AWS Lambda dev with Python

A story of a pigeon

I work on Socorro which is the crash ingestion pipeline for Mozilla's products.

The pipeline starts at the collector which handles incoming HTTP POST requests, pulls out the payload, futzes with it a little, and then saves it to AWS S3. Socorro then processes some of those crashes in the processor. The part that connects the two is called Pigeon. It was intended as a short-term solution to bridge the collector and the processor, but it's still around a year later and the green grass grows all around all around and the green grass grows all around.

Pigeon is an AWS Lambda function that triggers on S3 ObjectCreated:Put events, looks at the filename, and then adds things to the processing queue depending on the filename structure. We called it Pigeon for various hilarious reasons that are too mundane to go into in this blog post.

It's pretty basic. It doesn't do much. It was a short term solution we thought we'd throw away pretty quickly. I wrote some unit tests for the individual parts of it and a "client" that invoked the function in a faux AWS Lambda like way. That was good enough.

But then some problems

Pigeon was written with Python 2 because at the time AWS Lambda didn't have a Python 3 runtime. That changed--now there's one with Python 3.6.

In January, I decided to update Pigeon to work with Python 3.6. I tweaked the code, tweaked the unit tests, and voila--it was done! Then we deployed it to our -stage environment where it failed epically in Technicolor glory (but no sound!) and we had to back it out and return to the Python 2 version.

What happened? I'll tell you what happened--we had a shit testing environment. Sure, we had tests, but they lacked several things:

  1. At no point do we test against the build artifact for Pigeon. The build artifact for AWS Lambda jobs in Python is a .zip file that includes the code and all the libraries that it uses.

  2. The tests "invoke" Pigeon with a "client", but it was pretty unlike the AWS Lambda Python 3.6 runtime.

  3. Turns out I had completely misunderstood how I should be doing exception handling in AWS Lambda.

So our tests tested some things, but missed some important things and a big bug didn't get caught before going to -stage.

It sucked. I felt chagrinned. I like to think I have a tolerance for failure since I do it a lot, but this felt particularly fail-y and some basic safeguards would have prevented it from happening.

Fleshing out AWS Lambda in Python project

We were thinking of converting another part of the Socorro pipeline to AWS Lambda, but I put that on hold until I had wrapped my head around how to build a development environment that included scaffolding for testing AWS Lambda functions in a real runtime.

Miles or Brian mentioned aws-sam-local. I looked into that. It's written in Go, they suggest installing it with npm, it does a bunch of things, and it has some event generation code. But for the things I needed, it seemed like it would just be a convenience cli for docker-lambda.

I had been aware of docker-lambda for a while, but hadn't looked at the project recently. They added support for passing events via stdin. Their docs have examples of invoking Lambda functions. That seemed like what I needed.

I took that and built the developer environment scaffolding that we've got in Pigeon now. Further, I decided to use this same model for future AWS Lambda function development.

How does it work?

Pigeon is a Python project, so it uses Python libraries. I maintain those requirements in a requirements.txt file.

I install the requirements into a ./build directory:

$ pip install --ignore-installed --no-cache-dir -r requirements.txt -t build/

I copy the Pigeon source into that directory, too:

$ cp build/

That's all I need for the runtime to use.

The tests are in the tests/ directory. I'm using pytest and in the file have this at the top:

import os
import sys

# Insert build/ directory in sys.path so we can import pigeon

I'm using Docker and docker-compose to aid development. I use a test container which is a python:3.6 image with the test requirements installed in it.

In this way, tests run against the ./build directory.

Now I want to be able to invoke Pigeon in an AWS Lambda runtime so I can debug issues and also write an integration test.

I set up a lambda-run container that uses the lambci/lambda:python3.6 image. I mount ./build as /var/task since that's where the AWS Lambda runtime expects things to be.

