# Cluster Infrastructure Refinement — Design

**Date**: 2026-03-16
**Component**: Cluster Infrastructure (`docs/requirements/Component-ClusterInfrastructure.md`)
**Status**: Approved — superseded in part: the keep-oldest SBR decision was replaced by the auto-down decision 2026-07-21 (`docs/plans/2026-07-21-auto-down-availability-decision.md`).

## Problem

The Cluster Infrastructure doc covered topology and failover behavior but lacked specification for the split-brain resolver strategy, seed node configuration, failure detection timing, and dual-node failure recovery.

## Decisions

### Split-Brain Resolver
- **Keep-oldest** strategy. The longest-running node stays active on partition; the younger node downs itself.
- Stable-after duration: 15 seconds — prevents premature downing during startup or transient instability.
- Quorum-based strategies rejected because they cause total cluster shutdown on any partition in a two-node cluster.

### Seed Node Configuration
- **Both nodes are seed nodes.** No startup ordering dependency. Whichever node starts first forms the cluster.

### Failure Detection Timing
- Heartbeat interval: **2 seconds**.
- Failure threshold: **10 seconds** (5 missed heartbeats).
- Total failover time: **~25 seconds** (10s detection + 15s stable-after + singleton restart).
- All values configurable. Defaults balance failover speed with stability.

### Dual-Node Recovery
- **Automatic recovery**, no manual intervention. First node up forms a new cluster from seed configuration.
- Site clusters rebuild from SQLite (deployed configs, S&F buffer). Alarm states re-evaluate from live data.
- Central cluster rebuilds from MS SQL. No message buffer state to recover.

## Affected Documents

| Document | Change |
|----------|--------|
| `docs/requirements/Component-ClusterInfrastructure.md` | Added 3 new sections: Split-Brain Resolution, Failure Detection Timing, Dual-Node Recovery. Updated Node Configuration to clarify both-as-seed. |

## Alternatives Considered

- **Static-quorum / keep-majority**: Rejected — both cause total cluster shutdown on partition in a two-node cluster. Unacceptable for SCADA availability.
- **Single designated seed node**: Rejected — creates startup ordering dependency for no benefit in a two-node cluster.
- **Manual recovery on dual failure**: Rejected — system already persists all state needed for automatic recovery.
- **Fast detection (1s/5s)**: Rejected — too sensitive; brief network hiccups would trigger unnecessary failovers and full actor hierarchy rebuilds.
- **Conservative detection (5s/30s)**: Rejected — 30 seconds of data collection downtime is too long for SCADA.
- **Shorter stable-after (10s)**: Rejected — matching the failure threshold risks downing nodes that are slow to respond (GC pause, heavy load).
