Crate datum_cluster 

Cluster membership for Datum.

This crate is intentionally a membership layer only. It wraps foca 1.x as the private SWIM engine and keeps all foca identities, configuration, timers, and notifications out of the public API. The public model mirrors the useful Akka Cluster member states Datum needs before placement arrives: MemberState::Joining, MemberState::Up, MemberState::Leaving, MemberState::Exiting, MemberState::Down, and MemberState::Removed.

The implementation is split into two planes. The membership actor owns the registry, state transitions, Signal current-state publication, lossless Subscription event publication, and downing decisions. The gossip plane is one Tokio task that owns the UDP socket and foca instance; it receives datagrams, drains foca runtime sends/timers/notifications, and never spawns an actor per packet.

Current-state reads use Signal<ClusterState> because operators usually want the latest immutable view without replay. Membership changes use Subscription<MemberEvent> because active subscribers must observe every accepted state transition in sequence. Subscription is bounded and backpressures the publisher instead of dropping membership events.

Failure detection and downing are deliberately separate. Foca suspicion marks a member unreachable; a DowningProvider decides when that unreachable member becomes MemberState::Down. The default TimeoutDowning is simple and useful for tests and small deployments, but it is not a split-brain resolver: v0.10 has no quorum, lease, or majority strategy. A split-brain resolution provider is a named follow-up before placement can make strong singleton or sharding claims.

C3 placement uses a deterministic coordinator convention rather than an election protocol: every node picks the oldest reachable Up member from its local C1 view, tie-broken by node id. C1 does not assign Akka up numbers, so v0.10 uses the propagated member incarnation as the age key. This is deterministic for a shared view, but it is not fencing: under a partition plus timeout downing, both sides can transiently have a coordinator. Quorum or lease fencing is a v0.11+ follow-up.

UDP is the v0.10 gossip transport. QUIC plus mTLS is deferred for the control plane and future secure gossip transport; the public API is transport-neutral so that change does not expose foca internals.

Structs

ClusterConfig

Configuration for one Datum cluster member.

ClusterNode

Running Datum cluster membership node.

ClusterState

Immutable current membership snapshot.

Member

Datum-owned member record. No foca types appear in this model.

MemberEvent

Ordered member event.

Signal

Latest-value state cell with a lock-free synchronous snapshot and a coalesced change feed.

Subscription

Latest-value state cell with a bounded every-change feed.

TimeoutDowning

Down a member after it has remained unreachable for a fixed timeout.

Enums

ClusterError

Errors returned by datum-cluster.

MemberEventKind

Kind of MemberEvent.

MemberState

Akka-style member lifecycle state used by Datum membership.

Constants

VERSION

The datum-cluster crate version.

Traits

DowningProvider

Pluggable policy that decides when an unreachable member is down.

Type Aliases

ClusterResult

Result alias for datum-cluster.