I created a shell script for invoking Pigeon:


docker-compose run \
    --rm \
    -v "$PWD/build":/var/task \
    --service-ports \
    lambda-run pigeon.handler $@

That's based on the docker-lambda invoke examples.

Let's walk through that:

  1. It runs the lambda-run container with the services it depends on as defined in my docker-compose.yml file.

  2. It mounts the ./build directory as /var/task because that's where the runtime expects the code it's running to be.

  3. The DOCKER_LAMBDA_USE_STDIN=1 environment variable causes it to look at stdin for the event. That's pretty convenient.

  4. It runs invokes pigeon.handler which is the handler function in the pigeon Python module.

I have another script that generates fake AWS S3 ObjectCreated:Put events. I cat the result of that into the invoke shell script. That runs everything nicely:

$ ./bin/ --key v2/raw_crash/000/20180313/00007bd0-2d1c-4865-af09-80bc00180313 > event.json
$ cat event.json | ./bin/
Starting socorropigeon_rabbitmq_1 ... done
START RequestId: 921b4ecf-6e3f-4bc1-adf6-7d58e4d41f47 Version: $LATEST
{"Timestamp": 1523588759480920064, "Type": "pigeon", "Logger": "antenna", "Hostname": "300fca32d996", "EnvVersion": "2.0", "Severity": 4, "Pid": 1, "Fields": {"msg": "Please set PIGEON_AWS_REGION. Returning original unencrypted data."}}
{"Timestamp": 1523588759481024512, "Type": "pigeon", "Logger": "antenna", "Hostname": "300fca32d996", "EnvVersion": "2.0", "Severity": 4, "Pid": 1, "Fields": {"msg": "Please set PIGEON_AWS_REGION. Returning original unencrypted data."}}
{"Timestamp": 1523588759481599232, "Type": "pigeon", "Logger": "antenna", "Hostname": "300fca32d996", "EnvVersion": "2.0", "Severity": 6, "Pid": 1, "Fields": {"msg": "number of records: 1"}}
{"Timestamp": 1523588759481796864, "Type": "pigeon", "Logger": "antenna", "Hostname": "300fca32d996", "EnvVersion": "2.0", "Severity": 6, "Pid": 1, "Fields": {"msg": "looking at key: v2/raw_crash/000/20180313/00007bd0-2d1c-4865-af09-80bc00180313"}}
{"Timestamp": 1523588759481933056, "Type": "pigeon", "Logger": "antenna", "Hostname": "300fca32d996", "EnvVersion": "2.0", "Severity": 6, "Pid": 1, "Fields": {"msg": "crash id: 00007bd0-2d1c-4865-af09-80bc00180313 in dev_bucket"}}
{"Timestamp": 1523588759497482240, "Type": "pigeon", "Logger": "antenna", "Hostname": "300fca32d996", "EnvVersion": "2.0", "Severity": 6, "Pid": 1, "Fields": {"msg": "00007bd0-2d1c-4865-af09-80bc00180313: publishing to socorrodev.normal"}}
END RequestId: 921b4ecf-6e3f-4bc1-adf6-7d58e4d41f47
REPORT RequestId: 921b4ecf-6e3f-4bc1-adf6-7d58e4d41f47 Duration: 101 ms Billed Duration: 200 ms Memory Size: 1536 MB Max Memory Used: 28 MB


Then I wrote an integration test that cleared RabbitMQ queue, ran the invoke script with a bunch of different keys, and then checked what was in the processor queue.

Now I've got:

  • tests that test the individual bits of Pigeon

  • a way to run Pigeon in the same environment as -stage and -prod

  • an integration test that runs the whole setup

A thing I hadn't mentioned was that Pigeon's documentation is entirely in the README. The docs cover setup and development well enough that I can hand this off to normal people and future me. I like simple docs. Building scaffolding such that docs are simple makes me happy.


You can see the project at

Want to comment? Send an email to willkg at bluesock dot org. Include the url for the blog entry in your comment so I have some context as to what you're talking about.