Metrics vs Logs vs Traces: A 2026 Decision Guide

Each telemetry type has a sweet spot. The decision tree that picks the right type per use case, with cost and cardinality trade-offs.

When metrics win

Metrics are the right tool for aggregate, low-cardinality questions. Pre-aggregated values are cheap to store and fast to query; the trade is loss of per-event detail.

• Aggregate questions. "How is the service performing overall?" Pre-aggregated time series answer in milliseconds at fixed cost.
• Cardinality is the limit. Bounded label sets stay cheap; user_id or request_id as labels explodes cost; push those to logs.
• SLO surface. Latency, error-rate, saturation gauges; the metric is the SLO; alerting fires from the same series.
• Dashboard fit. Time-series shape matches dashboard panels natively; no aggregation step at query time.

When logs win

Logs are the right tool for specific, high-cardinality questions. Volume is the cost, not cardinality; the trade is slower aggregate queries.

• Specific questions. "What happened to this one request?" Structured logs answer quickly with per-event detail.
• High cardinality is fine. User ID, request ID, full URL all live happily as log fields; cost scales with volume not cardinality.
• Tune log levels. INFO in prod by default; DEBUG behind a flag; sample noisy routes; the volume bill is the constraint.
• Structured shape. JSON or key-value form; queryable with grep-like tools or proper log search; unstructured logs are a tax.

When traces win

Traces are the right tool for cross-service, dependency-graph questions. They show the call path in motion; the trade is sampling complexity and cost.

• Cross-service questions. "Where in the chain did this slow down?" Traces show the dependency graph in motion across services.
• Sample wisely. Head-based sampling is cheap but blind; tail-based captures slow traces and saves cost on fast ones.
• Span attributes. Request ID, customer ID, route; the same fields that link a trace to logs and metrics.
• OTel SDK. Auto-instrumentation rolls traces out without per-line changes; vendor lock-in is reduced by the standard wire format.