Where things run: placement and discovery

Two questions about location, and they are different questions:

Discovery may be stale and nothing depends on its being right. A request that reaches a host that no longer owns the actor is answered FENCED by the fence in the services chapter, and the ownership epoch is what makes that answer true. Discovery is an optimisation of where to look, not a source of safety.

Discovery

Discovery has an associated Resource (what is being asked about) and Location (what comes back), and one async method:

discover(what: Resource) -> Result<Vec<Location>, DiscoveryError>

Two implementations exist, and they answer at different scopes:

• LocalSupervisor answers for its own node only. It is asked about an ActorRef and returns its own host id if the actor is Running. A draining, retired, stopped or absent actor is NotFound, which means “not here”, not “nowhere”.
• QuicDirectory answers with a URL, from a record of which host each placed actor was put on. See the actor system.

Placement

Placement takes a resource and returns the hosts that should run it. The one scheduler is the LAVA scheduler:

• It is the placement, scoring and ordering of the LAVA paper: placement (§4.3), NILAS scoring (§4.2) and LARS ordering (§4.4).
• Time is caller-supplied, monotonic. Exits are explicit events. A predicted expiry never removes a job.
• It knows hosts, their capacity and whether each is schedulable. It places a job, can migrate one, and can begin draining a host once an approval gate covers every job on it.
• A host can be made unschedulable without removing it. That is what the actor system does for a host membership suspects.

Around it, in the same area:

• CRUSH placement, behind its own feature because it is a dependency of its own. It is a stub.
• Prequal is not placement of actors; it picks which replica gets a request, by asking a few replicas how they are doing and sending to the one that looks best now. It is a pure decision: no sockets, no clock, no thread.
• Monitoring holds the Vivaldi network coordinates and a node’s published, numbered resources. This book does not go into it.

Evidence

All of this is plain Rust, not verified. The scheduling code is compiled into the dependency-free build, but none of it is under Verus; the tests are the evidence. Of this area, only Id and IdSet carry proofs. This chapter makes no claim of the kind the protocol chapters do, and the protocols do not depend on placement being correct: a badly placed actor is slow or unavailable, never doubly owned.

Next: the actor system.