fastapi-logging

name	fastapi-logging
description	Configure structured JSON logging with correlation IDs and request context for FastAPI

FastAPI Logging & Correlation IDs

Overview

This skill covers setting up structured JSON logging with correlation ID middleware for request tracing across the application.

Create logging.py

Create src/app/logging.py:

import logging
import sys
from typing import Any

from pythonjsonlogger import jsonlogger

from app.config import settings


class CustomJsonFormatter(jsonlogger.JsonFormatter):
    """
    Custom JSON formatter that adds standard fields to all log records.
    
    Output format:
    {
        "timestamp": "2025-01-05T12:00:00.000Z",
        "level": "INFO",
        "message": "Request completed",
        "logger": "app.api.v1.items",
        "correlation_id": "uuid",
        ...additional fields
    }
    """

    def add_fields(
        self,
        log_record: dict[str, Any],
        record: logging.LogRecord,
        message_dict: dict[str, Any],
    ) -> None:
        super().add_fields(log_record, record, message_dict)

        # Standard fields
        log_record["timestamp"] = self.formatTime(record, self.datefmt)
        log_record["level"] = record.levelname
        log_record["logger"] = record.name

        # Remove default fields we're replacing
        log_record.pop("levelname", None)
        log_record.pop("name", None)

        # Add correlation_id from context if available
        from app.middleware.correlation_id import get_correlation_id

        correlation_id = get_correlation_id()
        if correlation_id:
            log_record["correlation_id"] = correlation_id


def setup_logging() -> None:
    """
    Configure application logging.
    
    Call this early in application startup, before creating the FastAPI app.
    
    Configures:
    - JSON format for production (LOG_JSON_FORMAT=true)
    - Human-readable format for development
    - Log level from settings
    - Suppresses noisy third-party loggers
    """
    log_level = getattr(logging, settings.log_level.upper(), logging.INFO)

    # Root logger configuration
    root_logger = logging.getLogger()
    root_logger.setLevel(log_level)

    # Remove existing handlers
    root_logger.handlers.clear()

    # Create handler
    handler = logging.StreamHandler(sys.stdout)
    handler.setLevel(log_level)

    if settings.log_json_format:
        # JSON format for production
        formatter = CustomJsonFormatter(
            fmt="%(timestamp)s %(level)s %(name)s %(message)s",
            datefmt="%Y-%m-%dT%H:%M:%S.%f",
        )
    else:
        # Human-readable format for development
        formatter = logging.Formatter(
            fmt="%(asctime)s | %(levelname)-8s | %(name)s | %(message)s",
            datefmt="%Y-%m-%d %H:%M:%S",
        )

    handler.setFormatter(formatter)
    root_logger.addHandler(handler)

    # Suppress noisy loggers
    logging.getLogger("uvicorn.access").setLevel(logging.WARNING)
    logging.getLogger("uvicorn.error").setLevel(logging.WARNING)
    logging.getLogger("sqlalchemy.engine").setLevel(
        logging.INFO if settings.database_echo else logging.WARNING
    )
    logging.getLogger("httpx").setLevel(logging.WARNING)
    logging.getLogger("httpcore").setLevel(logging.WARNING)


def get_logger(name: str) -> logging.Logger:
    """
    Get a logger instance for a module.
    
    Usage:
        logger = get_logger(__name__)
        logger.info("Something happened", extra={"user_id": "123"})
    
    Args:
        name: Logger name (typically __name__)
        
    Returns:
        Configured logger instance
    """
    return logging.getLogger(name)

Create middleware/correlation_id.py

Create src/app/middleware/correlation_id.py:

import contextvars
from uuid import uuid4

from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
from starlette.requests import Request
from starlette.responses import Response

# Context variable to store correlation ID for the current request
_correlation_id_ctx: contextvars.ContextVar[str | None] = contextvars.ContextVar(
    "correlation_id",
    default=None,
)

# Header name for correlation ID
CORRELATION_ID_HEADER = "X-Correlation-ID"


def get_correlation_id() -> str | None:
    """
    Get the correlation ID for the current request context.
    
    Returns None if called outside of a request context.
    
    Usage:
        from app.middleware.correlation_id import get_correlation_id
        
        correlation_id = get_correlation_id()
        logger.info("Processing", extra={"correlation_id": correlation_id})
    """
    return _correlation_id_ctx.get()


def set_correlation_id(correlation_id: str) -> None:
    """
    Set the correlation ID for the current context.
    
