Lua-based Pre-Processing

Use Lua for complex Turing-complete PR Automations

PR automations normally work through deterministic templates defined in YAML. This goes a long way toward handling the code generation needed for a large set of repeatable platform engineering tasks, but occasionally you need to go further by:

generating code based on other configuration files in a repo
adding content-based error handling
auto-inferring inputs based on complex, custom rules

For all these cases, Plural's Lua integration lets developers fill the gap with a sandboxed, Turing-complete programming language.

Configuring a lua-based PR Automation

The simplest example of a Lua-based PR automation is below:

yaml

apiVersion: deployments.plural.sh/v1alpha1
kind: PrAutomation
metadata:
  name: lua-updater
spec:
  name: lua-updater
  documentation: Updates the console image version
  repositoryRef:
    name: infra
    namespace: infra
  git:
    ref: main
    folder: lua
  lua:
    external: true # sources lua code from the infra/infra GitRepository's main branch in the lua folder. 
    folder: '.'
  scmConnectionRef:
    name: plural
  title: "Updating images to for {{ context.component }}"
  message: "Updating images to for {{ context.component }} to use {{ context.images }}"
  identifier: pluralsh/plrl-up-demos
  configuration:
  - name: component
    type: STRING
    documentation: The component to update
  - name: images
    type: STRING
    documentation: The images to update

This will use all Lua code in the configured folder, which can use all the capabilities described in our core Lua docs here. Some example Lua code might look like:

lua

local images, err = encoding.jsonDecode(context.images)
if err then
    error("Failed to decode images: " .. tostring(err))
end

lutFile, err = fs.read("images/lut.yaml")
if err then
    error("Failed to read lut file: " .. tostring(err))
end

lut, err = encoding.yamlDecode(lutFile)
if err then
    error("Failed to decode lut file: " .. tostring(err))
end

overlays = {}

for _, image in ipairs(images) do
    local repo, tag = image:match("^(.-):(.+)$")
    if not repo then
        error("Invalid image: " .. tostring(image))
    end

    if lut[repo] then
        tag = tag:match("([^@]+)")
        if tag then
            local overlayContent = {images = {}}
            overlayContent["images"][lut[repo]] = {version = tag}
            table.insert(overlays, {
                file = "images/registry.yaml",
                yaml = encoding.yamlEncode(overlayContent)
            })
        end
    end
end

if #overlays > 0 then
    prAutomation = {
        updates = {
            yamlOverlays = overlays
        }
    }
end

Lua PR Automation outputs

Lua scripts should generate one of two fields: