Add TIL post about Postgresql 55000 error

Author: andreyuhai

_posts/2024-01-29-postgresql-55000-error.md

@@ -0,0 +1,26 @@
+---
+title: PostgreSQL 55000 error
+date: 2023-11-21
+tags: [til, postgresql]
+description: What was the root cause of PostgreSQL 55000 error in our case
+---
+
+We had a Kafka consumer which was crashing with the error below
+
+```elixir
+(Postgrex.Error ERROR 55000 (object_not_in_prerequisite_state) recovery is in progress
+
+    hint: WAL control functions cannot be executed during recovery.)
+```
+
+In that consumer module, there was a function which was eventually trying to get the LSN from the DB by querying something like
+
+```
+SELECT pg_last_wal_replay_lsn()::varchar
+```
+
+It turns out we were actually querying a replica where WAL was enabled, figured that out quite quickly by asking in #postgresql IRC.
+
+> 10:49 < Berge> Note that a replica (using WAL replication) is considered to be in recovery
+
+Thank you Berge! TIL!