JDBC: logical replication and LSN feedback

["Yason TR" <yason.tr@xxxxxxx>]

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.

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


-- 
Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general