrelid) LIKE '%CURRENT_TIME%' OR
>> pg_get_expr(d.adbin, d.adrelid) LIKE '%CURRENT_DATE%' OR
>> pg_get_expr(d.adbin, d.adrelid) LIKE '%LOCALTIME%' OR
>> pg_get_expr(d.adbin, d.adrelid) LIKE '%LOCALTIMESTAMP%') AND
>> (a.atttypid = 'timestamp'::regtype::oid OR a.atttypid = 'timestamp
>> with time zone'::regtype::oid OR a.atttypid = 'date'::regtype::oid OR
>> a.atttypid = 'time'::regtype::oid OR a.atttypid = 'time with time
>> zone'::regtype::oid) , false) FROM pg_catalog.pg_class c,
>> pg_catalog.pg_attribute a LEFT JOIN pg_catalog.pg_attrdef d ON
>> (a.attrelid = d.adrelid AND a.attnum = d.adnum) WHERE c.oid =
>> a.attrelid AND a.attnum >= 1 AND a.attisdropped = 'f' AND c.oid =
>> to_regclass('"t2"') ORDER BY a.attnum
>> 2020-03-10 09:08:36: pid 12361: LOG: DB node id: 0 backend pid: 12459
>> statement: insert into t2 values(1);
>>
>> So my question is, did you use pg_terminate_backend() or kill the postgres backend process on node 1 by using kill command?
>>
>>>> Have you seen any failover event of node 1 and then it failback while the test? If process 29684 copied the down status of node 1 when failover happened but failed to change the local status to "up" when node 1 failed back, then this could be an expiation.
>>>
>>> Not as far as I remember. If node 1 went down, it would always stay down until I manually used some kind of recovery (as in - I never was anything in the log about failback).
>>>
>>>> If your test environment's network is more unstable than the data center, it is likely to have more chance of failover. Of course this is just a guess.
>>>
>>> I asked our sysadmin to move the second DB server (node 1) to the same physical machine in virtualization where is the first DB server (node 0), so I'll see if there is any difference.
>>>
>>> Vladimír Láznička
>>>
>>> -----Original Message-----
>>> From: Tatsuo Ishii <ishii at sraoss.co.jp>
>>> Sent: Thursday, March 5, 2020 1:18 PM
>>> To: Láznička Vladimír <Vladimir.Laznicka at cca.cz>
>>> Cc: pengbo at sraoss.co.jp; pgpool-general at pgpool.net
>>> Subject: Re: [pgpool-general: 6896] Re: Inconsistency between DB
>>> nodes with native replication
>>>
>>>> Do you think there is a possibility that only one specific child
>>>> process of pgpool thinks that one of the DB nodes is dead?
>>>
>>> Yes. Pgpool-II maintains backend status (up, down ,quarantine etc.) in shared memory and also each pgpool process has local copy of it (usually each pgpool looks into the local copy status). So if for some reason the local status becomes out of sync with the shared memory status, we might see the phenomena.
>>>
>>> Have you seen any failover event of node 1 and then it failback while the test? If process 29684 copied the down status of node 1 when failover happened but failed to change the local status to "up" when node 1 failed back, then this could be an expiation.
>>>
>>>> Is there a way to find about it? I guess I could set the logging to
>>>> some debug level, but I don't even want to imagine the log volume in
>>>> that case, since it is already hundreds of megabytes in size.
>>>
>>> As far as I know there's no way to find out the phenomena without turning on debug log. But, yes, it would create huge amount of log.
>>>
>>>> On the side note I also have the test running on other environment
>>>> (in customer's data center) and it did not fail so far (since 25th
>>>> of
>>>> February) so it may come from some specific setting on those servers
>>>> (pgpool.conf is set the same apart from IP addresses and number of
>>>> child processes generated at startup).
>>>
>>> If your test environment's network is more unstable than the data center, it is likely to have more chance of failover. Of course this is just a guess.
>>>
>>> Best regards,
>>> --
>>> Tatsuo Ishii
>>> SRA OSS, Inc. Japan
>>> English: http://www.sraoss.co.jp/index_en.php
>>> Japanese:http://www.sraoss.co.jp
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