The process that runs one broadcast group — the supervised owner of a single epidemic-broadcast context: its own mailbox, spanning-tree state and outstanding-lazy table. Group identity is the handler module, so each broadcast handler runs in its own group and independent gossip streams never share a tree or a mailbox. The public entry points for broadcasting and managing groups are in partisan_broadcast.

What the shell owns, and what it delegates

Tree construction and repair are delegated to a pluggable tree engine (partisan_broadcast_engine); the shell owns everything else — the gen_server process and mailbox, the membership poll, handler dispatch, the message transport, the periodic ticks, and the rolling-upgrade router. The engine is pure and returns actions the shell executes, so an alternate engine drops in with no change here.

Two engines exist: Plumtree (partisan_plumtree_engine, the default) and Thicket (partisan_thicket_engine). The shell drives an engine in one of two dispatch modes, read from the engine's dispatch_mode/0:

• typed — the shell decodes each inbound wire message and calls a Plumtree-shaped callback, passing the handler's verdicts (is this novel? is this i_have stale?). This is the default path.
• raw — the shell hands the engine whole wire messages via handle_message/2 and executes its deliver/fetch actions against the group's single handler. This is how a self-describing engine such as Thicket runs.

Non-blocking delivery

A handler may keep its heavy apply off the tree process: claim/2 runs on the tree process and does only the fast, atomic novelty check, while handle_broadcast/2 runs the apply in the handler's own process, delivered asynchronously. A slow apply then never blocks the tree or the other handlers. A handler that implements only merge/2 keeps the synchronous path.

Membership

The shell reads membership from a lock-free snapshot (partisan_membership), polling its version on each tick and reconciling the engine's peer sets when it changes, rather than subscribing to a synchronous event bus that would block the membership oracle.

Rolling upgrade

Partisan's own control-plane group keeps a fixed registered name, so membership keeps converging in a mixed-version cluster, and a compatibility router forwards legacy-addressed messages for application handlers to their per-group process.

Reading guide

start_link/2 and init/1 build the group and select the engine; the handle_cast clauses dispatch inbound wire messages to the engine (typed or raw); handle_info drives the lazy/repair and exchange ticks; run_engine/2 runs an engine callback and executes the actions it returns. Broadcasting and the debug queries are near the top.