WSGI servers (Gunicorn, uWSGI) in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run a Flask app with Gunicorn using multiple workers?

Consider a Flask app started with Gunicorn using the command gunicorn -w 4 app:app. What is the effect of specifying -w 4?

A Gunicorn runs 4 instances of the Flask app in the same process, but only one handles requests at a time.

B Gunicorn runs a single process with 4 threads, sharing the same memory space.

C Gunicorn runs 4 separate worker processes, each handling requests independently, improving concurrency.

D Gunicorn runs 4 worker processes but only one can handle requests at any moment.

💡 Hint

Think about how Gunicorn uses workers to handle multiple requests simultaneously.

Gunicorn creates multiple worker processes (4 in this case). Each worker runs independently and can handle requests concurrently, improving performance under load.

📝 Syntax

intermediate

Which Gunicorn command correctly binds the server to port 8080?

You want to start a Flask app with Gunicorn and bind it to port 8080 on all interfaces. Which command is correct?

A gunicorn -p 8080 app:app

B gunicorn -b localhost:8080 app:app

C gunicorn --bind 127.0.0.1:8080 app:app

D gunicorn --bind 0.0.0.0:8080 app:app

💡 Hint

Binding to 0.0.0.0 means listening on all network interfaces.

The --bind option specifies the IP and port. 0.0.0.0:8080 means listen on all interfaces at port 8080.

🔧 Debug

advanced

Why does uWSGI fail to start with this config snippet?

Given this uWSGI config snippet:

[uwsgi]
module = app:app
master = true
processes = 4
socket = 127.0.0.1:5000

What is the likely cause of failure?

Flask

[uwsgi]
module = app:app
master = true
processes = 4
socket = 127.0.0.1:5000

A The socket option requires 'http-socket' instead of 'socket' for HTTP serving.

B The 'module' option should be 'app.app' not 'app:app'.

C The 'master' option must be false to run multiple processes.

D The 'processes' option must be set to 1 for uWSGI to start.

💡 Hint

Check if the socket type matches the expected protocol for HTTP serving.

Using socket creates a raw socket expecting a WSGI server behind a proxy. To serve HTTP directly, http-socket is needed.

❓ state_output

advanced

What is the output when using Gunicorn with multiple workers and a global counter?

Consider this Flask app:

from flask import Flask
app = Flask(__name__)
counter = 0

@app.route('/')
def index():
    global counter
    counter += 1
    return str(counter)

If you run it with gunicorn -w 3 app:app and make 3 requests, what is the possible output sequence?

Flask

from flask import Flask
app = Flask(__name__)
counter = 0

@app.route('/')
def index():
    global counter
    counter += 1
    return str(counter)

A The responses could be any combination of 1, 2, or 3 repeated because each worker has its own counter.

B The responses will be 1, 2, 3 in order because the counter is shared across workers.

C All responses will be 3 because Gunicorn synchronizes the counter across workers.

D The app will crash because global variables cannot be used with Gunicorn.

💡 Hint

Think about how global variables behave in multiple processes.

Each Gunicorn worker is a separate process with its own memory. The global counter is not shared, so each worker increments its own counter independently.

🧠 Conceptual

expert

Why is using uWSGI with Emperor mode beneficial in production?

What is the main advantage of running uWSGI in Emperor mode for managing multiple Flask apps?

A Emperor mode allows uWSGI to run without configuration files, simplifying setup.

B Emperor mode automatically manages multiple uWSGI instances, restarting them if they crash, improving reliability.

C Emperor mode enables uWSGI to serve static files faster than Nginx.

D Emperor mode disables worker processes to reduce memory usage.

💡 Hint

Think about how production servers handle multiple apps and failures.

Emperor mode supervises multiple uWSGI instances, automatically restarting them if they fail, which helps keep apps running smoothly in production.