TEP-0087: Custom Tasks Graduation [Problem Statement]

Today, `Custom Tasks` shared by the Tekton community are all *experimental* and we haven't
defined a process to promote them beyond *experimental*.

As such:
- users can't depend on the `Custom Tasks` because they not *stable* (they can change any time).
- contributors don't have a process to stabilize their `Custom Tasks` or integrate them natively
to the Tekton Pipelines API.

In this TEP, we aim to:
- define the graduation path for `Custom Tasks` from *experimental* to *stable* to *integrated*.
- provide infrastructure for *stable* `Custom Tasks` that are officially supported by Tekton.

See also [TEP-0002: Enable Custom Tasks](https://github.com/tektoncd/community/blob/main/teps/0002-custom-tasks.md).

teps/0087-custom-tasks-graduation.md

@@ -0,0 +1,211 @@
+---
+status: proposed
+title: Custom Tasks Graduation
+creation-date: '2021-09-28'
+last-updated: '2021-09-28'
+authors:
+- '@jerop'
+see-also:
+- TEP-0002
+---
+
+# TEP-0087: Custom Tasks Graduation
+
+<!--
+**Note:** When your TEP is complete, all of these comment blocks should be removed.
+
+To get started with this template:
+
+- [ ] **Fill out this file as best you can.**
+  At minimum, you should fill in the "Summary", and "Motivation" sections.
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+  appropriate Working Group.
+- [ ] **Create a PR for this TEP.**
+  Assign it to people in the SIG that are sponsoring this process.
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+  Avoid getting hung up on specific details and instead aim to get the goals of
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+  start with the high-level sections and fill out details incrementally in
+  subsequent PRs.
+
+Just because a TEP is merged does not mean it is complete or approved.  Any TEP
+marked as a `proposed` is a working document and subject to change.  You can
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+
+```
+<<[UNRESOLVED optional short context or usernames ]>>
+Stuff that is being argued.
+<<[/UNRESOLVED]>>
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+focused.  If you disagree with what is already in a document, open a new PR
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+
+If there are new details that belong in the TEP, edit the TEP.  Once a
+feature has become "implemented", major changes should get new TEPs.
+
+The canonical place for the latest set of instructions (and the likely source
+of this file) is [here](/teps/NNNN-TEP-template/README.md).
+
+-->
+
+<!--
+This is the title of your TEP.  Keep it short, simple, and descriptive.  A good
+title can help communicate what the TEP is and should be considered as part of
+any review.
+-->
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+<!--
+A table of contents is helpful for quickly jumping to sections of a TEP and for
+highlighting any additional information provided beyond the standard TEP
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+Ensure the TOC is wrapped with
+  <code>&lt;!-- toc --&rt;&lt;!-- /toc --&rt;</code>
+tags, and then generate with `hack/update-toc.sh`.
+-->
+
+<!-- toc -->
+- [Summary](#summary)
+- [Motivation](#motivation)
+  - [Goals](#goals)
+  - [Non-Goals](#non-goals)
+  - [Use Cases](#use-cases)
+- [Requirements](#requirements)
+- [References](#references)
+<!-- /toc -->
+
+## Summary
+
+<!--
+This section is incredibly important for producing high quality user-focused
+documentation such as release notes or a development roadmap.  It should be
+possible to collect this information before implementation begins in order to
+avoid requiring implementors to split their attention between writing release
+notes and implementing the feature itself.
+
+A good summary is probably at least a paragraph in length.
+
+Both in this section and below, follow the guidelines of the [documentation
+style guide]. In particular, wrap lines to a reasonable length, to make it
+easier for reviewers to cite specific portions, and to minimize diff churn on
+updates.
+
+[documentation style guide]: https://github.com/kubernetes/community/blob/master/contributors/guide/style-guide.md
+-->
+
+Today, `Custom Tasks` shared by the Tekton community are all *experimental* and we haven't 
+defined a process to promote them beyond *experimental*. 
+
+As such:
+- users can't depend on the `Custom Tasks` because they not *stable* (they can change any time).
+- contributors don't have a process to stabilize their `Custom Tasks` or integrate them natively 
+to the Tekton Pipelines API.
+
+In this TEP, we aim to:
+- define the graduation path for `Custom Tasks` from *experimental* to *stable* to *integrated*.
+- provide infrastructure for *stable* `Custom Tasks` that are officially supported by Tekton.
+
+## Motivation
+
+<!--
+This section is for explicitly listing the motivation, goals and non-goals of
+this TEP.  Describe why the change is important and the benefits to users.  The
+motivation section can optionally provide links to [experience reports][] to
+demonstrate the interest in a TEP within the wider Tekton community.
+
+[experience reports]: https://github.com/golang/go/wiki/ExperienceReports
+-->
+
+As described in [TEP-0002: Enable Custom Tasks](0002-custom-tasks.md), we introduced `Custom Tasks` that are defined as
