# Communication Layer Refinement — Design

**Date**: 2026-03-16
**Component**: Central–Site Communication (`docs/requirements/Component-Communication.md`)
**Status**: Approved — transport decisions superseded by the ClusterClient→gRPC migration (`docs/plans/2026-07-22-clusterclient-to-grpc-plan.md`); the "no buffering at central" decision still stands.

## Problem

The Communication Layer doc defined 8 message patterns clearly but lacked specification for timeouts, transport configuration, reconnection behavior, message ordering guarantees, and connection failure handling.

## Decisions

### Message Timeouts
- **Per-pattern timeouts with sensible defaults**, overridable in configuration.
- Deployment and system-wide artifacts: 120 seconds (script compilation can be slow).
- Lifecycle commands, integration routing, recipe/command delivery, remote queries: 30 seconds.
- Uses the Akka.NET ask pattern; timeout results in failure to caller.

### Transport Configuration
- **Akka.NET built-in reconnection** with explicitly configured transport heartbeat interval and failure detection threshold.
- No custom reconnection logic — framework handles it.
- Settings explicitly documented rather than relying on framework defaults, for predictability in a SCADA context.

### Connection Failure Behavior
- **In-flight messages get a timeout error** — caller retries manually. No buffering at central. Consistent with existing design principle.
- Automatic retry rejected due to risk of duplicate processing (e.g., site may have applied a deployment before the connection dropped).

### Message Ordering
- **Per-site ordering guaranteed** — relies on Akka.NET's built-in per-sender/per-receiver ordering. No custom sequencing logic needed.

### Debug Stream Interruption
- **Stream dies on any disconnect** (failover or network blip). Engineer reopens the debug view manually.
- Auto-resume rejected — adds complexity for a transient diagnostic tool.

## Affected Documents

| Document | Change |
|----------|--------|
| `docs/requirements/Component-Communication.md` | Added 4 new sections: Message Timeouts, Transport Configuration, Message Ordering, Connection Failure Behavior |

## Alternatives Considered

- **Global timeout for all patterns**: Rejected — deployment involves compilation and needs more time than a simple query.
- **Default Akka.NET transport settings**: Rejected — relying on undocumented defaults is risky for SCADA; explicit configuration ensures predictable behavior.
- **Automatic retry of in-flight messages**: Rejected — risks duplicate processing and contradicts the no-buffering-at-central principle.
- **No ordering guarantee**: Rejected — Akka.NET provides this for free; the design already implicitly relies on it.
- **Auto-resume debug streams on reconnection**: Rejected — adds state tracking complexity for a transient diagnostic feature.
