From: | "Yason TR" <yason(dot)tr(at)gmx(dot)com> |
---|---|
To: | pgsql-general(at)postgresql(dot)org |
Subject: | JDBC: logical replication and LSN feedback |
Date: | 2017-09-19 13:37:31 |
Message-ID: | trinity-3e0e6193-3a88-442e-9b4f-1366a43c68f3-1505828251806@3c-app-mailcom-bs15 |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-general pgsql-jdbc |
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
From | Date | Subject | |
---|---|---|---|
Next Message | Job | 2017-09-19 13:47:16 | Insert large number of records |
Previous Message | Yason TR | 2017-09-19 13:34:28 |
From | Date | Subject | |
---|---|---|---|
Next Message | Achilleas Mantzios | 2017-09-19 14:14:16 | Re: JDBC: logical replication and LSN feedback |
Previous Message | Dave Cramer | 2017-09-18 20:50:02 | Re: Properties, defaults, PR 900, etc |