Locust EP 2: Understanding Locust Wait Times with Complete Examples

17 November 2024 at 07:43

Locust is an excellent load testing tool, enabling developers to simulate concurrent user traffic on their applications. One of its powerful features is wait times, which simulate the realistic user think time between consecutive tasks. By customizing wait times, you can emulate user behavior more effectively, making your tests reflect actual usage patterns.

In this blog, we’ll cover,

What wait times are in Locust.
Built-in wait time options.
Creating custom wait times.
A full example with instructions to run the test.

What Are Wait Times in Locust?

In real-world scenarios, users don’t interact with applications continuously. After performing an action (e.g., submitting a form), they often pause before the next action. This pause is called a wait time in Locust, and it plays a crucial role in mimicking real-life user behavior.

Locust provides several ways to define these wait times within your test scenarios.

FastAPI App Overview

Here’s the FastAPI app that we’ll test,


from fastapi import FastAPI

# Create a FastAPI app instance
app = FastAPI()

# Define a route with a GET method
@app.get("/")
def read_root():
    return {"message": "Welcome to FastAPI!"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str = None):
    return {"item_id": item_id, "q": q}

Locust Examples for FastAPI

1. Constant Wait Time Example

Here, we’ll simulate constant pauses between user requests


from locust import HttpUser, task, constant

class FastAPIUser(HttpUser):
    wait_time = constant(2)  # Wait for 2 seconds between requests

    @task
    def get_root(self):
        self.client.get("/")  # Simulates a GET request to the root endpoint

    @task
    def get_item(self):
        self.client.get("/items/42?q=test")  # Simulates a GET request with path and query parameters

2. Between wait time Example

Simulating random pauses between requests.


from locust import HttpUser, task, between

class FastAPIUser(HttpUser):
    wait_time = between(1, 5)  # Random wait time between 1 and 5 seconds

    @task(3)  # Weighted task: this runs 3 times more often
    def get_root(self):
        self.client.get("/")

    @task(1)
    def get_item(self):
        self.client.get("/items/10?q=locust")

3. Custom Wait Time Example

Using a custom wait time function to introduce more complex user behavior


import random
from locust import HttpUser, task

def custom_wait():
    return max(1, random.normalvariate(3, 1))  # Normal distribution (mean: 3s, stddev: 1s)

class FastAPIUser(HttpUser):
    wait_time = custom_wait

    @task
    def get_root(self):
        self.client.get("/")

    @task
    def get_item(self):
        self.client.get("/items/99?q=custom")

Full Test Example

Combining all the above elements, here’s a complete Locust test for your FastAPI app.


from locust import HttpUser, task, between
import random

# Custom wait time function
def custom_wait():
    return max(1, random.uniform(1, 3))  # Random wait time between 1 and 3 seconds

class FastAPIUser(HttpUser):
    wait_time = custom_wait  # Use the custom wait time

    @task(3)
    def browse_homepage(self):
        """Simulates browsing the root endpoint."""
        self.client.get("/")

    @task(1)
    def browse_item(self):
        """Simulates fetching an item with ID and query parameter."""
        item_id = random.randint(1, 100)
        self.client.get(f"/items/{item_id}?q=test")

Running Locust for FastAPI

Run Your FastAPI App
Save the FastAPI app code in a file (e.g., main.py) and start the server


uvicorn main:app --reload

By default, the app will run on http://127.0.0.1:8000.

2. Run Locust
Save the Locust file as locustfile.py and start Locust.


locust -f locustfile.py

3. Configure Locust
Open http://localhost:8089 in your browser and enter:

Host: http://127.0.0.1:8000
Number of users and spawn rate based on your testing requirements.

4. Run in Headless Mode (Optional)
Use the following command to run Locust in headless mode


locust -f locustfile.py --headless -u 50 -r 10 --host http://127.0.0.1:8000`

-u 50: Simulate 50 users.

-r 10: Spawn 10 users per second.

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