Kafka

You probably heard about property testing (AKA property-based testing), a testing technique where test inputs are randomly generated in great quantity and particular desired properties of the code under test are checked. It’s very practical and I’m a big proponent of using it where it makes sense.

In this post, I will tell you how I used property testing with the Proptest library in Rust to ensure the correctness of a bunch of generated serialization/deserialization code for the Apache Kafka protocol.

The post assumes you’re familiar with Rust. However, to follow it you don’t need any knowledge about Kafka or its protocol. I will explain the problem I was working on briefly, but will not go into much detail.

Introduction

Apache Kafka has a sizeable custom binary protocol that is versioned, with various data types, optional fields, etc. Unfortunately, it doesn’t use a well-known serialization format like Protobuf. The protocol message schema is described in JSON. The actual Java code that does serialization and deserialization is generated from this description¹.

I created a library kafka_wire_protocol. It’s a generated de-/serialization code for the Kafka protocol, but for Rust (and in the future, for Go). The correctness of the generated code is paramount. I applied many testing techniques to it: quick unit tests, integration tests against a real Kafka instance, fuzzying. And also kafka_wire_protocol is a good example of software that can benefit from property testing. About this is going to be this post.

You don’t need to familiarize yourself with kafka_wire_protocol to understand what will be discussed. I will give examples from the code, but they will be self-contained. However, if you’re interested, feel free to check out this repo and follow the instruction in README to run the code generator, compile, and run the tests.

I worked with the Apache Kafka protocol on the low level quite a bit. It wasn’t easy to start doing this following the official guide only and I read the code a lot. With this post, I want to give you a head start by guiding you step by step from primitive values to meaningful requests.

We will use Python as the programming language. However, the code will be zero-dependency and easily portable to the language of your choice.