+CRDs that are executed using [`Runs`](https://github.com/tektoncd/pipeline/blob/main/docs/runs.md). These `Custom Tasks` 
+have reconciling controllers that watch for `Runs` referencing their types and updates their status. `Custom Tasks` 
+enable extensibility in Tekton - users can implement functionality that's not supported natively in Tekton Pipelines. 
+
+Hitherto, we have implemented several `Custom Tasks` in [Tekton Experimental Repository](https://github.com/tektoncd/experimental):
+- [Common Expression Language Custom Tasks](https://github.com/tektoncd/experimental/tree/main/cel) - provides support 
+for Common Expression Language (CEL) in Tekton Pipelines.
+- [Wait Custom Tasks](https://github.com/tektoncd/experimental/tree/main/wait-task) - enables waiting for some amount 
+of time between `Tasks`.
+- [Task Looping Custom Tasks](https://github.com/tektoncd/experimental/tree/main/task-loops) - enables running a 
+`Task` in a loop with varying `Parameter` values.
+- [Pipelines in Pipelines Custom Tasks](https://github.com/tektoncd/experimental/tree/main/pipelines-in-pipelines) - 
+provide support for composing and executing `Pipelines` in `Pipelines`.
+
+These `Custom Tasks` implementations are all *experimental*. Notwithstanding, they are in use in critical projects, such 
+as [Kubeflow Pipelines on Tekton](https://developer.ibm.com/blogs/kubeflow-pipelines-and-tekton-advances-data-workloads/).
+Providing stability levels and progression for `Custom Tasks` will enable users to rely on them and empower contributors 
+to safely update them. 
+
+We may decide to promote some `Custom Tasks` to top level such that their functionality is supported natively in the 
+Tekton Pipelines API. On the other hand, we may decide not to natively support some functionalities provided in some 
+`Custom Tasks` but we need to provide stability guarantees. Thus, we need to provide an incremental graduation path and 
+stability levels for `Custom Tasks`. 
+
+### Goals
+
+<!--
+List the specific goals of the TEP.  What is it trying to achieve?  How will we
+know that this has succeeded?
+-->
+
+1. Provide a graduation path for `Custom Tasks` from *experimental* to *stable* and *integrated*.
+
+2. Provide infrastructure for *stable* `Custom Tasks` that are officially supported by Tekton.
+
+### Non-Goals
+
+<!--
+What is out of scope for this TEP?  Listing non-goals helps to focus discussion
+and make progress.
+-->
+
+1. Promote `Custom Tasks` feature itself from `alpha` to `beta` - we will address this separately soon.
+
+2. Provide CLI support for `Custom Tasks` provided as *stable* integrations - we can explore this later.
+
+### Use Cases
+
+<!--
+Describe the concrete improvement specific groups of users will see if the
+Motivations in this doc result in a fix or feature.
+
+Consider both the user's role (are they a Task author? Catalog Task user?
+Cluster Admin? etc...) and experience (what workflows or actions are enhanced
+if this problem is solved?).
+-->
+
+1. As a contributor, I want to promote my `Custom Tasks` from *experimental* to *stable* and, possibly, *integrated*. 
+I need a process that I can follow with clear requirements for graduation to the next level. 
+
+2. As a user, I need to use *stable* `Custom Tasks` that I can rely on. 
+
+## Requirements
+
+<!--
+Describe constraints on the solution that must be met. Examples might include
+performance characteristics that must be met, specific edge cases that must
+be handled, or user scenarios that will be affected and must be accomodated.
+-->
+
+- Define graduation requirements and path for `Custom Tasks` from *experimental* to *stable* and *integrated*.
+
+- Provide a catalog of `Custom Tasks` that are *stable* integrations with necessary infrastructure and processes, such 
+as thorough testing and frequent releases.
+
+## References
+
+<!--
+Use this section to add links to GitHub issues, other TEPs, design docs in Tekton
+shared drive, examples, etc. This is useful to refer back to any other related links
+to get more details.
+-->
+
+- [TEP-0002: Enable Custom Tasks](0002-custom-tasks.md)
+- [TEP-0056: Pipelines in Pipelines](0056-pipelines-in-pipelines.md)
+- [Pipelines in Pipelines Custom Tasks](https://github.com/tektoncd/experimental/tree/main/pipelines-in-pipelines)
+- [Common Expression Language Custom Tasks](https://github.com/tektoncd/experimental/tree/main/cel)
+- [Wait Custom Tasks](https://github.com/tektoncd/experimental/tree/main/wait-task)
+- [Task Looping Custom Tasks](https://github.com/tektoncd/experimental/tree/main/task-loops)
+- [Kubeflow Pipelines on Tekton](https://developer.ibm.com/blogs/kubeflow-pipelines-and-tekton-advances-data-workloads/) 

teps/README.md

@@ -223,3 +223,4 @@ This is the complete list of Tekton teps:
 |[TEP-0073](0073-simplify-metrics.md) | Simplify metrics | proposed | 2021-06-23 |
 |[TEP-0080](0080-support-domainscoped-parameterresult-names.md) | Support domain-scoped parameter/result names | implemented | 2021-08-19 |
 |[TEP-0084](0084-endtoend-provenance-collection.md) | end-to-end provenance collection | proposed | 2021-09-16 |
+|[TEP-0087](0087-custom-tasks-graduation.md) | Custom Tasks Graduation | proposed | 2021-09-28 |