The Dispatch Boundary
Part of a series on #80 — Extract P4 channel agent dispatch SPI to casehub-blocks. Previous: The First Blocks.
P1–P3 were utility extractions — move the class, update imports, done. P4 was the first one that forced a design question: where does the boundary between the reusable pattern and the app-specific wiring actually sit?
DraftHouse’s ChannelAgentDispatcher does three things: routes a sub-task request to the matching handler (first-match on handles() predicates), invokes an LLM via DebateAgentProvider, and posts the result back to the channel via MessageService. The routing loop is generic. The LLM invocation is generic. But the error dispatch — encoding a SUB_TASK_ERROR with the debate protocol’s metadata format and posting it via MessageService — is entirely DraftHouse-specific.
The first decision was the dependency boundary. MessageService lives in casehub-qhorus (runtime), but blocks only depends on casehub-qhorus-api. Taking a MessageService parameter would pull the entire qhorus runtime into blocks as a compile dependency. Instead, the dispatcher takes Consumer<MessageDispatch> for posting messages and Function<AgentTask, String> for LLM invocation. DraftHouse passes messageService::dispatch and debateAgentProvider::analyse as method references. The blocks class never sees the runtime types.
The second decision was error dispatch. The base class logs the error. DraftHouse overrides onError() to encode the debate protocol’s SUB_TASK_ERROR metadata header (entryType, subTaskId, taskType, agent) and post it to the channel. A future app — say, an AML case review — would override the same method with its own error format. The pattern is: blocks owns the routing loop; apps own the protocol encoding.
The CDI wrinkle was minor but predictable. Quarkus needs a no-args constructor to proxy @ApplicationScoped beans. The blocks base class is a plain library class, not a CDI bean — but subclasses will be. A protected no-args constructor in the base avoids the DeploymentException without exposing an invalid construction path to callers.
DraftHouse’s dispatcher went from 126 lines of self-contained logic to a 70-line subclass that wires debate-specific concerns onto a 90-line generic base. The six concrete handlers (VerifyHandler, ArbitrateHandler, etc.) didn’t change at all — they implement ChannelAgentHandler from blocks now, but the interface is identical. Twenty-six files touched, all import changes.