Server-Side Swift: Testing on Linux

Testing is an important part of the software development process. Writing unit tests and automating them as much as possible allows you to develop and evolve your applications quickly.

In this tutorial, you’ll learn:

Why Test on Linux?

Getting Started

Testing is a key part of any app development and deployment. You’ll find this helpful tutorial that covers how to test Vapor apps and how to write tests for server-side Swift projects.

Testing on Linux is important as well. You’d usually deploy your app to an operating system that is different from the one you used for development. However, in this case it’s vital that you test your app in the same environment that you deploy in. Why is it important?

Foundation on Linux isn’t the same as Foundation on macOS. On macOS, it still uses the Objective-C framework, which has been thoroughly tested over the years. Linux uses the pure Swift Foundation framework, which isn’t as robust. The implementation status list shows that many features remain unimplemented on Linux. If you use these features, your app may crash. Although the situation improves constantly, you must still ensure everything works as expected on Linux.

Start by downloading the materials for this tutorial, using the Download Materials button at the top or bottom of this tutorial. The starter project contains a Vapor app with some tests. Open Package.swift. You’ll see a test target defined in the targets array:

• How to test your server-side Swift apps on Linux.
• What the differences are between testing on macOS and Linux.
• How to use Docker and Docker Compose to easily test your apps locally on Linux.
Note: This tutorial requires Docker. Check out Docker on macOS: Getting Started to learn the basics of and how to install Docker.

Why Test on Linux?

Testing is a key part of any app development and deployment. You’ll find this helpful tutorial that covers how to test Vapor apps and how to write tests for server-side Swift projects.

Testing on Linux is important as well. You’d usually deploy your app to an operating system that is different from the one you used for development. However, in this case it’s vital that you test your app in the same environment that you deploy in. Why is it important?

Foundation on Linux isn’t the same as Foundation on macOS. On macOS, it still uses the Objective-C framework, which has been thoroughly tested over the years. Linux uses the pure Swift Foundation framework, which isn’t as robust. The implementation status list shows that many features remain unimplemented on Linux. If you use these features, your app may crash. Although the situation improves constantly, you must still ensure everything works as expected on Linux.

Getting Started

Start by downloading the materials for this tutorial, using the Download Materials button at the top or bottom of this tutorial. The starter project contains a Vapor app with some tests. Open Package.swift. You’ll see a test target defined in the targets array:

.testTarget(name: "AppTests", dependencies: [
  .target(name: "App"),
  .product(name: "XCTVapor", package: "vapor"),
])

This defines a testTarget type with a dependency on App and XCTVapor (providing Vapor test helpers on top of XCTest). Tests must live in the Tests/ directory. In this case, that’s Tests/AppTests.

Checking the Tests

The tests connect to a PostgreSQL database, with the configuration defined in configure.swift in Sources/App. The tests are expecting a database on port 5432 called vapor-test, which accepts a username of vapor_username and a password of vapor_password. This helps avoid conflicting Docker containers when running the app locally.

In Terminal, enter the following:

docker run --name postgres-test -e POSTGRES_DB=vapor-test \
  -e POSTGRES_USER=vapor_username -e POSTGRES_PASSWORD=vapor_password \
  -p 5433:5432 -d postgres

This:

• Runs a new container named postgres-test.
• Specifies the database name, username and password through environment variables.
• Allows apps to connect to the Postgres server by mapping the external port 5433 to the container’s port 5432.
• Runs the server in the background as a daemon.
• Uses the Docker image named postgres for this container. If the image isn’t present on your machine, Docker automatically downloads it.

Then run the tests. In Terminal, make sure you’re in the starter directory, then enter the following:

swift test

This pulls down the required dependencies, compiles the project and runs the tests. You’ll see all the tests complete successfully:

To clean up, in Terminal, run the following commands:

# 1
docker stop postgres-test
# 2
docker rm postgres-test

These commands:

1. Stop the postgres-test container in which you ran your tests.
2. Delete the postgres-test container now that it’s stopped.

Running Tests on Linux

Test Discovery on Linux

On macOS, swift test still uses Objective-C to find the tests to run. On Linux, there’s no Objective-C runtime to discover your XCTestCases.
However, since Swift 5.1 you can run Swift tests on Linux with a flag to automatically discover tests. And because your project uses Swift 5.3, you can use this new flag:

swift test --enable-test-discovery

This command can be run on Linux machines and doesn’t require you to create or generate a LinuxMain.swift class before it can run tests on Linux.

It will discover and run your tests. Because this flag is available on Linux, you can use it in your continuous integration flow as well.

Early feedback is always valuable in software development, and running tests on Linux is no exception. Using a Continuous Integration system to automatically test on Linux is vital, but what happens if you want to test on Linux on your Mac? You’ll do that now with Docker.