Uvicorn server basics in FastAPI - Performance & Optimization

Performance: Uvicorn server basics

MEDIUM IMPACT

Uvicorn affects how fast your FastAPI app starts and handles requests, impacting server response time and throughput.

Running a FastAPI app with a single worker vs multiple workers

FastAPI

uvicorn main:app --host 0.0.0.0 --port 8000 --workers 4

Multiple workers allow parallel request handling, improving throughput and responsiveness.

📈 Performance GainHandles multiple requests concurrently, reducing wait time and improving INP.

Running a FastAPI app with a single worker vs multiple workers

FastAPI

uvicorn main:app --host 0.0.0.0 --port 8000

Single worker can only handle one request at a time, causing delays under load.

📉 Performance CostBlocks handling multiple requests, increasing response time under concurrency.

Performance Comparison

Pattern	Concurrency	CPU Usage	Response Latency	Verdict
Single worker	1 request at a time	Low CPU usage	High under load	[X] Bad
Multiple workers	Multiple requests concurrently	Higher CPU usage	Lower latency	[OK] Good

Rendering Pipeline

Uvicorn runs the ASGI server that receives HTTP requests, passes them to FastAPI, and sends back responses. It manages event loops and worker processes.

→Request Handling

→Concurrency Management

→Response Sending

⚠️ BottleneckSingle worker limits concurrency, causing request queuing and slower response times.

Core Web Vital Affected

INP

Uvicorn affects how fast your FastAPI app starts and handles requests, impacting server response time and throughput.

Optimization Tips

1Use multiple workers in Uvicorn to improve request concurrency.

2Monitor server response times to detect bottlenecks.

3Adjust worker count based on CPU cores and expected load.

Performance Quiz - 3 Questions

What is the main performance benefit of running Uvicorn with multiple workers?

A It decreases the memory usage per request.

B It reduces the size of the server binary.

C It allows handling multiple requests at the same time.

D It improves the visual layout of the webpage.

DevTools: Network

How to check: Open browser DevTools, go to Network tab, make multiple requests to your API, and observe response times.

What to look for: Look for increased response times or queued requests indicating server bottlenecks.

Practice

1. What is the main purpose of running uvicorn with a FastAPI app?

easy

A. To start a server that handles web requests for the FastAPI app

B. To compile the FastAPI app into an executable file

C. To create a database connection for the FastAPI app

D. To generate API documentation automatically

Solution

1. Step 1: Understand what Uvicorn does

   Uvicorn is an ASGI server that runs your FastAPI app so it can receive and respond to web requests.

2. Step 2: Identify the purpose of running Uvicorn

   Running Uvicorn starts the server that listens for HTTP requests and sends responses from your FastAPI app.

3. Final Answer:

   To start a server that handles web requests for the FastAPI app -> Option A

4. Quick Check:

   Uvicorn runs server = A [OK]

Hint: Remember: Uvicorn runs the server for your app [OK]

Common Mistakes:

• Thinking Uvicorn compiles code
• Confusing server start with database setup
• Assuming Uvicorn generates docs automatically

2. Which of the following is the correct command to run a FastAPI app located in main.py with auto-reload enabled?

easy

A. uvicorn main:app reload

B. uvicorn main:app --reload

C. uvicorn app:main --reload

D. uvicorn main.py --reload

Solution

1. Step 1: Understand Uvicorn command syntax

   The correct syntax is uvicorn module_name:app_variable. Here, the module is main and the app variable is app.

2. Step 2: Check the options for auto-reload

   The option --reload enables auto-reload during development.

3. Final Answer:

   uvicorn main:app --reload -> Option B

4. Quick Check:

   Correct syntax with --reload = D [OK]

Hint: Use module:app format with --reload flag [OK]

Common Mistakes:

• Including .py extension in module name
• Swapping module and app names
• Missing double dashes before reload

3. Given this command: uvicorn app.main:app --host 0.0.0.0 --port 8080, what will be the effect?

medium

A. The app runs on localhost at port 80

B. The app runs only on 127.0.0.1 at port 8080

C. The app runs on all network interfaces at port 8080

D. The app runs on all interfaces at default port 8000

Solution

1. Step 1: Analyze the host option

   The host 0.0.0.0 means the server listens on all network interfaces, not just localhost.

2. Step 2: Analyze the port option

   The port is explicitly set to 8080, so the app will listen on port 8080.

3. Final Answer:

   The app runs on all network interfaces at port 8080 -> Option C

4. Quick Check:

   Host 0.0.0.0 + port 8080 = C [OK]

Hint: 0.0.0.0 means all interfaces, port sets listening port [OK]

Common Mistakes:

• Assuming 0.0.0.0 means localhost only
• Ignoring the port option
• Confusing default port 8000 with specified port

4. You run uvicorn main:app --reload but get an error: ModuleNotFoundError: No module named 'main'. What is the likely cause?

medium

A. The app variable is not named app inside main.py

B. Uvicorn requires Python 2, but you have Python 3

C. The --reload flag is not supported on your system

D. The file main.py is missing or not in the current directory

Solution

1. Step 1: Understand the error message

   The error says the module main cannot be found, meaning Python can't locate main.py.

2. Step 2: Check file location

   If main.py is missing or not in the current directory where the command runs, this error occurs.

3. Final Answer:

   The file main.py is missing or not in the current directory -> Option D

4. Quick Check:

   ModuleNotFoundError means missing file/module = B [OK]

Hint: Check if main.py exists in current folder [OK]

Common Mistakes:

• Assuming app variable name causes ModuleNotFoundError
• Blaming --reload flag for module errors
• Confusing Python version requirements

5. You want to run your FastAPI app with Uvicorn so it reloads on code changes and listens only on localhost at port 5000. Which command is correct?

hard

A. uvicorn app:app --reload --host 127.0.0.1 --port 5000

B. uvicorn app:app --reload --host 0.0.0.0 --port 5000

C. uvicorn app:app --host 127.0.0.1 --port 5000

D. uvicorn app:app --reload --port 127.0.0.1 --host 5000

Solution

1. Step 1: Identify the reload option

   The --reload flag enables auto-reload on code changes, so it must be included.

2. Step 2: Set host and port correctly

   Listening only on localhost means host should be 127.0.0.1. The port must be set to 5000 using --port 5000.

3. Step 3: Check option order and syntax

   Options order does not affect functionality, but the syntax must be correct: --host followed by IP, --port followed by port number.

4. Final Answer:

   uvicorn app:app --reload --host 127.0.0.1 --port 5000 -> Option A

5. Quick Check:

   Reload + localhost + port 5000 = A [OK]

Hint: Use --reload with --host 127.0.0.1 and --port 5000 [OK]

Common Mistakes:

• Using 0.0.0.0 for localhost only
• Swapping host and port values
• Omitting --reload for auto-reload