Monitoring and Observability for Caches: Tools, Metrics, and Alerts

Caches hide complexity—until they fail. Observability gives you the signals needed to maintain healthy cache performance, diagnose regressions, and avoid outages. This article lists the must-have metrics, sensible alert thresholds, and tooling patterns for cache observability.

"You can’t fix what you can’t measure. Cache instrumentation is essential for reliability and performance tuning."

Essential Metrics

• Cache Hit Ratio: Percentage of requests served from cache. Track by resource type (static vs API) and region.
• Origin Request Rate: Number of requests hitting origin per second. Spikes here often indicate cache misses or purge storms.
• TTFB: Time to First Byte for cached vs origin-served responses.
• Purge/Invalidation Latency: Time for a purge to propagate to edge PoPs.
• Eviction Rates: How often objects are being evicted due to memory pressure.
• Stale Responses Served: Count of times stale data was served (if you support stale-while-revalidate).

Useful Derived Metrics

• Cache Efficiency: Ratio of bytes served from cache vs bytes requested; helps measure bandwidth savings.
• Cost per Request: Combine egress costs and origin compute to understand cost impact.
• Cache Warmth: Hit ratio over a rolling window after deploys or cache clears.

Alerting Guidelines

• Alert on sustained drops in cache hit ratio (e.g., >20% drop sustained for 5 minutes).
• Alert on unexpected origin request rate spikes that exceed normal baselines.
• Alert when purge propagation latency exceeds a threshold for your SLA (e.g., >60 seconds in critical regions).
• Alert on elevated eviction rates coupled with high memory usage.

Tracing and Distributed Context

Propagate a correlation id through requests to tie origin logs to cache behavior. Distributed tracing helps identify if a request was served by cache, validated with the origin, or fetched from origin—making root cause analysis straightforward.

Tools and Platforms

Consider the following:

• Prometheus + Grafana for time series metrics and dashboards.
• OpenTelemetry for traces and context propagation.
• CDN-native dashboards for edge-specific metrics like PoP-level hit ratios and purge metrics.
• Log aggregation for detailed origin logs and conditional request analysis.

Dashboards You Need

1. Global cache hit ratio by resource type.
2. Origin request rate and response times.
3. PoP-level heatmap for hit ratios and latency.
4. Purge latency over time and last purge timestamps.

Playbook Examples

When hit ratio drops in a region:

1. Check recent deploys or purge events.
2. Inspect Vary headers and Cookie usage to ensure personal data hasn’t accidentally been cached.
3. Compare TTFB for cached vs origin to confirm cache miss patterns.
4. If purges occurred, validate purge success and propagation logs.

Final Thoughts

Observability transforms cache management from guesswork into a measurable discipline. Combine metrics, traces, logs, and healthy alerting to keep your caches performing predictably. Investing in observability pays dividends in both reliability and cost optimization.