Why on_start and on_stop are Essential for Locust Users

In this chapter, we’ll cover,

1. What on_start and on_stop do.

2. Why they are important.

3. Practical examples of using these methods.

4. Running and testing Locust scripts.

What Are on_start and on_stop?

• on_start: This method is executed once when a new simulated user starts. It's commonly used for tasks like logging in or setting up the environment.

• on_stop: This method is executed once when a simulated user stops. It’s often used for cleanup tasks like logging out.

These methods are executed only once per user during the lifecycle of a test, as opposed to tasks that are run repeatedly.

Why Use on_start and on_stop?

1. Simulating Real User Behavior: Real users often start a session with an action (e.g., login) and end it with another (e.g., logout).

2. Initial Setup: Some tasks require initializing data or setting up user state before performing other actions.

3. Cleanup: Ensure that actions like logout are performed to leave the system in a clean state.

Examples

Basic Usage of on_start and on_stop

In this example, we just print on start and `on stop` for each user while running a task.

from datetime import datetime
from locust import User, task, between, constant, constant_pacing

class MyUser(User):

    wait_time = between(1, 5)

    def on_start(self):
        print("on start")

    def on_stop(self):
        print("on stop")

    @task
    def print_datetime(self):
        print(datetime.now())