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Property-based testing stateful systems: a tutorial

Property-based testing (PBT), i.e. generating random inputs and checking some property of the output, of pure programs is an established practice by now. It's taught in introductory university classes and it's part of test suites in industry.

Most real world programs are not pure though, they are stateful. While it's often possible to structure your program in such a way that the impure stuff is done in main, e.g. read the contents of a file, and then passed on to a pure function, e.g. a parser, it's not always possible. Consider a long-running program that interacts with the filesystem and with other programs over the network, e.g. some kind of web service or a distributed database. It's difficult to split such a program up into doing a little bit of impure stuff at the start, then hand it over to a pure function (which we can apply PBT on).

Given this it's perhaps a bit surprising that there are relatively few resources about applying PBT to stateful systems. This repository is an attempt to close that gap and try to make PBT stateful systems more common.

The goals we'd like to achieve are:

Show how to test stateful (i.e. impure/monadic) programs using property-based testing;
Show how we can do concurrent testing to help uncover problems such as race conditions;
Show how we can build bigger systems in a modular way by applying the property-based testing equivalent of integration and contract tests;
Show how to use fault injection and so called simulation testing to "end-to-end" test distributed systems;
Introduce the reader to related work and open problems in the area along the way.

In the interest of brevity, we assume that the reader already has:

Enough familiarity with Haskell to be able to read simple programs, for example if you can follow along in the Learn You a Haskell for Great Good! tutorial, then you should be fine;
Some experience with property-based testing of non-stateful (i.e. pure) programs. For example as explained in the official QuickCheck manual or in the following tutorial;
Basic knowledge of state machines (i.e. Mealy / Moore machines and transducers).

Other than that this tutorial is striving to be as self-contained as possibly as well as accessible to non-Haskell programmers.

Structure

The tutorial is split up into five parts (so far), and each part has the following structure:

Motivation: explains why we are doing what we are about to do;
Plan: how we will do it;
Code: a concrete implementation of the idea (in case you get stuck when trying to implement it yourself);
Discussion: common questions or objections;
Exercises: things the authors were to lazy to do, but they know how to;
Problems: things the authors don't know how to do (yet);
See also: links to further reading about the topic or related topics;
Summary: the most important take away.

The parts build upon each other. We start by modelling and testing a simple counter using a state machine in part 1, we then reuse the same state machine model to test the counter for thread-safety using linearisability in part 2. In part 3 we will implement a queue and a web service that uses said queue, the state machine model for the queue and the real implementation of the queue will be contract tested to ensure that the model is faithful to the implementation, subsequently while testing the web service we will use the model in place of the real queue. In part 4 we introduce fault injection to the queue allowing us to test how the web service performs when its dependency fails. Finally, in part 5, we combine all the above ideas in what, sometimes is called simulation testing, to test a distributed system that uses replicated state machines.

Usage

This repository contains literate Haskell code in src. If you want to interact with it, install ghcup and then type cabal repl. Alternatively, if you are using the nix package manager, then running nix-shell in the root directory should give you the right ghc version and all other dependencies you might need.

The literate code is transformed into markdown using pandoc in tools/generate_markdown.sh and the markdown is put inside the docs directory for easier browsing.

The following is a link to the first part of the generate markdown, at the end it will link to the second part and so on. Or you can use the table of contents above or the docs directory to jump to desired part straight away.

Contributing

Any feedback, suggestions for improvement or questions are most welcome via the issue tracker!

See the CONTRIBUTING.md file for more detailed guidelines regarding contributing.

License