Structured Logging

Structured Logging - Observability Patterns

Consistent, machine-readable logging across the full stack. The backend uses structlog with JSON output; the frontend uses a custom Logger class. This skill codifies conventions for both and adds correlation IDs, log context, and level guidelines.

Description

Enforces JSON-structured logging with correlation IDs, consistent log levels, and contextual metadata across the FastAPI backend (structlog) and Next.js frontend (Logger class). Covers sensitive data redaction, request tracing, and observability best practices.

When to Apply

Positive Triggers

• Adding logging to new modules or API endpoints
• Reviewing existing log statements for consistency
• Implementing request tracing or correlation IDs
• Debugging production issues via log analysis
• Setting up log aggregation or monitoring pipelines
• User mentions: "logging", "logs", "observability", "tracing", "monitoring", "debug"

Negative Triggers

• Implementing error response formats (use error-taxonomy instead)
• Designing metrics/dashboards (use metrics-collector when available)
• Configuring CI/CD pipelines (use ci-cd-patterns when available)

Core Directives

Always Structured, Never Unstructured

# GOOD: Structured with context
logger.info("Document created", document_id=doc.id, user_id=user.id)

# BAD: Unstructured string interpolation
logger.info(f"Document {doc.id} created by user {user.id}")

# BAD: print() for logging
print(f"Created doc: {doc.id}")

Log Levels

What NOT to Log

• Passwords, tokens, API keys, or session IDs
• Full request/response bodies (log summaries instead)
• Personal information beyond what's needed for debugging
• High-frequency events without sampling (e.g., every heartbeat)

Backend Patterns (structlog)

Existing Setup

The project configures structlog in apps/backend/src/utils/logging.py:

• Debug mode: ConsoleRenderer() (human-readable)
• Production mode: JSONRenderer() (machine-readable)
• Context vars: merge_contextvars enables request-scoped context

Getting a Logger

from src.utils import get_logger

logger = get_logger(__name__)

# Logger name becomes the "logger" field in JSON output
# e.g., "logger": "src.api.routes.documents"

Correlation IDs

Add a middleware that generates a correlation ID per request and binds it to structlog context:

import uuid
import structlog
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.requests import Request


class CorrelationIdMiddleware(BaseHTTPMiddleware):
    """Attach a correlation ID to every request for log tracing."""

    async def dispatch(self, request: Request, call_next):
        correlation_id = request.headers.get(
            "X-Correlation-ID",
            str(uuid.uuid4())
        )

        # Bind to structlog context (available to all loggers in this request)
        structlog.contextvars.clear_contextvars()
        structlog.contextvars.bind_contextvars(
            correlation_id=correlation_id,
        )

        response = await call_next(request)
        response.headers["X-Correlation-ID"] = correlation_id
        return response

Register in apps/backend/src/api/main.py:

from .middleware.correlation import CorrelationIdMiddleware

app.add_middleware(CorrelationIdMiddleware)

Request Logging

Log every API request with timing:

import time
from src.utils import get_logger

logger = get_logger(__name__)


class RequestLoggingMiddleware(BaseHTTPMiddleware):
    """Log request method, path, status, and duration."""

    async def dispatch(self, request: Request, call_next):
        start = time.perf_counter()
        response = await call_next(request)
        duration_ms = (time.perf_counter() - start) * 1000

        logger.info(
            "Request completed",
            method=request.method,
            path=request.url.path,
            status=response.status_code,
            duration_ms=round(duration_ms, 2),
        )
        return response

Agent Execution Logging

Log agent lifecycle events consistently:

logger = get_logger(__name__)


async def execute_agent(agent_name: str, task: str):
    logger.info("Agent started", agent=agent_name, task=task[:100])

    try:
        result = await agent.run(task)
        logger.info(
            "Agent completed",
            agent=agent_name,
            status="success",
            duration_ms=result.duration_ms,
        )
        return result
    except TimeoutError:
        logger.error(
            "Agent timed out",
            agent=agent_name,
            error_code="AGENT_RUNTIME_TIMEOUT",
        )
        raise
    except Exception as exc:
        logger.error(
            "Agent failed",
            agent=agent_name,
            error_code="AGENT_RUNTIME_FAILED",
            error=str(exc),
        )
        raise

