use kapp as a Go Module

[universam1]

Describe the problem/challenge you have

We'd be very excited if we would not have to shell out to Kapp from an orchestration tool but use as a Go module, which would dramatically improve the error handling and resilience.

Describe the solution you'd like

Similar to https://github.com/vmware-tanzu/carvel-ytt/blob/develop/examples/integrating-with-ytt/apis.md#as-a-go-module there could be general commitment to an exported pkg or API, that is subject to change as long as Kapp version is < 1

Anything else you would like to add:

Issue created on positive initial feedback from https://kubernetes.slack.com/archives/CH8KCCKA5/p1659086135812959

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[universam1]

Due to repo move a go.mod replace is required:

replace (
	github.com/k14s/kapp => github.com/vmware-tanzu/carvel-kapp v0.50.0
)

[praveenrewar]

Due to repo move a go.mod replace is required:

Ah, yes. @benmoss is working on a PR to update this :)

[universam1]

as @praveenrewar mentioned the current requirement to build is k8s.io/client-go < v0.23

From my test a simple approach is to leverage the cobra command including all Kapp app per se. Disadvantage here is that the commands are to be generated via a string generator which is a dynamic not very safe interfacing. Example

import (
	"github.com/cppforlife/go-cli-ui/ui"
	"github.com/k14s/kapp/pkg/kapp/cmd"
)
	confUI := ui.NewConfUI(ui.NewNoopLogger())
	command := cmd.NewDefaultKappCmd(confUI)

	args := []string{
		"app-group",
		"deploy",
		"-g", "all",
		"-n", "default",
		"--directory", "myfolder",
		"-c",
		"--apply-default-update-strategy", "fallback-on-replace",
		"--yes"}
	command.SetArgs(args)
	command.Execute()

I'll research what it takes to do an explicit call to the pkg

[universam1]

Alternative approach is to avoid spf13/cobra as dependency and call the structs and methods natively. Advantage is less dependencies and thus a little smaller binary, but with the rather challenging requirement to disassabmle all the default values of the structs.

Example for app-group only run:

	confUI := ui.NewConfUI(ui.NewNoopLogger())
	defer confUI.Flush()
	confUI.EnableNonInteractive()
	confUI.EnableColor()

	nonInteractiveUI := ui.NewNonInteractiveUI(confUI)

	configFactory := cmdcore.NewConfigFactoryImpl()
	configFactory.ConfigurePathResolver(func() (string, error) { return kubeConfig, nil })
	configFactory.ConfigureContextResolver(cmdcore.NewKubeconfigContextFlag().Value)
	configFactory.ConfigureYAMLResolver(cmdcore.NewKubeconfigYAMLFlag().Value)

	depsFactory := cmdcore.NewDepsFactoryImpl(configFactory, nonInteractiveUI)
	o := cmdag.NewDeployOptions(nonInteractiveUI, depsFactory, kapplogger.NewTODOLogger())

	o.AppGroupFlags.Name = "all"

	o.AppGroupFlags.NamespaceFlags.Name = "kube-system"
	o.DeployFlags.Directory = serviceFolder
	o.AppFlags.DiffFlags.Changes = false
	o.AppFlags.DeployFlags.OverrideOwnershipOfExistingResources = true
	o.AppFlags.DeployFlags.ExistingNonLabeledResourcesCheck = true
	o.AppFlags.DeployFlags.ExistingNonLabeledResourcesCheckConcurrency = 100

	o.AppFlags.ApplyFlags.ApplyIgnored = false
	o.AppFlags.ApplyFlags.Wait = true
	o.AppFlags.ApplyFlags.WaitIgnored = false
	o.AppFlags.DeleteApplyFlags.ApplyIgnored = false
	o.AppFlags.DeleteApplyFlags.Wait = true
	o.AppFlags.DeleteApplyFlags.WaitIgnored = true

	o.AppFlags.ApplyFlags.ApplyingChangesOpts.Timeout = 15 * time.Minute
	o.AppFlags.ApplyFlags.ApplyingChangesOpts.CheckInterval = time.Second
	o.AppFlags.ApplyFlags.ApplyingChangesOpts.Concurrency = 5

	o.AppFlags.ApplyFlags.WaitingChangesOpts.Timeout = 15 * time.Minute
	o.AppFlags.ApplyFlags.WaitingChangesOpts.ResourceTimeout = time.Minute
	o.AppFlags.ApplyFlags.WaitingChangesOpts.CheckInterval = time.Second
	o.AppFlags.ApplyFlags.WaitingChangesOpts.Concurrency = 5

	o.AppFlags.ApplyFlags.AddOrUpdateChangeOpts.DefaultUpdateStrategy = "fallback-on-replace"
	err = o.Run()

[renuy]

@praveenrewar to see the feasibility and figure out a way to make these functions as APIs

[dprotaso]

Following up here - it would be good for kapp to be a go module then it could be used as a library in an operator-like tool etc.