409cc62309
Cluster Infrastructure: add min-nr-of-members=1 requirement for single-node operation after failover. Add graceful shutdown / CoordinatedShutdown section for fast singleton handover during planned maintenance. Site Runtime: add explicit supervision strategies per actor type (Resume for coordinators, Stop for short-lived execution actors). Stagger Instance Actor startup to prevent reconnection storms. Add Tell-vs-Ask usage guidance per Akka.NET best practices (Tell for hot path, Ask for system boundaries only). Data Connection Layer: add Connection Actor Model section documenting the Become/Stash pattern for connection lifecycle state machine. Health Monitoring: add dead letter count as a monitored metric. Host: add REQ-HOST-8a for dead letter monitoring (subscribe to EventStream, log at Warning level, report as health metric).
76 lines
4.6 KiB
Markdown
76 lines
4.6 KiB
Markdown
# Component: Health Monitoring
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## Purpose
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The Health Monitoring component collects and reports operational health metrics from site clusters to the central cluster, providing engineers with visibility into the status of the distributed system.
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## Location
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Site clusters (metric collection and reporting). Central cluster (aggregation and display).
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## Responsibilities
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### Site Side
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- Collect health metrics from local subsystems.
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- Periodically report metrics to the central cluster via the Communication Layer.
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### Central Side
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- Receive and store health metrics from all sites.
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- Detect site connectivity status (online/offline) based on heartbeat presence.
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- Present health data in the Central UI dashboard.
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## Monitored Metrics
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| Metric | Source | Description |
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|--------|--------|-------------|
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| Site online/offline | Communication Layer | Whether the site is reachable (based on heartbeat) |
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| Active/standby node status | Cluster Infrastructure | Which node is active, which is standby |
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| Data connection health | Data Connection Layer | Connected/disconnected/reconnecting per data connection |
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| Tag resolution counts | Data Connection Layer | Per connection: total subscribed tags vs. successfully resolved tags |
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| Script error rates | Site Runtime (Script Actors) | Frequency of script failures |
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| Alarm evaluation error rates | Site Runtime (Alarm Actors) | Frequency of alarm evaluation failures |
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| Store-and-forward buffer depth | Store-and-Forward Engine | Pending messages by category (external, notification, DB write) |
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| Dead letter count | Akka.NET EventStream | Messages sent to actors that no longer exist — indicates stale references or timing issues |
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## Reporting Protocol
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- Sites send a **health report message** to central at a configurable interval (default: **30 seconds**).
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- Each report is a **flat snapshot** containing the current values of all monitored metrics, a **monotonic sequence number**, and the **report timestamp** from the site. Central replaces the previous state for that site only if the incoming sequence number is higher than the last received — this prevents stale reports (e.g., delayed in transit or from a pre-failover node) from overwriting newer state.
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- **Offline detection**: If central does not receive a report within a configurable timeout window (default: **60 seconds** — 2x the report interval), the site is marked as **offline**. This gives one missed report as grace before marking offline.
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- **Online recovery**: When central receives a health report from a site that was marked offline, the site is automatically marked **online**. No manual acknowledgment required — the metrics in the report provide immediate visibility into the site's condition.
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## Error Rate Metrics
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Script error rates and alarm evaluation error rates are calculated as **raw counts per reporting interval**:
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- The site maintains a counter for each metric that increments on every failure.
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- Each health report includes the count since the last report. The counter resets after each report is sent.
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- Central displays these as "X errors in the last 30 seconds" (or whatever the configured interval is).
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- **Script errors** include all failures that prevent a script from completing successfully: unhandled exceptions, timeouts, recursion limit violations, and any other error condition.
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- **Alarm evaluation errors** include all failures during alarm condition evaluation.
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- For detailed diagnostics (error types, stack traces, affected instances), operators use the **Site Event Log Viewer** — the health dashboard is for quick triage, not forensics.
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## Central Storage
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- Health metrics are held **in memory** at the central cluster for display in the UI.
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- No historical health data is persisted — the dashboard shows current/latest status only.
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- Site connectivity history (online/offline transitions) may optionally be logged via the Audit Log or a separate mechanism if needed in the future.
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## No Alerting
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- Health monitoring is **display-only** for now — no automated notifications or alerts are triggered by health status changes.
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- This can be extended in the future.
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## Dependencies
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- **Communication Layer**: Transports health reports from sites to central.
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- **Data Connection Layer (site)**: Provides connection health metrics.
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- **Site Runtime (site)**: Provides script error rate and alarm evaluation error rate metrics.
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- **Store-and-Forward Engine (site)**: Provides buffer depth metrics.
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- **Cluster Infrastructure (site)**: Provides node role status.
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## Interactions
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- **Central UI**: Health Monitoring Dashboard displays aggregated metrics.
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- **Communication Layer**: Health reports flow as periodic messages.
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