JSON Output Format

In production, each log line is a single JSON object:

{
  "timestamp": "2026-02-13T09:30:00.000Z",
  "level": "info",
  "event": "Request completed",
  "logger": "src.api.middleware.logging",
  "correlation_id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
  "method": "POST",
  "path": "/api/documents",
  "status": 201,
  "duration_ms": 42.5
}

Frontend Patterns (Logger)

Existing Setup

The project has a Logger class in apps/web/lib/logger.ts:

• Level filtering via LOG_LEVEL env var
• ISO timestamp formatting
• JSON context serialisation

Usage Convention

import { logger } from '@/lib/logger';

// Business events
logger.info('Document created', { documentId: doc.id, userId: user.id });

// Warnings
logger.warn('API response slow', { endpoint: '/api/agents', durationMs: 2500 });

// Errors (always include the error object)
logger.error('Failed to fetch documents', error, { userId: user.id });

// Debug (stripped in production via LOG_LEVEL)
logger.debug('API response', { status: response.status, body: data });

Correlation ID Propagation

Pass the correlation ID from backend responses to subsequent requests:

let correlationId: string | null = null;

export async function apiRequest(path: string, options?: RequestInit) {
  const headers: Record<string, string> = {
    'Content-Type': 'application/json',
    ...(correlationId ? { 'X-Correlation-ID': correlationId } : {}),
  };

  const response = await fetch(`${BACKEND_URL}${path}`, {
    ...options,
    headers: { ...headers, ...options?.headers },
  });

  // Capture correlation ID from response
  correlationId = response.headers.get('X-Correlation-ID');

  return response;
}

Replacing console.log

All console.log, console.error, and console.warn calls should use the logger instead:

// BAD
console.log('User logged in');
console.error('Failed to load', error);

// GOOD
logger.info('User logged in', { userId: user.id });
logger.error('Failed to load', error, { component: 'Dashboard' });

Log Context Standards

Required Fields

Every log entry should include (automatically via middleware or manually):

Recommended Fields (per domain)

Logging Checklist

When adding or reviewing logging:

• Use get_logger(__name__) (backend) or logger import (frontend)
• Use structured key-value context, not f-strings
• Correct log level (ERROR/WARNING/INFO/DEBUG)
• No secrets, tokens, or passwords in log output
• Error logs include error_code from error-taxonomy where applicable
• High-frequency operations use DEBUG level (not INFO)

Anti-Patterns

Checklist

• JSON-structured log output configured for production (structlog JSONRenderer)
• Correlation IDs propagated via X-Correlation-ID header
• Sensitive data redacted from all log output
• Log levels consistently applied per the level guidelines table
• All console.log calls replaced with logger methods in frontend code
• Error logs include error_code from the error taxonomy

Response Format

[AGENT_ACTIVATED]: Structured Logging
[PHASE]: {Implementation | Review | Configuration}
[STATUS]: {in_progress | complete}

{logging analysis or implementation guidance}

[NEXT_ACTION]: {what to do next}

Integration Points

Error Taxonomy

Error logs should include error_code from the error taxonomy:

logger.error("Agent failed", error_code="AGENT_RUNTIME_FAILED", agent=name)

Council of Logic (Shannon Check)

• Log messages must be concise — maximum signal, minimum noise
• Avoid logging the same event at multiple levels
• Use sampling for high-frequency events (e.g., log 1 in 100 health checks)

Australian Localisation (en-AU)

• Timestamps: ISO 8601 (UTC) in log output, DD/MM/YYYY in human reports
• Spelling: behaviour, colour, organisation, analyse, centre, serialisation
• Compliance: Logs must not contain data subject to Privacy Act 1988 without justification