# SQ-020: Cluster Membership (Gossip)

**Status:** `[ ] TODO`
**Blocked by:** SQ-009, SQ-019
**Priority:** Medium

## Description

Nodes discover each other via seed list and maintain a membership list through periodic heartbeats.

## Files to Create/Modify

- `crates/sq-cluster/src/lib.rs` - module exports
- `crates/sq-cluster/src/membership.rs` - seed list, join, heartbeat, failure detection
- `crates/sq-server/src/grpc/cluster.rs` - ClusterService Join/Heartbeat RPC impl
- `crates/sq-server/src/cli/serve.rs` - add --seeds CLI flag

## Configuration

```
SQ_SEEDS=node1:6060,node2:6060   # Seed node addresses
SQ_NODE_ID=node-1                 # Unique node ID
SQ_HEARTBEAT_INTERVAL_MS=5000    # Heartbeat every 5s
SQ_FAILURE_THRESHOLD=3           # Missed heartbeats before suspected
```

## Membership State Machine

```
Unknown -> Alive (on Join response or Heartbeat)
Alive -> Suspected (missed 3 heartbeats)
Suspected -> Dead (suspected for 30 seconds)
Dead -> Alive (on successful re-Join)
```

## Acceptance Criteria

- [ ] Start 3 nodes with seed list, all discover each other
- [ ] Status RPC shows all 3 nodes as "alive"
- [ ] Stop one node, others detect it as "suspected" then "dead"
- [ ] Restart dead node, it re-joins and becomes "alive"
- [ ] Node with no seeds starts as single-node cluster