Ability to set a specific number of simulated users per Locust class

[barrywhart]

We are using Locust to test an internal web service where there are two sources of load:

• Varying load generated by end users. Existing Locust works well for this; we simply enter the number of simulated users in the Locust UI before launching the test.
• Steady load generated by internal clients. This load does not vary because there is a queue between the clients and the service, and the queue always has a known number of works.

For the latter case, it would be useful to have a way to explicitly set the number of users rather than having Locust compute it from the weights.

[barrywhart]

Here is a preliminary implementation of this feature for discussion/review. It is implemented as a monkey patch which extends the existing behavior of LocustRunner.weight_locusts(). If this suggestion sounds useful and the implementation looks like a good start, I'll be happy to implement this more properly (i.e. as changes to Locust, rather than a monkey patch) and create a PR for it.

##########################################################################
# Monkey patch the code that creates locusts (i.e. simulated users) so
# we can implement a behavior that works better for Orion. Orion is an API,
# not a web site with users, so the Locust concepts don't necessarily make
# sense for us.
##########################################################################

old_weight_locusts = runners.LocustRunner.weight_locusts


def weight_locusts(self, amount, stop_timeout=None):
    """
    Returns a list of classes from self.locust_classes, one entry per "user"
    that should be launched.

    The default implementation weights all the classes based on their "weight"
    field. This version implements a "force_users" property on the Locust
    classes. If that property is present, that *exact* number of users will
    be created. The remaining classes will be allocated normally, based on
    amount.

    :param self:
    :param amount:
    :param stop_timeout:
    :return:
    """
    # First handle the "force_users" classes.
    force_users_result = []
    for locust in self.locust_classes:
        force_users = getattr(locust, 'force_users', None)
        if force_users:
            force_users_result += [locust for x in xrange(0, force_users)]

            # Set weight to 0 so the "parent" function will ignore this class.
            locust.weight = 0

    # Now do normal parent processing.
    result = old_weight_locusts(self, amount, stop_timeout)
    return force_users_result + result

runners.LocustRunner.weight_locusts = weight_locusts

[aldenpeterson-wf]

I am closing this as it appears the monkey-patch approach works - LMK if I need to reopen this.

[zoharm]

Hi all, apologies if I am incorrectly resurrecting this, I tried out this monkey patch (simply added it to the locustfile.py) but not surprisingly got "name 'runners' is not defined"

What am I doing wrong? Any help please?

Thank you!

[Grimlek]

Here is an example for locust 1.5.3. Definitely not a perfect solution but something to get anyone started if they stumble upon this.

from locust import events, runners

"""
    Monkey Patch Users class to allow specifying a specific number of users. 
    
    Add max_users_count attribute to user class.
"""
weight_old_users = runners.Runner.weight_users


def weight_users(self, amount):
    # now do normal parent processing and get bucket
    bucket = weight_old_users(self, amount)

    for user in self.user_classes:
        max_users_count = getattr(user, 'max_users_count', None)
        if max_users_count:
            bucket_users = [x for x in bucket if repr(x) == repr(user)]
            
            # modify bucket to force a limited number of users
            # in the event locust count is modified in the UI, don't add more users to new greenlets (quick approach)
            if len(bucket_users) > max_users_count and len(self.user_greenlets) == 0:
                new_bucket = [x for x in bucket if repr(x) != repr(user)]
                bucket_forced_users = bucket_users[-max_users_count:]
                bucket = new_bucket + bucket_forced_users
            elif len(self.user_greenlets) > 0:
                new_bucket = [x for x in bucket if repr(x) != repr(user)]
                bucket = new_bucket
    return bucket


runners.Runner.weight_users = weight_users
#############################################################
#              END MONKEY PATCH
#############################################################