Generating Deterministic .zip Files For Terraform CloudGov Deployments

Ensuring consistent and reproducible deployments is a critical aspect of modern software development. In the realm of infrastructure as code (IaC), where tools like Terraform are used to manage and provision resources, this need for determinism extends to the artifacts generated during the deployment process. One common artifact is the .zip file, often used to package application code or configuration files. This article delves into the importance of generating .zip files deterministically, particularly within the context of the terraform-cloudgov modules, and provides a detailed exploration of a solution to achieve this goal.

The Importance of Deterministic .zip File Generation

Deterministic .zip file generation is crucial for minimizing spurious or blocked application deployments. When a .zip file is generated, various factors can influence its content and structure, including file modification times, compression algorithms, and the order in which files are added to the archive. If these factors are not controlled, even minor changes in the environment or the build process can lead to different .zip files being generated from the same source code.

This non-determinism can have several negative consequences:

• Spurious deployments: If a new .zip file is generated even when the underlying code hasn't changed, it can trigger unnecessary deployments, consuming resources and potentially disrupting services.
• Blocked deployments: Deployment pipelines often rely on checksums or other mechanisms to verify the integrity of deployment artifacts. If a .zip file is generated non-deterministically, these checks may fail, blocking deployments even when the code is valid.
• Debugging challenges: When deployments fail due to non-deterministic .zip file generation, it can be difficult to diagnose the root cause, as the issue may not be directly related to the code itself.
• Wasted resources: Non-deterministic builds can lead to wasted time and resources, especially in large organizations with complex deployment pipelines.

Therefore, ensuring that .zip files are generated deterministically is essential for reliable and efficient deployments.

The Challenge in terraform-cloudgov

The terraform-cloudgov modules, like many IaC projects, rely on .zip files to package and deploy applications. The prepare scripts within this repository are responsible for generating these .zip files. However, without specific measures in place, these scripts can produce non-deterministic .zip files, leading to the issues mentioned above.

To address this challenge, a solution was implemented to ensure deterministic .zip file generation within the terraform-cloudgov modules. This solution involves controlling the factors that can influence the .zip file's content and structure.

The Solution: Ensuring Deterministic .zip File Generation

The core of the solution lies in addressing the factors that contribute to non-deterministic .zip file generation. These factors primarily revolve around file metadata, compression methods, and file ordering within the archive. The implemented solution addresses these issues by:

1. Setting modification timestamps: .zip files store modification timestamps for each file within the archive. These timestamps, if not controlled, can vary between builds, leading to different .zip files. The solution sets the modification timestamps of all files added to the .zip archive to a consistent value, ensuring that this factor does not contribute to non-determinism.
2. Using a consistent compression method: The .zip format supports various compression methods. Using different methods or varying compression levels can result in different .zip files. The solution enforces the use of a single, consistent compression method (typically DEFLATE) with a fixed compression level.
3. Sorting files before adding them to the archive: The order in which files are added to the .zip archive can also affect the final result. The solution sorts the files alphabetically before adding them, ensuring a consistent ordering.
4. Normalizing file paths: File paths within the .zip archive can be represented in different ways (e.g., with or without leading slashes). The solution normalizes file paths to a consistent format, preventing variations due to path representation.

The implemented solution, as demonstrated in the provided pull request (this working solution), effectively addresses these factors, resulting in deterministic .zip file generation.

The code modifications involve using a library or built-in functionality that allows setting specific timestamps and compression options while creating the .zip archive. By ensuring these parameters remain constant across different runs, the resulting .zip files will be identical, provided the input files are the same.

Implementation Details

The solution typically involves modifying the script responsible for creating the .zip file. This script often uses a library or utility to handle the zipping process. The key steps in the implementation are:

• Importing necessary libraries: The script imports the libraries required for file system operations and .zip archive creation.
• Defining the target directory: The script identifies the directory containing the files to be zipped.
• Creating the .zip archive: The script creates a new .zip archive file.
• Iterating through files: The script iterates through all files within the target directory.
• Setting modification time: For each file, the script sets the modification time to a predetermined constant value. This ensures that the timestamp stored in the .zip archive is consistent across builds.
• Adding files to the archive: The script adds each file to the .zip archive, using a consistent compression method and level.
• Closing the archive: The script closes the .zip archive, finalizing the process.

The specific code implementation may vary depending on the programming language and libraries used. However, the underlying principles remain the same: control file metadata, compression, and ordering to ensure determinism.

Applying the Solution to All Prepare Scripts

To ensure consistent .zip file generation across the terraform-cloudgov repository, the deterministic .zip creation code needs to be applied to all prepare scripts. This involves identifying all scripts responsible for creating .zip files and incorporating the necessary code modifications. The provided task list highlights the need to add this code to all prepare scripts in the repository.

This can be achieved through a systematic approach:

1. Identifying prepare scripts: Use the provided search query or other methods to identify all prepare scripts within the repository.
2. Analyzing each script: Examine each script to determine how it creates .zip files.
3. Implementing the solution: Incorporate the deterministic .zip file generation code into each script, ensuring that file metadata, compression, and ordering are controlled.
4. Testing: Thoroughly test each modified script to verify that it generates .zip files deterministically.

By systematically applying the solution to all prepare scripts, the terraform-cloudgov repository can ensure consistent and reliable .zip file generation across all modules.

Acceptance Criteria: Validating Deterministic .zip Generation

To ensure the effectiveness of the implemented solution, specific acceptance criteria need to be defined and validated. These criteria should clearly demonstrate that the generated .zip files are indeed deterministic and that changes to the underlying code result in different .zip files.

The provided acceptance criteria outline a clear testing procedure:

GIVEN I have called the prepare script with an existing checkout of a directory

AND I named that file orig.zip

• WHEN I run the prepare script again AND I name the output file repeat.zip THEN the sha256 values of orig.zip and repeat.zip are the same
• WHEN I make an empty commit AND I run the prepare script again AND I name the output file updated.zip THEN the sha256 values of orig.zip and updated.zip are different

These criteria effectively test the core requirements:

• Determinism: The first criterion ensures that running the prepare script multiple times with the same input results in identical .zip files (same sha256 values).
• Change detection: The second criterion ensures that even a minor change (an empty commit) results in a different .zip file (different sha256 values).

By validating these criteria, the team can confidently confirm that the implemented solution effectively addresses the issue of non-deterministic .zip file generation.

Conclusion

Generating .zip files deterministically is crucial for ensuring consistent and reliable deployments in IaC projects like terraform-cloudgov. By controlling file metadata, compression methods, and file ordering, the implemented solution effectively addresses the challenges of non-deterministic .zip file generation. Applying this solution to all prepare scripts within the repository and validating the defined acceptance criteria will ensure that the terraform-cloudgov modules generate consistent .zip files, minimizing spurious deployments and improving the overall deployment process.

This approach not only enhances the reliability of deployments but also contributes to a more streamlined and efficient development workflow. By eliminating the variability in .zip file generation, developers can focus on code changes and improvements without being concerned about unexpected deployment failures caused by non-deterministic builds. The principles discussed in this article can be applied to various other projects and scenarios where deterministic artifact generation is a critical requirement.