handling different versions of Postgres of unit

src/charm.py

@@ -92,6 +92,7 @@
 
 PRIMARY_NOT_REACHABLE_MESSAGE = "waiting for primary to be reachable from this unit"
 EXTENSIONS_DEPENDENCY_MESSAGE = "Unsatisfied plugin dependencies. Please check the logs"
+DIFFERENT_VERSIONS_PSQL_BLOCKING_MESSAGE = "Please select the correct version of postgresql to use. No need to use different versions of postgresql."
 
 Scopes = Literal[APP_SCOPE, UNIT_SCOPE]
 
@@ -519,6 +520,8 @@ def _on_peer_relation_changed(self, event: HookEvent):
 
         self._update_new_unit_status()
 
+        self._validate_database_version()
+
     # Split off into separate function, because of complexity _on_peer_relation_changed
     def _start_stop_pgbackrest_service(self, event: HookEvent) -> None:
         # Start or stop the pgBackRest TLS server service when TLS certificate change.
@@ -1577,6 +1580,18 @@ def client_relations(self) -> List[Relation]:
                 relations.append(relation)
         return relations
 
+    def _validate_database_version(self):
+        """Checking that only one version of Postgres is used."""
+        peer_db_version = self.app_peer_data.get("database-version")
+
+        if self.unit.is_leader() and peer_db_version is None:
+            self.app_peer_data.update({"database-version": self._patroni.get_postgresql_version()})
+            return
+
+        if peer_db_version != self._patroni.get_postgresql_version():
+            self.unit.status = BlockedStatus(DIFFERENT_VERSIONS_PSQL_BLOCKING_MESSAGE)
+        return
+
 
 if __name__ == "__main__":
     main(PostgresqlOperatorCharm)

tests/integration/ha_tests/test_self_healing.py

@@ -578,6 +578,8 @@ async def test_deploy_zero_units(ops_test: OpsTest, charm):
     logger.info("connect to DB and create test table")
     await create_test_data(connection_string)
 
+    # TODO: implement a test using the store with a different version of Postgres
+
     unit_ip_addresses = []
     primary_storage = ""
     for unit in ops_test.model.applications[app].units: