8KB limit of Postgres NOTIFY

You can setup Rails ActionCable to use Postgres instead of Redis by adjusting the adapter configuration:

# config/cable.yml

production:
  adapter: postgresql
  url: <%= ENV["DATABASE_URL"] %>
  channel_prefix: myapp_production

This uses the Postgres LISTEN and NOTIFY commands.

As of writing, the NOTIFY payload is limited to 8000 bytes per default. If you're using Hotwire this may not always be enough and you will see error messages like this

PG::InvalidParameterValue (ERROR:  payload string too long)

The solutions available to you are the following:

• Configure Postgres for larger payloads (not possible on Heroku)
• Use a fallback strategy for larger payloads as implemented in the actioncable-enhanced-postgresql-adapter gem.
• Use Redis instead (with its own limits)