Re: An issue in JDBC replication API

Hi,

On Sat, Jun 20, 2020 at 7:37 PM Dave Cramer <davecramer(at)postgres(dot)rocks>
wrote:

>
> Dave Cramer
> www.postgres.rocks
>
>
> On Fri, 19 Jun 2020 at 19:37, Dave Cramer <davecramer(at)postgres(dot)rocks>
> wrote:
>
>> Hi Abbas,
>>
>> Thanks for this I seem to recall something about this.
>>
>> Dave Cramer
>> www.postgres.rocks
>>
>>
>> On Fri, 19 Jun 2020 at 16:40, Abbas Butt <abbas(dot)butt(at)enterprisedb(dot)com>
>> wrote:
>>
>>> Hi,
>>> I have found that the JDBC replication API does not handle the keep
>>> alive messages sent by the server correctly.
>>> A replication client cannot ignore even a single keep alive message from
>>> the server and it must respond to it.
>>> If it does not the server will stop WAL sender with the following error:
>>> LOG: terminating walsender process due to replication timeout
>>> LOG: disconnection: session time: 0:02:00.173 user=replicant
>>> database=edb host=127.0.0.1 port=57900
>>>
>>> and this is exactly what is happening when I use JDBC replication API.
>>> The JDBC driver is ignoring the server's keep alive messages and hence
>>> the server stops the WAL sender and drops the connection.
>>>
>>> Attached please find a sample program that I have mostly copied from the
>>> documentation available here
>>>
>>> https://jdbc.postgresql.org/documentation/head/replication.html#logical-replication
>>>
>>> When we run this program and do not do any activity on the database
>>> server (I have tested with server version 12 and JDBC 42.2.14), after some
>>> time (wal_sender_timeout/2 seconds to be precise) the server stops the
>>> WAL sender process and drops the connection.
>>> To use the program follow these steps:
>>>
>>> Issue the following commands to the database server:
>>> ./createuser --superuser --replication -h 127.0.0.1 -p 7777 -U postgres
>>> replicant
>>> select pg_drop_replication_slot('radium');
>>> CREATE TABLE numbers(a int PRIMARY KEY, b varchar(255));
>>> GRANT SELECT on numbers to replicant;
>>> INSERT INTO numbers VALUES(10, 'ten'),(20,'twenty'),(30,'thirty');
>>> CREATE PUBLICATION barium for TABLE numbers;
>>>
>>> To run the program use the following commands:
>>> mvn clean
>>> mvn package
>>> java -cp
>>> target/Lithium-1.0-SNAPSHOT.jar:/home/abbas/tmp/postgresql-42.2.14.jar
>>> com.edb.App
>>>
>>> If my analysis is correct, can we please have this issue fixed in the
>>> driver?
>>>
>>> Best Regards
>>>
>>
>
> I'll have a look, but we do attempt to handle it here
> https://github.com/pgjdbc/pgjdbc/blob/f89e62cdce4895f163dc95353cd31614347624b6/pgjdbc/src/main/java/org/postgresql/core/v3/replication/V3PGReplicationStream.java#L137
>

I had a look at that part of the code. Assuming that the server needs reply
only if replyRequired (last byte of the keep alive message) is 1 is
unfortunately not correct. Server is expecting a reply irrespective of the
replyRequired value. I have tested with libpq as well and I can confirm
that the last byte of the keep alive message should simply be ignored by
the client, and a reply must be sent to each keep alive message.

>
> Dave
>>>
>>
>

--
--
*Abbas*
Architect

Ph: 92.334.5100153
Skype ID: gabbasb
www.enterprisedb.co <http://www.enterprisedb.com/>m
<http://www.enterprisedb.com/>

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