Bosh Templating Language RFC

toc/rfc/rfc-draft-add-new-bosh-release-templating-language.md

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+# Meta
+[meta]: #meta
+- Name: Bosh Templating Language
+- Start Date: 2024-10-21
+- Author(s): jpalermo
+- Status: Draft
+- RFC Pull Request: [community#1004](https://github.com/cloudfoundry/community/pull/1004)
+
+
+## Summary
+
+Introduce new custom templating language that can be used by bosh releases. Ideally gather feedback from release
+authors as part of the RFC process to define the MVP of the language.
+
+## Problem
+
+Currently all bosh release templates are rendered using ERB using the Bosh Director's Ruby process, or whatever
+Ruby process is available when a `bosh create-env` is being run.
+
+We've had multiple release breakages over the years as we upgrade the Bosh Director Ruby version and the Ruby
+language changes in a way that release templates were not expecting.
+
+A secondary problem is that template rendering can be slow when rendering templates for large CF deployments.
+
+## Proposal
+
+Write a library/cli that defines a strict grammar suitable for basic template rendering within Bosh releases.
+
+A proof of concept can be found [here](https://github.com/jpalermo/bosh-template-renderer).
+
+We would NOT be removing ERB rendering as an option anytime in the near future. We would update many of the Bosh
+Releases in the Foundational Infrastructure Working Group to use the new templates and then target releases
+that are on Diego Cells as large CF deployments render those same templates a large number of times.
+
+Having a grammar under our control means we can ensure it does not change or break through golang and dependency
+upgrades.
+
+The Bosh Director is written in Ruby, so it will be able to shell out to the CLI to render templates. While
+the Bosh CLI is written in golang so it will be able to import the library and use it directly.
+
+The grammar would be kept relatively small. We will not be providing a direct replacement for ERB. Many validations
+and data transformations are happening within ERB templates right now that should be happening at different layers
+of the deployment process.
+
+Data transformation should be happening within the release runtimes or pre-start scripts. Validation can also happen
+there, although it's possible we want to expand the Bosh Release `spec` files to provide some level of validation so
+errors can be caught early in the deploy process (that work is NOT part of this RFC).