Optimize CI Builds By Replacing Nix-shell With Check Phases And Nix-build

Hey guys! Ever felt like your Continuous Integration (CI) builds are taking forever, especially when dealing with Nix packages? You're not alone! In this article, we'll dive deep into how we can optimize our CI pipelines by replacing those slow nix-shell <..> invocations with more efficient check phases and nix-build calls. This is a game-changer, particularly when you're working with projects like kiosk or OCaml, where build times can quickly spiral out of control. So, buckle up, and let's get started!

The Problem with nix-shell in CI

Let's address the elephant in the room: the traditional approach of using nix-shell in CI. While nix-shell is fantastic for local development, it often falls short when it comes to CI environments. The main issue lies in the way nix-shell handles dependencies. When you invoke nix-shell, it essentially creates a completely isolated environment, which is awesome for reproducibility. However, in a CI context, this often translates to redundantly copying hundreds of Nix packages just to re-run a single test command, especially for specialized systems like kiosk or languages like OCaml. Think about it – your CI system is diligently downloading and setting up packages that might already exist in the Nix store or a binary cache. This is not only time-consuming but also puts a significant strain on your CI infrastructure, wasting valuable resources and making the entire process feel sluggish. The overhead becomes even more pronounced when dealing with large projects or when multiple CI jobs are running concurrently. Each job ends up pulling in the same dependencies, leading to a bottleneck that impacts the overall efficiency of your development workflow. Moreover, the verbose nature of nix-shell can clutter the CI logs, making it harder to pinpoint the actual cause of test failures or build issues. In short, while nix-shell has its merits, it's not the ideal solution for the fast-paced and resource-conscious world of CI.

Enter Check Phases and nix-build

So, what's the alternative? The solution lies in leveraging Nix's built-in mechanisms for defining and executing tests: check phases and nix-build calls. These features offer a more streamlined and efficient approach to CI, particularly when you're aiming for speed and resource optimization. A check phase, in Nix parlance, is essentially a designated section within your derivation that specifies how to run tests. By defining tests as check phases, you're essentially telling Nix: "Hey, after building this package, run these tests to make sure everything's working as expected." This approach allows Nix to intelligently manage dependencies and caching. If the derivation (the recipe for building your package) and its dependencies haven't changed, Nix can simply skip the build and check phases altogether, relying on pre-built binaries from the cache. This is a huge win for CI because it means that your tests only run when necessary, saving you a ton of time and resources. The beauty of using nix-build in conjunction with check phases is that it allows you to explicitly control the build process. Instead of relying on the implicit behavior of nix-shell, you can directly instruct Nix to build a specific derivation and run its check phase. This level of control is crucial for CI environments, where predictability and repeatability are paramount. Furthermore, defining tests as check phases makes your build process more transparent and modular. It clearly separates the build and test steps, making it easier to understand and maintain your Nix expressions. This also opens up opportunities for more granular caching strategies, where you can cache the results of individual check phases, further optimizing your CI pipeline.

How Check Phases and nix-build Optimize CI

Let's break down exactly how check phases and nix-build calls optimize your CI builds. The core benefit stems from Nix's intelligent caching mechanism. When you define tests as check phases, Nix can determine if a derivation (the blueprint for building a package) already exists in the cache. This check is crucial because it drastically reduces redundant computations. If the derivation is cached and its dependencies haven't changed, Nix can skip the entire build process, including the test execution. This is a stark contrast to the nix-shell approach, where the environment is often recreated from scratch, even if the underlying packages are already available. This cache-aware behavior is particularly beneficial for projects with a large number of dependencies, or when you're working in a monorepo setup. Imagine a scenario where you have a complex application with dozens of libraries and tools. With nix-shell, every CI job might end up rebuilding and reinstalling these dependencies, even if they haven't changed. Check phases, on the other hand, allow Nix to efficiently reuse cached builds, making your CI runs significantly faster. Another crucial aspect of optimization is the reduction in unnecessary package copying. nix-shell often involves copying packages into a temporary environment, which adds overhead to the build process. With check phases and nix-build, Nix can directly operate on packages in the Nix store, eliminating the need for copying. This not only saves time but also reduces disk I/O, which can be a bottleneck in some CI environments. Moreover, check phases encourage a more modular and declarative approach to testing. By explicitly defining tests as part of your derivation, you make your build process more transparent and maintainable. This also allows for better integration with CI tools, as you can easily track the status of individual check phases and identify potential failures more quickly.

