Autosizing web services

Molotov, the load testing tool I've developed, comes now with an autosizing feature. When the --sizing option is used, Molotov will slowly ramp-up the number of workers per process and will stop once there are too many failures per minute.

The default tolerance for failure is 5%, but this can be tweaked with the --sizing-tolerance option.

Molotov will use 500 workers that are getting ramped up in 5 minutes, but you can set your own values with --workers and --ramp-up if you want to autosize at a different pace.

See all the options at

This load testing technique is useful to determine what is the limiting resource for a given application: RAM, CPU, I/O or Network.

Running Molotov against a single node that way can help decide what is the best combination of RAM, CPU, Disk and Bandwidth per node to deploy a project. In AWS that would mean helping chosing the size of the VM.

To perform this test you need to deploy the app on a dedicated node. Since most of our web services projects at Mozilla are now available as Docker images, it becomes easy to automate that deployment when we want to test the service.

I have created a small script on the top of Molotov that does exactly that, by using Amazon SSM (Systems Manager). See

Amazon SSM

SSM is a client-server tool that simplifies working with EC2 nodes. For instance, instead of writing a low-level script using Paramiko that drives EC2 instances through SSH, you can send batch commands through SSM to any number of EC2 instances, and get back the results asynchronously.

SSM integrates with S3 so you can get back your commands results as artifacts once they are finished.

Building a client around SSM is quite easy with Boto3. The only tricky part is waiting for the results to be ready.

This is my SSM client:

Deploying and running

Based on this SSM client, my script is doing the following operations on AWS:

  • Deploy (or reuse) an EC2 Instance that has an SSM agent and a Docker agent running
  • Run the Docker container of the service on that EC2 instance
  • Run a Docker container that runs Glances (more on this later)

Once the EC2 instance has the service up and running, it's ready to be used via Molotov.

The script takes a github repo and run it, using moloslave Once the test is over, metrics are grabbed via SSM and the results are presented in a fancy HTML 5 page where you can find out what is the bottleneck of your service

Example with Kinto

Kinto is a Python service that provides a rest-ish API to read write schemaless JSON documents. Running a load test on it using Molotov is pretty straightforward. The test script adds data, browses it and verifies that the Kinto service returns things correctly. And Kinto has a docker image published on Docker hub.

I've run the sizing script using that image on a t2.micro instance. Here are the results:

You can see that the memory is growing throughout the test, because the Docker image uses a memory database and the test keeps on adding data -- that is also why the I/O is sticking to 0.

If you double-click on the CPU metrics, you can see that the CPU reaches almost 100% at the end of the test before things starts to break.

So, for a memory backend, the limiting factor for Kinto is the CPU, which makes sense. If we had had a bottleneck on I/O, that would have been an indication that something was wrong.

Another interesting test would be to run it against a Postgres RDS deployment instead of a memory database.

Collecting Metrics with Glances

The metrics are collected on the EC2 box using Glances ( which runs in its own Docker container and has the ability to measure other docker images running on the same agent. see

In other words, you can follow the resource usage per docker container, and in our case that's useful to track the container that runs the actual service.

My Glances docker container uses this image: which runs the tool and spits out the metrics in a CSV file I can collect via SSM once the test is over.

Vizualizing results

I could have send the metrics to an Influxdb or Grafana system, but I wanted to create a simple static page that could work locally and be passed around as a test artifact.

That's where Plotly ( comes in handy. This tool can turn a CSV file produced by Glances into a nice looking HTML5 page where you can toggle between metrics and do other nice stuff.

I have used Pandas/Numpy to process the data, which is probably overkill given the amount of processed lines, but their API are a natural fit to work with Plotly.

See the small class I've built here:


The new Molotov sizing feature is pretty handy as long as you can automate the deployment of isolated nodes for the service you want to test -- and that's quite easy with Docker and AWS.

Autosizing can give you a hint on how an application behaves under stress and help you decide how you want to initially deploy it.

In an ideal world, each one of our services has a Molotov test already, and running an autosizing test can be done with minimal work.

In a super ideal world, everything I've described is part of the continuous deployement process :)