Hi all,
I am developing an application which connects to a logical replication slot, to consume the WAL events. These WAL
eventsare 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'tcalling `setAppliedLSN(stream.getLastReceiveLSN())` and/or `setFlushedLSN(stream.getLastReceiveLSN())` enough to
acknowledgean 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
resultof `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
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