4.5Logging Dos and Don'ts for Testability

Do	Don't
Log at every significant state transition	Log raw request/response bodies (PII risk)
Include correlation IDs (trace_id, request_id)	Use string interpolation for log messages
Use consistent event names across services	Log at DEBUG level in production
Include timing data for operations	Log secrets, tokens, or passwords
Separate business events from technical events	Create logs only useful during development
Use structured fields for every variable value	Embed values in the message string

Anti-Pattern: Values in Message Strings

# BAD -- values embedded in string, impossible to query
log.info(f"User {user_id} placed order {order_id} for ${total}")

# GOOD -- values as structured fields, queryable
log.info("order_placed", user_id=user_id, order_id=order_id, total_cents=total)