    Typically called by middleware, but can be used in tests
    or background tasks.
    """
    _correlation_id_ctx.set(correlation_id)


class CorrelationIdMiddleware(BaseHTTPMiddleware):
    """
    Middleware that manages correlation IDs for request tracing.
    
    Behavior:
    1. Checks for X-Correlation-ID header in incoming request
    2. If present, uses that ID (for distributed tracing)
    3. If absent, generates a new UUID
    4. Stores ID in context variable (accessible via get_correlation_id())
    5. Adds ID to response headers
    
    This enables tracing requests across:
    - Multiple services (when ID is passed in headers)
    - Log aggregation systems
    - Error tracking systems
    """

      () -> Response:
        
        correlation_id = request.headers.get(CORRELATION_ID_HEADER)
          correlation_id:
            correlation_id = (uuid4())

        
        set_correlation_id(correlation_id)

        
        response =  call_next(request)

        
        response.headers[CORRELATION_ID_HEADER] = correlation_id

         response

Create middleware/init.py

Create src/app/middleware/__init__.py:

from app.middleware.correlation_id import (
    CORRELATION_ID_HEADER,
    CorrelationIdMiddleware,
    get_correlation_id,
    set_correlation_id,
)

__all__ = [
    "CORRELATION_ID_HEADER",
    "CorrelationIdMiddleware",
    "get_correlation_id",
    "set_correlation_id",
]

Usage in Application

In main.py

from app.logging import setup_logging
from app.middleware import CorrelationIdMiddleware

# Setup logging BEFORE creating app
setup_logging()

def create_app() -> FastAPI:
    app = FastAPI(...)
    
    # Add correlation ID middleware
    app.add_middleware(CorrelationIdMiddleware)
    
    return app

In Services/Routers

from app.logging import get_logger

logger = get_logger(__name__)

class ItemService:
    async def create(self, obj_in: ItemCreate) -> Item:
        logger.info(
            "Creating item",
            extra={
                "item_name": obj_in.name,
                "category_id": str(obj_in.category_id),
            },
        )
        
        item = await self._repository.create(obj_in)
        
        logger.info(
            "Item created successfully",
            extra={"item_id": str(item.id)},
        )
        
        return item

In Exception Handlers

from app.middleware.correlation_id import get_correlation_id

async def app_exception_handler(request: Request, exc: AppException):
    correlation_id = get_correlation_id()
    
    logger.warning(
        "Application error",
        extra={
            "error_code": exc.error_code,
            "correlation_id": correlation_id,
        },
    )
    
    return JSONResponse(
        content={
            "detail": exc.message,
            "correlation_id": correlation_id,
            ...
        }
    )

Log Output Examples

JSON Format (Production)

{
  "timestamp": "2025-01-05T12:00:00.123456",
  "level": "INFO",
  "logger": "app.items.service",
  "message": "Creating item",
  "correlation_id": "550e8400-e29b-41d4-a716-446655440000",
  "item_name": "Widget",
  "category_id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890"
}

Human-Readable Format (Development)

2025-01-05 12:00:00 | INFO     | app.items.service | Creating item

Logging Best Practices

1. Use Appropriate Log Levels

Level	Use Case
DEBUG	Detailed diagnostic information
INFO	Routine operations, state changes
WARNING	Unexpected but handled situations
ERROR	Errors that need attention
CRITICAL	System failures

2. Include Contextual Information

# Good - includes context
logger.info(
    "Order processed",
    extra={
        "order_id": str(order.id),
        "total": order.total,
        "items_count": len(order.items),
    },
)

# Bad - no context
logger.info("Order processed")

3. Log at Boundaries

• Log at entry/exit of services
• Log external API calls
• Log database operations (sparingly)

4. Don't Log Sensitive Data

# Bad - logs password
logger.info(f"User login: {user.email}, password: {user.password}")

# Good - no sensitive data
logger.info("User login attempt", extra={"email": user.email})

5. Use Structured Fields

# Good - structured
logger.error(
    "Database query failed",
    extra={
        "query": "SELECT * FROM items",
        "duration_ms": 1500,
        "error": str(e),
    },
)

# Bad - unstructured
logger.error(f"Database query failed: {e}")

Request Logging Middleware (Optional)

For detailed request/response logging:

import time
from app.logging import get_logger

logger = get_logger("app.http")

class RequestLoggingMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        start_time = time.perf_counter()
        
        response = await call_next(request)
        
        duration_ms = (time.perf_counter() - start_time) * 1000
        
        logger.info(
            "Request completed",
            extra={
                "method": request.method,
                "path": request.url.path,
                "status_code": response.status_code,
                "duration_ms": round(duration_ms, 2),
            },
        )
        
        return response