ytt

ytt, short for YAML Templating Tool, is an open-source tool for templating, patching, and merging YAML files. It is part of the Carvel suite of tools and is commonly used to generate Kubernetes manifests from a combination of templates, data values, and overlays. ytt focuses on YAML-aware processing, aiming to produce valid, deterministic output even when many inputs are combined.

Key features include a templating model that uses data values supplied from separate YAML files, as well as a system of overlays and patches that can modify existing YAML without duplicating content. It supports library functions and user-defined functions to transform data during rendering, enabling reuse and composability across manifests. The tool is designed to work well in declarative workflows, helping teams manage complex Kubernetes configurations with predictable results.

Usage typically involves providing one or more input paths to templates and data values. A rendered result is produced by the ytt command line, usually by specifying -f with directories or files containing templates and additional -f inputs for values or overlays. This makes ytt suitable for integration into CI/CD pipelines and GitOps workflows, where a single source of truth can generate environment-specific manifests from a common template base.

In practice, ytt is used to render Kubernetes manifests by combining templates with environment-specific data values and patches. It is often used alongside other Carvel tools for image replacement and distribution, forming part of a broader approach to declarative Kubernetes configuration management.