Writing load tests

Writing load tests can be done with Requests, WebTest or ws4py. Loads provides a test case class that includes bridges to the three libraries.


Loads uses Gevent to spawn concurrent users. Most of the time, Gevent will play nicely with your tests and make sure that they are run asynchronously - but in case Loads is not sending the load it’s supposed to, it probably means some of your code is blocking the Gevent loop.

Read Writing asynchronous tests to troubleshoot this issue.

Using Requests

Requests is a popular library to query an HTTP service, and is widely used in the Python community.

Let’s say you want to load test the Elastic Search root page that’s running on your local host.

Write a test case like this one and save it in an file:

from import TestCase

class TestWebSite(TestCase):

    def test_es(self):
        res = self.session.get('http://localhost:9200')
        self.assertEqual(res.status_code, 200)

The TestCase class provided by Loads has a session attribute you can use to interact with an HTTP server. It’s a Session instance from Requests.

Now run loads-runner against it:

$ bin/loads-runner example.TestWebSite.test_es
[======================================================================]  100%

Hits: 1
Started: 2013-06-14 12:15:42.860586
Duration: 0.03 seconds
Approximate Average RPS: 39
Average request time: 0.01s
Opened web sockets: 0
Bytes received via web sockets : 0

Success: 1
Errors: 0
Failures: 0

This will execute your test just once - so you can control that your test works as expected.

Now, try to run it using 100 virtual users (-u), each of them running the test 10 times (–hits):

$ bin/loads-runner example.TestWebSite.test_es -u 100 --hits 10
[======================================================================]  100%
Hits: 1000
Started: 2013-06-14 12:15:06.375365
Duration: 2.02 seconds
Approximate Average RPS: 496
Average request time: 0.04s
Opened web sockets: 0
Bytes received via web sockets : 0

Success: 1000
Errors: 0
Failures: 0

Congrats, you’ve just sent a load of 1000 hits, using 100 virtual users.

Now let’s run a series of 10, 20 then 30 users, each one running 20 hits:

$ bin/loads-runner example.TestWebSite.test_something --hits 20 -u 10:20:30

That’s 1200 hits total.

You can use all Requests API to GET, PUT, DELETE, POST or do whatever you need on the service.

Don’t forget to control all responses with assertions, so you can catch any issue that may occur on your service on high load.

To do this, use the unit test assert methods provided by Python. Most services will break with 500s errors when they can’t cope with the load.

Using Loads with ws4py

Loads provides web sockets API through the ws4py library. You can initialize a new socket connection using the create_ws method provided in the test case class.

Run the file located in Loads’ examples directory, then write a test that uses a web socket against it:

from import TestCase

class TestWebSite(TestCase):

    def test_something(self):

        results = []

        def callback(m):

        ws = self.create_ws('ws://localhost:9000/ws',
                            protocols=['chat', 'http-only'],

        while len(results) < 2:

        self.assertEqual(results, ['something', 'happened'])

See ws4py documentation for more info.

Using Loads with WebTest

If you are a WebTest fan, you can use it instead of Requests. If you don’t know what WebTest is, you should have a look at it ;).

WebTest is really handy to exercise an HTTP service because it includes tools to easily control the responses status code and content.

You just need to use app instead of session in the test case class. app is a webtest.TestApp object, providing all the APIs to interact with an HTTP service:

from import TestCase

class TestWebSite(TestCase):

    server_url = "http://your-server-url"

    def test_something(self):
        self.assertTrue('tarek' in'/'))

Of course, because the server root URL will change during the tests, you can define it outside the tests, on the command line, with –server-url when you run your load test:

$ bin/loads-runner example.TestWebSite.test_something --server-url

Changing the server URL

It may happen that you need to change the server url when you’re running the tests. To do so, change the server_url attribute of the app object: = 'http://new-server'

Adding custom metrics

You can use the incr_counter method in your test case to increment a counter. This is useful if you want to count the number of times a particular event happens.

In this example, the tarek-was-there counter will be incremented everytime the test is successful:

from import TestCase

class TestWebSite(TestCase):

    def test_something(self):
        self.assertTrue('tarek' in'/'))

At the end of the test, you will be able to know how many times the counter was incremented.