Since Mercure v0.21, it's now possible to disable deprecated transport behavior (or rather, enable it with thedeprecated_transport flag) (seedunglas/mercure#1105). By default, Mercure is built with this flag.

FrankenPHP is built without thedeprecated_transport flag. This causes theMERCURE_TRANSPORT_URL env to be ignored. The default transport isbolt:///bolt.db.

This behavior shouldn't affect most users. However, there are exceptions:

• Those who used theMERCURE_TRANSPORT_URL environment variable may find that its value has no effect.

• When running as an unprivileged user, the following container startup error will occur:

  Error: loading initial config: loading new config: loading frankenphp app module: provision frankenphp: failed to provision caddy http: loading http app module: provision http: server srv0: setting up route handlers: route 0: loading handler modules: position 0: loading module 'subroute': provision http.handlers.subroute: setting up subroutes: route 2: loading handler modules: position 2: loading module 'mercure': provision http.handlers.mercure: provision http.handlers.mercure.bolt: "": invalid transport: open bolt.db: permission denied

I see solutions to this problem:

• Stop using theMERCURE_TRANSPORT_URL env and start using themercure.transport directive (seehttps://github.com/dunglas/mercure/blob/c7cf56e647e87aa814331e5ebf6672cae1ed2a54/docs/UPGRADE.md#017)
• Add thedeprecated_transport flag when building FrankenPHP
• Make the deprecated Mercure transport behavior the default

I suggest the first option.

@dunglas@maxhelias