💄 Surface better error when a publication is in an invalid state

Author: acco

docs/reference/databases.mdx

@@ -269,6 +269,34 @@ For Postgres 12 and 13, we do not support backfills, which rely heavily on the `
 
 If you require backfills, we recommend upgrading to Postgres 14 or later. If this is not an option, please [contact us](mailto:[email protected]) to discuss alternative approaches.
 
+## Known issues
+
+### Publication not recognized by replication slot
+
+When a publication is created *after* the replication slot, Postgres may report that the publication does not exist. Sequin will surface this as an “Issue with publication” health check.
+
+This is a [known Postgres issue](https://www.postgresql.org/message-id/18683-a98f79c0673be358%40postgresql.org).
+
+**Fix**: drop and re-create the replication slot *after* the publication exists:
+
+```sql
+-- Drop the slot
+SELECT pg_drop_replication_slot('sequin_slot');
+
+-- Re-create the slot
+SELECT pg_create_logical_replication_slot('sequin_slot', 'pgoutput');
+```
+
+**Prevention**: always create the publication first. If you need to change a publication later, alter it instead of dropping it:
+
+```sql
+-- Add or remove tables
+ALTER PUBLICATION sequin_publication ADD TABLE new_table;
+ALTER PUBLICATION sequin_publication DROP TABLE old_table;
+```
+
+See the full `ALTER PUBLICATION` documentation [here](https://www.postgresql.org/docs/current/sql-alterpublication.html).
+
 ## Monitoring
 
 Sequin will run health checks against your database connection automatically. You can view the status of the connection at any time in your database's "Overview" page.

lib/sequin/databases/databases.ex

@@ -388,7 +388,7 @@ defmodule Sequin.Databases do
   def verify_slot(%PostgresDatabase{} = database, %PostgresReplicationSlot{} = slot) do
     with_uncached_connection(database, fn conn ->
       with {:ok, _} <- Postgres.get_publication(conn, slot.publication_name),
-           {:ok, slot_info} <- Postgres.fetch_replication_slot(conn, slot.slot_name),
+           {:ok, slot_info} <- Postgres.get_replication_slot(conn, slot.slot_name),
            :ok <- validate_slot(database, slot_info) do
         Postgres.check_replication_permissions(conn)
       end

lib/sequin/health/health.ex

@@ -545,7 +545,10 @@ defmodule Sequin.Health do
         base_check
 
       connected_event.status == :fail ->
-        put_check_timestamps(%{base_check | status: :error, error: connected_event.error}, [
+        error_slug = Map.get(connected_event.error, :code)
+        error_slug = if is_binary(error_slug), do: String.to_existing_atom(error_slug), else: error_slug
+
+        put_check_timestamps(%{base_check | status: :error, error: connected_event.error, error_slug: error_slug}, [
           connected_event
         ])
 

lib/sequin/postgres/postgres.ex

@@ -459,8 +459,8 @@ defmodule Sequin.Postgres do
         "active" => active
       }
   """
-  @spec fetch_replication_slot(db_conn(), String.t()) :: {:ok, map()} | {:error, Error.t()}
-  def fetch_replication_slot(conn, slot_name) do
+  @spec get_replication_slot(db_conn(), String.t()) :: {:ok, map()} | {:error, Error.t()}
+  def get_replication_slot(conn, slot_name) do
     query = """
     select slot_name, active, database, slot_type
     from pg_replication_slots

lib/sequin/runtime/slot_processor_server.ex

@@ -907,29 +907,48 @@ defmodule Sequin.Runtime.SlotProcessorServer do
   defp on_connect_failure(%State{} = state, error) do
     conn = get_cached_conn(state)
 
-    error_msg =
-      case Postgres.fetch_replication_slot(conn, state.slot_name) do
-        {:ok, %{"active" => false}} ->
-          if is_exception(error) do
+    error_or_error_msg =
+      with {:ok, %{"active" => false}} <- Postgres.get_replication_slot(conn, state.slot_name),
+           {:ok, _pub} <- Postgres.get_publication(conn, state.publication) do
+        cond do
+          match?(%Postgrex.Error{postgres: %{code: :undefined_object, routine: "get_publication_oid"}}, error) ->
+            # Related to this: https://www.postgresql.org/message-id/18683-a98f79c0673be358%40postgresql.org
+            # Helpful error message shown in front-end.
+            Error.service(
+              service: :replication,
+              code: :publication_not_recognized,
+              message:
+                "Publication '#{state.publication}' is in an invalid state. You must drop and re-create the slot to use this publication with this slot."
+            )
+
+          is_exception(error) ->
             Exception.message(error)
-          else
-            inspect(error)
-          end
 
-        {:ok, %{"active" => true}} ->
+          true ->
+            inspect(error)
+        end
+      else
+        {:ok, %{"active" => true} = _slot} ->
           "Replication slot '#{state.slot_name}' is currently in use by another connection"
 
-        {:error, %Error.NotFoundError{}} ->
+        {:error, %Error.NotFoundError{entity: :replication_slot}} ->
           maybe_recreate_slot(state)
           "Replication slot '#{state.slot_name}' does not exist"
 
         {:error, error} ->
           Exception.message(error)
       end
 
+    error =
+      if is_binary(error_or_error_msg) do
+        Error.service(service: :replication, message: error_or_error_msg)
+      else
+        error_or_error_msg
+      end
+
     Health.put_event(
       state.replication_slot,
-      %Event{slug: :replication_connected, status: :fail, error: Error.service(service: :replication, message: error_msg)}
+      %Event{slug: :replication_connected, status: :fail, error: error}
     )
 
     :ok

lib/sequin_web/live/databases/show.ex

@@ -413,6 +413,26 @@ defmodule SequinWeb.DatabasesLive.Show do
     )
   end
 
+  defp maybe_augment_alert(%{error_slug: :publication_not_recognized} = check, database) do
+    publication_name = database.replication_slot.publication_name
+
+    # ""
+
+    Map.merge(
+      check,
+      %{
+        alertTitle: "Issue with publication",
+        alertMessage: """
+        There is an issue with publication `#{publication_name}`. The publication exists, but the replication slot does not recognize it. This is likely related to a [known issue](https://www.postgresql.org/message-id/18683-a98f79c0673be358%40postgresql.org) with Postgres. This issue occurs when you create a publication *after* creating a replication slot.
+
+        You'll need to drop and re-create your replication slot in order to use this publication with this slot.
+        """,
+        refreshable: false,
+        dismissable: false
+      }
+    )
+  end
+
   defp maybe_augment_alert(check, _database), do: check
 
   defp preload_database(database) do