How do I allocate fix number of users per single user class in locust - locust

Question

Suppose I have 3 separate user classes. I want to allocate fix number of users for each class. My code is as below.
class User_1(TaskSet):
# I need 3 users to execute the tasks within this user class
class User_2(TaskSet):
# I need only 1 user to execute the tasks within this user class
class User_3(TaskSet):
# I need only 1 user to execute the tasks within this user class
class API_User_Test(HttpUser):
#I already tried weighting the classes as below.
tasks = {Site_User_1: 3, User_2: 1, User_3: 1}
I've already tried weighting the classes as shown in the code above. But it doesn't work. Some times it will allocate more than 1 users for class User_2 or class User_3. Can someone tell me how to fix this issue.

Answer 1

A weight in Locust is just a statistical weight and is not guarantee. The weights determine how many times a task/user are put into a list to be selected from. When a new task/user is spawned, Locust randomly selects a task from the list. Given your weights:
tasks = {Site_User_1: 3, User_2: 1, User_3: 1}
Statistically speaking, spawning 5 users with weights 3/1/1 would get you 3/1/1 but it may not be that precise every time. While less likely, it's possible you could get 4/0/1 or 3/2/0 or 5/0/0.
From the Locust docs:
If the tasks attribute is specified as a list, each time a task is to be performed, it will be randomly chosen from the tasks attribute. If however, tasks is a dict - with callables as keys and ints as values - the task that is to be executed will be chosen at random but with the int as ratio. So with a task that looks like this:
{my_task: 3, another_task: 1}
my_task would be 3 times more likely to be executed than another_task.
Internally the above dict will actually be expanded into a list (and the tasks attribute is updated) that looks like this:
[my_task, my_task, my_task, another_task]
and then Python’s random.choice() is used pick tasks from the list.
If you absolutely have to have full control over exactly what users are running, I'd probably recommend having a single Locust user with a single task that contains your own logic on what to run. Create your own list of functions to call and iterate through it each time a new user is created. Might have to be external to the user as a global or something. But the idea is you manage the logic yourself and not Locust.
Edit:
Using the single user method to control what's running won't work well if you run on multiple workers as the workers don't communicate with each other. You may consider doing some more advanced things like sending messages between master and workers to coordinate, or use an external source like a database or other service the workers talk to to know what they should run.