Apko is a project developed by Chainguard aimed at building and deploying OCI artifacts (like containers and Helm charts) with a focus on supply chain security and reproducibility. It fundamentally shifts the paradigm of building images from traditional, layered approaches (like Dockerfiles) to a single-layer, declarative build process. This approach drastically reduces the attack surface and improves auditability. Instead of a series of instructions that modify layers, Apko takes a source directory and a blueprint file, and produces a final, flattened image.

The core innovation lies in its use of a blueprint file, written in YAML, which defines the desired state of the final image. This blueprint specifies the files to include, their destinations within the image, user/group ownership, permissions, and other metadata. Crucially, it *doesn't* define *how* to get there, only *what* the result should be. Apko then handles the complexities of assembling the image in a secure and reproducible manner. This declarative nature allows for easier verification and understanding of the build process. It also enables features like content-addressable builds, where the image hash is determined solely by the blueprint and source content, guaranteeing immutability.

Apko's architecture is built around several key components. The `apko` CLI is the primary interface for users, handling blueprint parsing, source directory processing, and image creation. A key internal component is the "flattening" engine, which takes the blueprint and source and constructs the final, single-layer image. This flattening process eliminates the historical layers inherent in traditional container images, removing potential vulnerabilities hidden within older layers and simplifying dependency tracking. Apko also integrates with existing container registries, allowing for seamless pushing and pulling of built images.

A significant benefit of Apko is its improved security posture. By eliminating layers, it removes the risk of secrets or vulnerabilities being inadvertently included in older, unused layers. The declarative blueprint provides a clear and auditable record of the image's contents and configuration. Furthermore, Apko supports signing images using tools like cosign, ensuring authenticity and integrity. The single-layer approach also simplifies vulnerability scanning, as all content is readily accessible for analysis.

Beyond basic image building, Apko supports more complex scenarios like building Helm charts. It can package application code, dependencies, and Kubernetes manifests into a single, reproducible OCI artifact. This simplifies deployment and management of applications on Kubernetes. The project is actively developed and includes features like support for different architectures (amd64, arm64) and integration with various build systems. Apko represents a compelling alternative to traditional container build tools, particularly for organizations prioritizing supply chain security and reproducibility in their software delivery pipelines.