Implementing Check Phases: A Practical Example

Alright, let's get our hands dirty and walk through a practical example of implementing check phases. Imagine you have a simple Nix package that builds a command-line tool. You want to ensure that this tool works correctly by running a few basic tests in your CI pipeline. First, you'll need to define a check phase within your Nix derivation. This is typically done using the checkPhase attribute set. Inside the checkPhase, you'll specify the commands to run your tests. For instance, if you're using a testing framework like Bats or shUnit2, you would include the necessary commands to execute your test suite. Let's say your default.nix file looks something like this:

{ pkgs ? import <nixpkgs> {} }:

pkgs.stdenv.mkDerivation {
  name = "my-cli-tool";
  src = ./.;
  buildPhase = "make";
  installPhase = "mkdir -p $out/bin && cp my-cli-tool $out/bin";
  checkPhase = ''
    ./my-cli-tool --version | grep "1.0.0"
  '';
}

In this example, the checkPhase runs the my-cli-tool with the --version flag and uses grep to verify that the output contains the expected version string. This is a simple but effective way to ensure that your tool is behaving as expected. Now, to run the check phase in your CI, you would use the nix-build command with the --check flag. For example:

nix-build --no-out-link --check

The --no-out-link flag prevents Nix from creating a symlink to the output directory, which is often unnecessary in CI environments. The --check flag tells Nix to build the derivation and then run its check phase. If the check phase fails, nix-build will exit with a non-zero status code, signaling a failure in your CI pipeline. This straightforward approach allows you to integrate testing seamlessly into your Nix builds, ensuring that your packages are always in a working state. Remember to tailor the checkPhase to your specific project and testing requirements. You can include more complex tests, such as integration tests or property-based tests, as needed. The key is to define these tests declaratively within your Nix derivation, making them an integral part of your build process.

Leveraging Passthru Targets for Testing

Beyond check phases, passthru targets offer another powerful mechanism for incorporating tests into your Nix builds. A passthru target is essentially an attribute set that you can add to your derivation to expose specific values or functionalities. This is incredibly useful for defining tests as separate derivations that can be built and run independently. Think of it this way: instead of embedding your tests directly within the checkPhase, you create a dedicated derivation for each test or test suite. This separation of concerns leads to a more modular and maintainable build process. To implement passthru targets for testing, you would typically create a new derivation that depends on your main package and defines its own checkPhase. This test derivation would then be added to the passthru attribute set of your main derivation. Let's extend our previous example to illustrate this concept. Suppose you want to create a separate test derivation that runs a more comprehensive suite of tests. You might create a file named test.nix with the following content:

{ pkgs ? import <nixpkgs> {}, my-cli-tool }:

pkgs.stdenv.mkDerivation {
  name = "my-cli-tool-test";
  src = ./test;
  nativeBuildInputs = [ pkgs.bats ];
  buildInputs = [ my-cli-tool ];
  checkPhase = ''
    bats test.bats
  '';
}

This derivation defines a test suite using the Bats testing framework. It depends on the my-cli-tool package and executes the tests defined in the test.bats file. Now, in your main default.nix file, you would add this test derivation to the passthru attribute set:

{ pkgs ? import <nixpkgs> {} }:

let
  myCliTool = pkgs.stdenv.mkDerivation {
    name = "my-cli-tool";
    src = ./.;
    buildPhase = "make";
    installPhase = "mkdir -p $out/bin && cp my-cli-tool $out/bin";
    checkPhase = ''
      ./my-cli-tool --version | grep "1.0.0"
    '';
  };
  test = import ./test.nix { inherit pkgs myCliTool; };

in
rec {
  inherit myCliTool;
  passthru.tests = test;
}

By adding the test derivation to passthru.tests, you can now build and run the tests independently using nix-build. To run the tests, you would use the following command:

nix-build --no-out-link .#myCliTool.passthru.tests --check

This command tells Nix to build the test derivation associated with the myCliTool package and run its check phase. This approach offers several advantages. It allows you to organize your tests into separate modules, making them easier to manage and maintain. It also enables more granular caching, as you can cache the results of individual test suites. Furthermore, passthru targets can be used to expose other functionalities of your package, such as documentation or examples, making them a versatile tool for building complex Nix expressions.

Case Studies: Kiosk and OCaml

Let's consider specific scenarios where replacing nix-shell with check phases and nix-build can make a significant difference: kiosk systems and OCaml projects. Kiosk systems, often used for public-facing applications, typically involve complex configurations and a wide range of dependencies. Building and testing these systems in CI can be a challenge, especially when using nix-shell. The redundant copying of packages can lead to long build times, making it difficult to iterate quickly and confidently. By leveraging check phases, you can ensure that only the necessary components are rebuilt and tested, drastically reducing CI run times. Imagine a kiosk system that relies on a specific version of a web browser, a display manager, and a set of custom applications. With check phases, you can define tests that verify the integration of these components, ensuring that the system behaves as expected. These tests can range from simple checks, such as verifying that the web browser launches correctly, to more complex integration tests that simulate user interactions. By caching the results of these tests, you can avoid unnecessary rebuilds and significantly speed up your CI pipeline. OCaml projects, with their sophisticated type system and often intricate build processes, also benefit greatly from this optimization. OCaml's build system, often powered by tools like Dune, can be quite resource-intensive, particularly for large projects with many dependencies. Using nix-shell in CI can exacerbate these issues, leading to slow and unreliable builds. Check phases and nix-build, on the other hand, allow you to leverage Nix's caching capabilities to their fullest extent. You can define check phases that run your OCaml test suite, ensuring that your code is thoroughly tested before it's deployed. Furthermore, passthru targets can be used to expose additional testing functionalities, such as running specific test suites or generating code coverage reports. By adopting this approach, you can streamline your OCaml CI workflow, making it faster, more reliable, and more maintainable. In both the kiosk and OCaml scenarios, the key takeaway is that check phases and nix-build provide a more efficient and scalable solution for CI, allowing you to build and test your projects with confidence.

Conclusion: Embrace Check Phases and nix-build for Faster CI

So, there you have it, folks! Replacing nix-shell invocations with check phases and nix-build calls can be a game-changer for your CI pipelines, especially when dealing with complex projects like kiosk systems or languages like OCaml. By leveraging Nix's intelligent caching mechanisms and modular build process, you can significantly reduce build times, optimize resource utilization, and improve the overall reliability of your CI. We've explored the problems with nix-shell in CI, delved into the benefits of check phases and nix-build, and even walked through practical examples of implementing these techniques. We've also seen how passthru targets can further enhance your testing capabilities. The transition from nix-shell to check phases and nix-build might seem daunting at first, but the long-term benefits are well worth the effort. Not only will you save time and resources, but you'll also gain a deeper understanding of Nix's build process and how to effectively leverage its features. So, go ahead, give it a try! Start by identifying areas in your CI pipeline where nix-shell is causing bottlenecks. Then, begin refactoring your Nix expressions to incorporate check phases and nix-build calls. You'll be amazed at the difference it makes. Remember, faster CI builds mean faster feedback loops, which ultimately lead to higher quality software. And that's something we can all get behind!