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Lua

Sveltos enables the execution of Lua code stored within ConfigMap or Secret Kubernetes resources. To allow Sveltos to identify and reference these resources within the Sveltos profiles, the annotation projectsveltos.io/lua annotation needs to be defined within the resources. Check out the example below. The Lua code gets executed, and the resulting output is used to deploy configurations to matching clusters.

This will instruct Sveltos to fetch the Secret imported-secret in the default namespace and replicate it to any env: prod cluster:

apiVersion: v1
kind: ConfigMap
metadata:
  annotations:
    projectsveltos.io/lua: ok
  name: lua
  namespace: default
data:
  lua.yaml: |-
    function evaluate()
      local secret = getResource(resources, "ExternalSecret")
      print(secret.metadata.name)
      local hs = {}
      local result = [[
    apiVersion: v1
    kind: Secret
    metadata:
      name: ]] .. secret.metadata.name .. [[

      namespace: ]] .. secret.metadata.namespace .. [[

    data:
    ]]
      for k, v in pairs(secret.data) do
        result = result .. "  " .. k .. ": " .. v .. "\n"
      end
      hs.resources = result
      return hs
    end

apiVersion: config.projectsveltos.io/v1beta1
kind: ClusterProfile
metadata:
  name: deploy-resources
spec:
  clusterSelector:
    matchLabels:
      env: prod
  templateResourceRefs:
  - resource:
      apiVersion: v1
      kind: Secret
      name: imported-secret
      namespace: default
    identifier: ExternalSecret
  policyRefs:
  - kind: ConfigMap
    name: lua
    namespace: default

Helper Functions:

Sveltos provides several helper functions to simplify resource manipulation within Lua scripts:

  • getResource(resources, "resource identifier"): Retrieves a specific resource by its identifier from the provided resource list. The resources represents all Kubernetes resources defined in the Spec.TemplateResourceRefs section of a Sveltos profile. The identifier must match an identifier previously defined in the same Spec.TemplateResourceRefs section.
  • getLabel(resource, "key"): Returns the value of the label with the specified key for a given resource.
  • getAnnotation(resource, "key"): Returns the value of the annotation with the specified key for a given resource.
  • base64Encode and base64Decode: For base64 encoding and decoding.
  • json.encode and json.decode: For JSON encoding and decoding.
  • strings functions: Provides a range of string manipulation functions such as Compare, Contains, HasPrefix, HasSuffix, Join, Replace, Split, ToLower, and ToUpper. These functions are based on the this library. To use these methods, call them as strings.ToUpper("mystring").

Some examples:

  local strings = require("strings")

  function evaluate()
    local secret = getResource(resources, "ExternalSecret") 
    print(strings.ToUpper(secret.metadata.name))
    local splitTable = strings.Split(secret.metadata.name, "-") -- metadata.name in the example imported-secret
    for i, v in ipairs(splitTable) do
      print("Element", i, ":", v)
    end
    local encoded = base64Encode(secret.metadata.name)

  local json = require("json")

  function evaluate()
    local secret = getResource(resources, "ExternalSecret") 
    local encoded = json.encode(secret)
    print(encoded)

Extending Lua Capabilities with Custom Helper Functions:

Sveltos allows users to extend its Lua scripting capabilities by defining custom helper functions. These functions are packaged as Lua code within a ConfigMap residing in the projectsveltos namespace.

To load and utilize the custom functions, thelua-methods argument must be provided to the addon-controller deployment. Once loaded, the custom methods become available whenever Lua code is executed by Sveltos.

Note

The below libraries are used. https://github.com/chai2010/glua-strings/ for string manipulation. https://github.com/layeh/gopher-json for JSON handling.