Gain insights into property-based testing with PropEr in Elixir. Learn foundational principles, custom data generators, and advanced concepts for effective real-world application testing.

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Property-based testing relies on specifying some property of code, unlike unit tests, which specify the expected output in response to some inputs. In this course, we'll cover the concepts we need to feel confident about using even the most advanced features of PropEr with Elixir.

We'll start with the basic and foundational principles of property-based testing, see what the framework offers us to get started, make our way through thinking in properties, write our own custom data generators, and then learn more advanced concepts. We'll also see how you can use property-based testing in a realistic project.

By the time we finish this course, we should feel comfortable testing real-world applications using properties.

Property-Based Testing with PropEr in Elixir

As with the last chapter, the parallel version of our system can be adapted by just declaring a new property and reusing the same model, to see if any glaring concurrency issues can be found.

Let's take a look at the below code and watch the property in action.


# Code

The `property "parallel stateful property"` has been highlighted in the following code.

import React from 'react';
import { shallow, configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';

configure({ adapter: new Adapter() });

import HelloWorld from './app';

var TestResult = function() {
    this.succeeded = false;
    this.reason = "";
    this.input = "";
    this.expected_output = "";
    this.actual_output = "";
}

export const executeTests = function() {

  var results = [];

  result = new TestResult();
  result.input = 'HelloWorld Component';
  result.expected_output = "span containing text 'Hello World'"

  let wrapper = shallow(<HelloWorld />);

  // Call your Challenge function here.

  let type = wrapper.type();
  let testSuccessful = true;
  let failureReason;

  if (type !== 'span') {
    testSuccessful = false;
    failureReason = "You need to render exactly one span HTML element";
  } else if (wrapper.props().children != "Hello World") {
     testSuccessful = false;
     failureReason = "You have rendered wrong message in your span element";
  }

  result.actual_output = wrapper.html();

  if (testSuccessful) {
    result.succeeded = true;
    result.reason = "Succeeded"
  } else {
    result.succeeded = false;
    result.reason = failureReason;
  }

  results.push(result);

  return results;
}


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Once again, there are few changes required beasides the name of the generator
function and the function to run commands. We can run it and see it pass. 

As we saw in the last chapter, the PropEr parallel tests aren’t great at finding concurrency issues, so we shouldn’t take the pass as proof of success. We can try to add `:erlang.yield()` calls in multiple areas, but this won’t necessarily uncover everything. Careful analysis can always help clarify things.

It turns out that, by default, any PostgreSQL query runs in an implicit transaction even if we don’t specify one. Those transactions run in `read committed` isolation levels and are all simple `SELECT` and `UPDATE` queries with no external data and no need for reads from other queries. This means the operations are safe for concurrency. The one case that could look funky is the last one we changed where we check whether a book exists before borrowing it. Since the two operations aren’t in a transaction, there could be a risk of bad results where the wrong kind of error is returned, but no operation would succeed where it shouldn’t.

In any case, these tests will help in the event that the implementation changes, or that an unsafe query usage or pooling takes place in lieu of the present one connection per request.

With all this, we now have a nearly complete integration test for the application’s interactions with the database. For now we’ll leave connectivity failures untouched because those may be more interesting to cover in another property instead of adding to the complexity of this one.


We've learned most everything there is know about stateful properties. All that’s left is a specialization for systems that represent finite state machines, which we’ll get to discover next.

Take a look at the parallel testing of the application. 

Parallel Tests

Take a look at the parallel testing of the application.

Foundations of Property-Based Testing

Writing Properties

Thinking in Properties

Custom Generators

Responsible Testing

Properties-Driven Development

Shrinking

Stateful Properties

Case Study: Bookstore

State Machine Properties

Conclusion

Parallel Tests