Design for Easier HTTP Load Testing

I’ve told two people about this trick in the last few days, so it is worth writing it up.

It is hard to get a good distribution of requests in your HTTP load tester, it usually requires a knowledge of valid keys or users and a model of the distribution of the accesses. This can all be built in the load tester, but that seems to be a big barrier, since I’ve rarely seen that happen. I’ve certainly never done it and I’ve needed it several times.

The easy way to do this is to add a “random choice” parameter to the app. The app already knows the legal set of keys or users and can quickly make a choice. You already know the language and the code in the app, and the changes are localized to the URL parameter parsing. Let’s say you have a back-end server that returns records.

An HTTP load tester can access the single randomizing URL over and over again, and fetch different records each time. This is a trivial load test script. In Jakarta JMeter, it is one of the samples.

This is really very easy to write inside of the server. Getting a random key looks something like this, assuming that we already have an instance of java.util.Random initialized and ready to go.

key = cache.keySet().get(random.nextInt(cache.keySet().size()))

In Python, you can use the default, shared instance of the random source and the choice() convenience method:

key = random.choice(cache.keys())

This can all be done in the code that parses the URL parameters. Once you have a random key, the remainder of the app executes with no changes.

If the app should be tested with a non-uniform distribution of accesses, that is also easy to do. Python’s random.paretovariate() looks especially good for Zipf (80/20 or “long tail”) distributions. Or you could duplicate that code in your favorite language:

def paretovariate(self, alpha):
"""Pareto distribution.  alpha is the shape parameter."""
# Jain, pg. 495
u = 1.0 - self.random()
return 1.0 / pow(u, 1.0/alpha)

For user logins, add an option to masquerade as a random user, or even a random user from certain classes (big profile, frequent login, new user …).

For testing search, I once made an especially fancy tester that would access a log of queries in order, but start at a different place for each client. This preserves the time locality of queries while giving each client a different set. I used a cookie to hold the per-client state, so that each client would access the queries in order from their starting place. It went roughly like this:

  1. If the client did not send a cookie, choose a random index in the log.
  2. Otherwise, read the cookie to get an index.
  3. Set the cookie to the next index.
  4. Wrap the index, modulo the log size.
  5. Run the search with the query at that index.

Now go test your software. I might need to use it someday.

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