Porch Lazy Dog (Lathund)

A quick reference Lazy Dog/Lathund of tips, tricks, workarounds, and shortcuts for developers working with Porch. Please raise a PR to add your Porch magic spells to the Lazy Dog.

Debugging Starlark scripts

Debugging Starlark scripts can be difficult, especially when running mutation pipelines in Porch. One technique is to put print statements in the code to print the value of variables. For example:

def set_package_name(resources, root_kptfile_path, root_package_name):
  for r in resources:
    resource_path = r["metadata"]["annotations"]["internal.config.kubernetes.io/package-path"]
    resource_name = r["metadata"]["annotations"]["config.kubernetes.io/path"]
    print("Resource Path: " + resource_path)
    print("Resource Name: " + resource_name)

But that’s not enough in Porch.

To force output of the result of partial rendering of failing pipelines in kpt, you must set the following annotation on the Kptfile:

apiVersion: kpt.dev/v1
kind: Kptfile
metadata:
  name: wordpress
  annotations:
    kpt.dev/save-on-render-failure: "true"

You also must set an annotation on the Package Revision so that Porch will push draft package revisions even when they fail.

kubectl annotate packagerevision <name> porch.kpt.dev/push-on-render-failure=true

If the mutation pipeline is passing, you won’t get any output from your print() statement because Porch assumes everything is OK and does not print any output. The easiest way to work around this is to put a deliberate runtime error into your Starlark script, which will cause an error and trigger the output:

  set_package_name(ctx.resource_list["items"], root_kptfile_path, root_package_name)

  i = 10/0 # Deliberate division by zero error

Dumping resources to disk while debugging rendering in Porch

It can be difficult to see what is happening with PackageRevisionResources during rendering, especially if a mutation pipeline is buggy. During debugging of rendering in Porch it can be convenient to dump the resources to disk so that regular comparison tools can be used to spot inconsistencies.

For example, the code fragment below calls a render:

		resources, _, err = th.renderMutation(draftMeta.GetNamespace()).apply(ctx, resources)
		if err != nil {
			klog.Error(err)
			return renderError(err)
		}

You can temporarily add a call to the WriteResourcesToFS() function to dump the “before” and “after” resources to disk for comparison.

		_, err = repository.WriteResourcesToFS(filesys.MakeFsOnDisk(), "/tmp/before", resources.Contents)
		if err != nil {
			klog.Error(err)
			return renderError(err)
		}

		resources, _, err = th.renderMutation(draftMeta.GetNamespace()).apply(ctx, resources)
		if err != nil {
			klog.Error(err)
			return renderError(err)
		}
		_, err = repository.WriteResourcesToFS(filesys.MakeFsOnDisk(), "/tmp/after", resources.Contents)
		if err != nil {
			klog.Error(err)
			return renderError(err)
		}