daily-email: add 2023-08-24

Testing multiple implementations with contract tests

src/content/daily-email/2023-08-24.md

@@ -0,0 +1,24 @@
+---
+title: >
+  Testing multiple implementations with contract tests
+pubDate: 2023-08-24
+permalink: >
+  archive/2023/08/24/testing-multiple-implementations-with-contract-tests
+tags:
+  - automated-testing
+  - test-driven-development
+---
+
+If you have multiple implementations of a service, as I [mentioned in yesterday's email](), you need to ensure they all provide the same functionality.
+
+You need to be able to run the same tests against each implementation and have them pass.
+
+## How do you do this?
+
+In PHP, I use a trait that contains the test methods and then have a test class for each implementation that uses the trait and sets up any test data.
+
+Then, each test class will run the methods from the contract test trait and ensure they all provide the same behaviour, regardless of how it does so - whether it communicates with an API, uses an SDK, or returns fake values.
+
+If one implementation doesn't return the same result as the others, its test will fail.
+
+If you add a new implementation, you create a new test class, use the trait and get the tests to pass.