Re: [GENERAL] JDBC: logical replication and LSN feedback - Mailing list pgsql-general

Sent: Tuesday, September 19, 2017 at 4:14 PM
From: "Achilleas Mantzios" <achill@matrix.gatewaynet.com>
To: pgsql-general@postgresql.org
Subject: Re: [GENERAL] JDBC: logical replication and LSN feedback

On 19/09/2017 16:37, Yason TR wrote:
> Hi all,
>
> I am developing an application which connects to a logical replication slot, to consume the WAL events. These WAL events are then forwarded to a MQ broker.
>
> The heart of the code can be seen as:
>
> while (true) {
> Connection connection = null;
> PGReplicationStream stream = null;
>
> try {
> connection = DriverManager.getConnection("jdbc:postgresql://localhost:5432/db", properties);
> stream = connection.unwrap(PGConnection.class).getReplicationAPI().replicationStream().logical().withSlotName("slot").start();
>
> while (true) {
> final ByteBuffer buffer = stream.read();
>
> // ... MQ logic here ... omitted ...
>
> stream.setAppliedLSN(stream.getLastReceiveLSN());
> stream.setFlushedLSN(stream.getLastReceiveLSN());
> }
> } catch (final SQLException e) {
> // ... log exception ... omitted ...
> } finally {
> // ... close stream and connection ... omitted ...
> }
> }
>
> I notice some behavior which I cannot explain and would like to understand so I can alter my code:
>
> - When I restart the application, I notice that the application is retrieving the last event from the previous run again. The result is that this event is sent twice to the MQ broker after a restart of the application. Why is that? Isn't calling `setAppliedLSN(stream.getLastReceiveLSN())` and/or `setFlushedLSN(stream.getLastReceiveLSN())` enough to acknowledge an event, so it will removed from the WAL log and it will not be resent?
>
> - When receiving an event, the corresponding LSN from that event (which is sent in the payload) is not the same as the result of `stream.getLastReceivedLSN()`. Why is that? Which one should I use? Maybe this is correlated to my first question.
>
> - What is the difference between `setAppliedLSN(LSN)` and `setFlushedLSN(LSN)`? The Javadocs are not really helpful here.

The stages of a wal location generally go like : sent -> write -> flush -> replay , at least in terms of physical replication.
I guess applied=replayed ?

Note that from the docs : https://jdbc.postgresql.org/documentation/head/replication.html#logical-replication
it says :
"
In the event that replication has been restarted, it's will start from last successfully processed LSN that was sent via feedback to database.
"

>
> FYI, I also asked this question on https://stackoverflow.com/questions/46301578/postgres-jdbc-logical-replication-lsn-feedback.
>
> Thanks a lot and kind regards,
>
> Yason TR
>
>

--
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt



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