Local Docker: server-side realtime.send() broadcasts not delivered (missing replication slot)

[Sallvainian]

Bug Report

Describe the bug

Server-side broadcasts via realtime.send() (called from SQL functions/triggers/RPCs) silently fail in local Supabase Docker. Messages are inserted into realtime.messages but never delivered to WebSocket clients.

Client-side broadcasts via channel.send() work correctly.

Originally filed as supabase/cli#4914 — redirected here per maintainer feedback.

Environment

• Supabase CLI: 2.75.0
• Realtime image: public.ecr.aws/supabase/realtime:v2.73.2
• Platform: macOS (Darwin 25.4.0)
• PostgreSQL wal_level: logical

To Reproduce

1. supabase start
2. Subscribe to a broadcast channel from a browser client:

   const channel = supabase.channel('my-channel', {
     config: { broadcast: { self: false }, private: true }
   });
   channel.on('broadcast', { event: 'my_event' }, (msg) => {
     console.log('Received:', msg);
   });
   channel.subscribe();

3. Call realtime.send() from SQL (e.g., inside an RPC):

   PERFORM realtime.send(
     '{"key": "value"}'::jsonb,
     'my_event',
     'my-channel',
     true
   );

4. Expected: Client receives the broadcast via WebSocket
5. Actual: Message is inserted into realtime.messages table but never delivered. No errors raised.

Root Cause Investigation

The local Docker setup has:

• ✅ supabase_realtime_messages_publication exists with correct partitioned tables
• ✅ wal_level = logical
• ✅ realtime.send() inserts into realtime.messages successfully (218+ rows accumulated)
• ❌ pg_replication_slots returns 0 rows — no consumer exists for the publication

The Realtime service (v2.73.2) doesn't create a replication slot to consume from supabase_realtime_messages_publication, so inserts into realtime.messages are never picked up and delivered to WebSocket subscribers.

Verification queries

-- Shows 0 rows — no replication slot exists
SELECT slot_name, plugin, active FROM pg_replication_slots;

-- Publication exists with correct tables
SELECT schemaname, tablename 
FROM pg_publication_tables 
WHERE pubname = 'supabase_realtime_messages_publication';

-- Messages are accumulating but never consumed
SELECT count(*) FROM realtime.messages;

Impact

• Any project using server-side realtime.send() (from triggers, RPCs, background jobs) will silently fail in local development
• The function appears to succeed (no errors), making this extremely hard to debug
• Client-side channel.send() works fine, masking the issue when some events use client-side and others use server-side delivery

Workaround

Use client-side channel.send() after RPC success instead of server-side realtime.send() within the RPC.