Monitoring Request Handling Across Uvicorn Workers

What will you learn?

In this detailed guide, you will master the art of tracking and managing requests handled by multiple Uvicorn workers. By understanding how to monitor request distribution, you can optimize your web application’s performance and ensure a seamless user experience.

Introduction to Problem and Solution

When deploying ASGI applications like FastAPI with Uvicorn, employing multiple workers for handling requests concurrently is standard practice. However, as traffic increases, it becomes crucial to monitor and manage how these requests are distributed across various workers. Without proper monitoring, identifying bottlenecks or uneven load distribution can be challenging.

To address this challenge effectively, we will delve into implementing logging and monitoring solutions tailored for real-time observation of request handling across all active Uvicorn workers. By integrating logging middleware within our application and utilizing external tools such as Prometheus and Grafana for visualization, we can gain valuable insights into our system’s performance. This insight enables us to make informed decisions regarding scaling and optimization strategies.


# Logging Middleware Implementation
from starlette.middleware.base import BaseHTTPMiddleware

class RequestLoggingMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        # Log incoming request here (e.g., using Python's logging module)
        response = await call_next(request)
        # Optionally log outgoing response details as well
        return response

# Add the middleware to your FastAPI app instance

# Configure Python's logging module appropriately to capture logs in console output or an external file.

# Copyright PHD


By incorporating a custom middleware like RequestLoggingMiddleware, each incoming request along with its corresponding response gets logged by our application. The beauty lies in the fact that each worker process executes this middleware independently for the requests it handles, providing granular visibility at a per-worker level.

For comprehensive monitoring: – Log Collection: Utilize centralized log management systems (e.g., ELK stack or Splunk) for aggregating logs from all instances. – Metrics Export: Employ Prometheus to expose custom metrics from your application (such as response times and queue lengths) via an endpoint scraped by Prometheus. – Visualization: Grafana dashboards can visualize these metrics showing per-worker data, enabling you to identify imbalances or potential issues in real-time.

This approach not only aids in tracking current load but also assists in capacity planning through historical data analysis, helping identify peak usage periods that may require additional resources.

    1. How do I configure multiple workers with Uvicorn? To run Uvicorn with multiple workers, use the command uvicorn myapp:app –workers <number_of_workers>. Replace <number_of_workers> with your desired worker count based on available CPU cores.

    2. What is ASGI? ASGI stands for Asynchronous Server Gateway Interface; it�s a specification detailing how asynchronous servers communicate with Python web applications. It serves as a foundation for frameworks like FastAPI and Starlette that enable high-performance async capabilities.

    3. Can I use Prometheus without Grafana? Yes! While Grafana offers advanced visualization capabilities for easier data interpretation, Prometheus itself provides basic graphing functionalities accessible via its web UI suitable for simpler needs.

    4. Is there any performance overhead when using logging middleware? While there is minimal overhead introduced by additional code execution including logging operations�especially if they’re synchronous/blocking�modern asynchronous logging libraries minimize impact while maintaining high throughput rates of ASGI applications.

    5. Do I need separate tools for log aggregation from multiple instances/workers? Ideally yes; centralizing logs from various sources simplifies management and enables advanced search/filtering features crucial during debugging sessions or post-mortem analyses following incidents or outages.


Mastering the monitoring of request handling across Uvicorn workers is essential for optimizing web application performance. By implementing effective logging mechanisms and leveraging tools like Prometheus and Grafana, you can gain valuable insights into your system’s behavior. This knowledge empowers you to make data-driven decisions on scaling resources efficiently to meet user demands effectively.

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