swactor/docs/distribution.md

DistributedNode

DistributedNode is the top-level integration type that composes SWIM membership, Kademlia routing, the actor directory, location cache, and repair infrastructure into a single public API.

Internal Composition

┌─ DistributedNode ────────────────────────────────────────────────────────┐
│                                                                          │
│  keypair: Keypair             ← ed25519 identity + signing               │
│                                                                          │
│  ┌── SWIM ─────────────────┐  ┌── Kademlia ────────────────────────────┐ │
│  │  SwimNode                │  │  RoutingTable  (256 k-buckets, K=20)  │ │
│  │    ├ MemberList (CRDT)   │  │  DirectoryShard  (local entries)      │ │
│  │    ├ SwimProbe (FSM)     │  │  RepairQueue  (death-triggered)       │ │
│  │    └ DisseminationQueue  │  │  RepublishTracker  (periodic)         │ │
│  └──────────────────────────┘  └────────────────────────────────────────┘ │
│                                                                          │
│  cache: LocationCache         ← LRU: ActorAddress → NodeId              │
│  tick_count: u64              ← monotonic clock                          │
│                                                                          │
└──────────────────────────────────────────────────────────────────────────┘

Node Lifecycle

  new(config)
      │
      v
  Generate Keypair ──► NodeId = public key bytes
      │
      v
  Initialize subsystems (SwimNode, RoutingTable, DirectoryShard, ...)
      │
      v
  join(seeds) ──► SendJoinRequest to each seed
      │
      v
  handle_join_response(members) ──► populate MemberList + RoutingTable
      │
      v
  ┌─────────────┐
  │  tick() loop │ ──► drives SWIM probes, processes membership changes,
  │  (caller)    │     runs periodic republish, drains repair queue
  └─────────────┘
      │
      v
  leave() ──► disseminate Dead for self, graceful shutdown

See distribution_minor_flows.svg for the join handshake, dissemination piggybacking, and membership change cascade.

Actor Registration

register_actor(addr, generation):

1. Sign a DirectoryEntry with the node's keypair.
2. Store in local DirectoryShard.
3. Insert into LocationCache (self is the host).
4. Register with RepublishTracker for periodic re-STORE.
5. Return the signed entry — caller STOREs to r-closest nodes.

See actor_resolution.svg for the full datapath.

Actor Resolution

resolve_actor(addr) implements a 3-tier lookup chain:

1. LRU cache — instant, O(1).
2. Local directory shard — checks entries this node holds.
3. Kademlia lookup — returns NeedsLookup with closest known nodes; caller drives iterative FIND_VALUE + quorum read.

On delivery failure, invalidate_cache(addr) evicts stale entries.

Membership Change Cascade

When SWIM detects a state change, handle_membership_change(node_id, state) propagates effects through all subsystems:

State	Actions
Alive	Insert/update in RoutingTable
Suspect	Keep in RoutingTable (could downprioritize)
Dead	Remove from RoutingTable, invalidate cache entries for that node, queue affected directory entries in RepairQueue

This cascade ensures that a single SWIM death detection triggers routing table cleanup, cache invalidation, and directory repair in one tick.

Where Things Live

Type	File	Role
DistributedNode	node.rs	Top-level integration facade
DistributedNodeConfig	node.rs	Node configuration
ResolveResult	node.rs	3-tier resolution outcomes
LocationCache	cache.rs	LRU actor→node cache
Keypair	crypto.rs	Ed25519 keypair + signing
NodeId	types.rs	32-byte node identity
MemberState	types.rs	Alive / Suspect / Dead
DirectoryEntry	types.rs	Signed actor→node binding
NodeRecord	types.rs	Wire-format membership record