Runs several A2A server replicas that share one durable task store and one event stream, so any replica can serve any request for any task. This demonstrates the multi-server support in a2a.server.cluster: send, resubscribe, and cancel all work regardless of which replica a request lands on — no sticky routing needed.

• Shared, versioned task store (VersionedDatabaseTaskStore) — concurrent writes across replicas are serialized by optimistic concurrency (CAS); no lost updates.
• Shared event stream (DatabaseTaskEventStream) — a subscription on one replica streams events produced by an agent running on another.
• Cancel via CAS — a cancel on replica C stops an agent running on replica A (A's next save fails the compare-and-swap and aborts).

Nothing extra by default: it uses a file-backed SQLite database (/tmp/a2a_cluster_demo.db) shared by all replica processes.

For a realistic setup, point every replica at a shared Postgres/MySQL:

export A2A_CLUSTER_DSN='postgresql+asyncpg://user:pass@localhost/a2a'

Start a 3-replica cluster (ports 41241, 41242, 41243):

python -m samples.clustermode.run_cluster --replicas 3 --base-port 41241

In another terminal, exercise it (one task, spread across replicas):

python -m samples.clustermode.exercise --host 127.0.0.1 --ports 41241,41242,41243

Expected output ends with:

OK: one task was started, observed, and cancelled across three different replicas.

Every replica builds its handler the same way — the only multi-server-specific part is passing a shared event_stream and a shared VersionedTaskStore:

handler = DefaultRequestHandler(
    agent_executor=SlowEchoAgent(...),
    task_store=VersionedDatabaseTaskStore(engine=..., create_table=False),
    agent_card=agent_card,
    event_stream=DatabaseTaskEventStream(engine=..., create_table=False),
)

Omit event_stream (and use a plain InMemoryTaskStore) and you get the ordinary single-process behaviour — the multi-server path is fully opt-in.