Structured Logging

Best Practice: Structured Logging

Structured logging means emitting each log entry as a machine-parseable record of key-value fields, usually JSON, instead of a free-form string. A structured entry carries fields like timestamp, level, service, trace_id, and event-specific keys. This makes logs searchable, filterable, and joinable with traces and metrics, rather than something you grep by hand.

It matters because at scale, plain-text logs are nearly impossible to query reliably. Consistent fields let you filter by service, correlate a request across systems via trace IDs, and build alerts on log attributes. During migrations, structured logs make it far easier to compare behavior between old and new systems.

Structured logs are the third pillar of observability alongside metrics and traces, and they are most powerful when correlated with the other two. When a log line carries the same trace_id as a distributed trace, you can jump from a failing request in a trace straight to the detailed logs for that exact request. Structured logging also tames cost: a backend can index specific fields, so you query by level, service, or customer_id in milliseconds rather than scanning text. It does require discipline, because a consistent schema across teams is what makes cross-service queries possible, and a single team logging free-form strings breaks that consistency.

Step-by-Step Implementation Guidance

1. Choose a structured format (JSON is the common default) and a logging library that emits it.
2. Define a standard field set every service must include: timestamp, level, service name, version, and trace/span IDs.
3. Adopt OpenTelemetry log conventions so field names match your traces and metrics.
4. Inject correlation IDs (trace_id, request_id) into every log line within a request context.
5. Log events, not sentences: put variable data in fields, keep the message stable.
6. Set consistent log levels and avoid logging secrets or PII.
7. Ship logs to a central store that indexes fields for fast querying.

Common Mistakes Teams Make When Ignoring This Practice

• Concatenating variables into message strings, making fields unqueryable.
• Omitting trace or request IDs, so logs cannot be correlated.
• Inconsistent field names across services.
• Logging sensitive data into plaintext stores.
• Over-logging at high volume with no levels, raising cost and noise.

Tools and Techniques That Support This Practice

• Structured loggers: Zap, slog, Serilog, structlog, Logback JSON encoders.
• OpenTelemetry Logs SDK and Collector.
• Centralized stores: Elasticsearch/OpenSearch, Loki, Splunk.
• Trace-log correlation via shared trace IDs.
• OpenTelemetry Collector for parsing, redaction, and routing of logs.
• Sampling and log-level controls to manage volume and cost.

How This Practice Applies to Different Migration Types

• Cloud Migration: Standard fields let you query old and new environments with the same searches, easing side-by-side comparison during cutover.
• Database Migration: Log query identifiers and outcomes as fields to audit data parity.
• SaaS Migration: Correlate vendor request IDs in structured fields for end-to-end tracing.
• Codebase Migration: Keep field names stable across a rewrite so log-based comparisons hold.

Checklist

• Structured format and logger chosen
• Standard required field set defined
• OpenTelemetry log conventions adopted
• Correlation IDs injected per request
• Events logged as fields, not strings
• Secrets and PII excluded from logs
• Logs centralized with field indexing