Forbid Provides Boundary

Intent

Show a first-class structural relation check: two components declare kind provides hyperedges into the same resource, and a forbid provides ... except rule rejects every provider except the allowed one.

Model

Matches fixtures/fail/forbid_provides_boundary/gateway.shape:

module rules

resource JsonRpcEndpoint

component Gateway {
}

component PublicApi {
}

relation GatewayProvidesRpc {
  kind provides
  connects Gateway -> JsonRpcEndpoint
}

relation PublicApiProvidesRpc {
  kind provides
  connects PublicApi -> JsonRpcEndpoint
}

rule gateway_only_rpc_ingress {
  forbid provides JsonRpcEndpoint except Gateway
}

Expected result

Save the model (for example shape/gateway.shape) and run shp check shape/gateway.shape. In this repository the same model is:

shp check fixtures/fail/forbid_provides_boundary/gateway.shape

error: forbidden provides

PublicApi provides JsonRpcEndpoint via relation PublicApiProvidesRpc.
rule gateway_only_rpc_ingress forbids provides JsonRpcEndpoint except Gateway.

caused by:
  - fixtures/fail/forbid_provides_boundary/gateway.shape: relation PublicApiProvidesRpc
  - fixtures/fail/forbid_provides_boundary/gateway.shape: rule gateway_only_rpc_ingress forbids provides JsonRpcEndpoint

Exit code 1.

GatewayProvidesRpc is allowed. PublicApiProvidesRpc is the offending hyperedge.

Inspect the hypergraph

shp graph all --kind provides fixtures/fail/forbid_provides_boundary/gateway.shape

Hypergraph

provides:
  provides GatewayProvidesRpc: Gateway (component) -> JsonRpcEndpoint (resource)
  provides PublicApiProvidesRpc: PublicApi (component) -> JsonRpcEndpoint (resource)

Both provides hyperedges terminate at JsonRpcEndpoint. Removing or redirecting PublicApiProvidesRpc, or widening the rule’s except list, is what makes the model pass.

Why it fails

provides relations are structural claims, not component-body dependencies. The rule filters the hypergraph for providers of JsonRpcEndpoint and rejects any component outside the exception list.