more postgres replication

_posts/2023-12-08-debezium.md

@@ -174,12 +174,25 @@ the latest insert lsn. So in this case, Debezium starts streaming changes from
 latest lsn.
 
 When Debezium reconnect, it tries to look up the offset checkpoint from Kafka.
-If found, it uses the offset to start replication streaming. See
+If found, it uses it to start replication streaming. See
 [code](https://github.com/debezium/debezium/blob/493e1e23b0633a4e4d990e43733e249343599af5/debezium-connector-postgres/src/main/java/io/debezium/connector/postgresql/PostgresStreamingChangeEventSource.java#L143-L143).
-If not found, for example, manually deleted, then it uses the lsn of
-replication slot. See
+However, on Postgres server side, it uses
+`max(confirmed_flush_lsn of slot, user passed in lsn)`. So it is not guaranteed
+that the lsn provided by Debezium will take effect. On the other hand, if not
+found, for example, manually deleted, then it uses the lsn of the replication
+slot. See
 [code](https://github.com/debezium/debezium/blob/493e1e23b0633a4e4d990e43733e249343599af5/debezium-connector-postgres/src/main/java/io/debezium/connector/postgresql/connection/PostgresReplicationConnection.java#L341-L341).
 
+#### How should we move the offset forward?
+
+It may happen that there is a burst of database updates and we want to skip
+them. Then we need a way to reset the offset to the latest lsn. A simple
+solution would be
+
+1. Stop Debezium.
+2. Delete replication slot `select pg_drop_replication_slot('xxxx');`
+3. Start Debezium.
+
 ### Source Structure
 
 Every Kafka message has a "source" field as below.
@@ -217,5 +230,8 @@ concatenating the two parts and converted to decimal. For example:
 C0/F72DAA60 => echo $((16#C0F72DAA60)) = 828780685920
 ```
 
+Note, the second part should take 8 bytes. For example, for `1/190D7D0` we
+should patch it to `1/0190D7D0`.
+
 The Java implementation is
 [here](https://github.com/pgjdbc/pgjdbc/blob/e33be5c0481c22f4242a5d7ef2d2c09c8a17179f/pgjdbc/src/main/java/org/postgresql/replication/LogSequenceNumber.java#L44-L44).

_posts/2024-08-25-postgres-replication.md

@@ -252,8 +252,8 @@ client, and stops after receiving a `CopyDone` message from the client. When
 there is no new WALs, the server process enters a
 [wait state](https://github.com/postgres/postgres/blob/a3e6c6f929912f928fa405909d17bcbf0c1b03ee/src/backend/replication/walsender.c#L2886).
 This wait is implemeted using epoll or kevent depending on the underlying OS,
-and it waits for two types of events: socket event from the client and
-conditional variable events on new WAL. Every time when a new WAL is flushed to
+and it waits for two types of events: socket events from the client and
+conditional variable events of new WAL. Every time when a new WAL is flushed to
 disk, this conditional variable is woke up. See
 [code](https://github.com/postgres/postgres/blob/a3e6c6f929912f928fa405909d17bcbf0c1b03ee/src/backend/access/transam/xlog.c#L2499).
 Note, many processes are running at the same time ^\_^.
@@ -267,9 +267,30 @@ $ ps -ef | grep -i postgres
 ```
 
 One special note about the start `lsn`. When the start `lsn` provided in the
-`START_REPLICATION` command is older than slot's `confirmed_flush_lsn`, then it
-is reset to `confirmed_flush_lsn`. See
+`START_REPLICATION` command is older than slot's `confirmed_flush_lsn`, it is
+reset to `confirmed_flush_lsn`. See
 [code](https://github.com/postgres/postgres/blob/a3e6c6f929912f928fa405909d17bcbf0c1b03ee/src/backend/replication/logical/logical.c#L567).
+When the start `lsn` is newer than slot's `confirmed_flush_lsn`, then WALs
+between `[confirmed_flush_lsn, provided lsn)` are ignored. The subtlety is that
+the walsender process still processes the WALs in range
+`[confirmed_flush_lsn, provided lsn]`, but these records are thrown away in
+streaming. The core code is
+[this function](https://github.com/postgres/postgres/blob/a3e6c6f929912f928fa405909d17bcbf0c1b03ee/src/backend/replication/logical/snapbuild.c#L433).
+The corresponding stack path is
+
+```
+PostgresMain
+  -> exec_replication_command
+    -> WalSndLoop
+      -> XLogSendLogical
+        -> ogicalDecodingProcessRecord
+          -> heap_decode
+            -> SnapBuildProcessChange
+              -> ReorderBufferSetBaseSnapshot
+                -> AssertTXNLsnOrder
+                  -> SnapBuildXactNeedsSkip
+```
+
 Also, when creating a new replication slot by
 `pg_create_logical_replication_slot`, it will set the `confirmed_flush_lsn` to
 `pg_current_wal_insert_lsn()`. See
@@ -294,7 +315,7 @@ postgres=# select pg_current_wal_insert_lsn();
  0/190D798
 ```
 
-Also, during the while loop, the client sends
+During the while loop, the client sends
 [Standby Status Update](https://github.com/postgres/postgres/blob/a3e6c6f929912f928fa405909d17bcbf0c1b03ee/src/bin/pg_basebackup/pg_recvlogical.c#L144)
 messages to the server to inform the written and flushed `lsn`. On the server
 side, the corresponding `lsn` in-memory fields are