ADR-0208: System extensions carry what the SDK does not

• Status: accepted
• Date: 2026-10-20
• Deciders: Mahdi Amirabdollahi

Context

ADR-0203 moved every data path onto OpenBroker and left some shared features setting up Artemis machinery through system.artemis: message capture and plugin taps, transfer staging and its acceptance probe, flow's route sampling, and queue, address and divert lifecycle. Lifecycle is plain SDK work (Operations on a kind), but a tap, a staging area, an acceptance check and a route sample are Rahdar compositions that each system builds differently.

Decision

• Rahdar's per-system interfaces live in platform.broker and extend SystemExtension, which names its system: MessageTaps, TransferStaging, TransferAcceptance, FlowSampler, and the existing subscription counters, message expiry and dead-letter destinations. Each system implements them in system.<id>; SystemExtensions picks the one for a cluster's system.
• Lifecycle runs on Operations.preview and execute, and routing reads on Resources, with no extension.
• Only system.artemis, app, broker configuration and setup review reach system.artemis; BoundaryRulesTest enforces it.
• A cluster whose system has no implementation answers the feature as unavailable, with the reason.

Consequences

• A new system adds implementations, not branches in shared features.
• Wire traffic is unchanged for the moved code; lifecycle on the SDK adds per-node reads before it acts.

Alternatives considered

• Taps and staging in OpenBroker: they are Rahdar's product policy (names, ring sizes, provenance), not a broker vocabulary every SDK user needs.