The consumer count came from roles_with_service, which builds dashboard card
targets and therefore requires a single canonical domain. web-app-matrix
declares domains.canonical as a mapping (synapse, element) and was dropped;
in a deployment where it was the only consumer the count reached zero, the
server started with -volume.max=7, the unnamed default collection took all
seven slots on first write, and every S3 PUT for the matrix collection failed
with "No writable volumes and no free volumes left". Synapse answered 500 on
media upload and no avatar ever reached the bucket. web-app-openwebui carries
the same mapping shape and the same latent failure.
Consumers are now enumerated by the binding itself: a role is a consumer when
its merged config binds the engine and marks it shared, whatever the role is
called and however many domains it serves. The S3 identity file and the volume
budget read that one list, so the identities granted write access and the
slots budgeted for their collections cannot drift apart.
Selection reads the merged applications config rather than meta/services.yml,
because an operator may bind a store through the applications inventory
variable alone; that path is covered by a test so a later optimisation cannot
quietly reintroduce a naming or declaration assumption.
kevinveenbirkenbach