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In this PR, we discuss the motivation, goals, non-goals and requirements for `Workspace` dependencies so that we can align on the problem before exploring the proposal and alternatives later `Tasks` can have either resource dependencies or ordering dependencies between them. Resource dependencies are based on `Results` and `Workspaces`, while ordering dependencies are defined using `runAfter` to sequence `Tasks`. Today, users cannot specify resource dependencies based on `Workspaces`, that is, a `Task` should execute and use a given `Workspace` before another `Task` executes and uses the same `Workspace`. We need to provide a way for users to specify resource dependencies based on `Workspaces` to ensure that failure and skipping strategies for common CI/CD use cases work and users don't get unexpected `Pipeline` failures when we roll out those features. Failure and skipping strategies: - [TEP-0059: Skip Guarded Task Only](https://github.com/tektoncd/community/blob/main/teps/0059-skip-guarded-task-only.md) - [TEP-0050: Ignore Task Failures](https://github.com/tektoncd/community/blob/main/teps/0050-ignore-task-failures.md) Issue: tektoncd/pipeline#3109
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| --- | ||
| status: proposed | ||
| title: Workspace Dependencies | ||
| creation-date: '2021-04-23' | ||
| last-updated: '2021-04-23' | ||
| authors: | ||
| - '@jerop' | ||
| --- | ||
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| # TEP-0063: Workspace Dependencies | ||
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| <!-- | ||
| **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. | ||
| These should be easy if you've preflighted the idea of the TEP with the | ||
| appropriate Working Group. | ||
| - [ ] **Create a PR for this TEP.** | ||
| Assign it to people in the SIG that are sponsoring this process. | ||
| - [ ] **Merge early and iterate.** | ||
| Avoid getting hung up on specific details and instead aim to get the goals of | ||
| the TEP clarified and merged quickly. The best way to do this is to just | ||
| 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 | ||
| denote sections that are under active debate as follows: | ||
| ``` | ||
| <<[UNRESOLVED optional short context or usernames ]>> | ||
| Stuff that is being argued. | ||
| <<[/UNRESOLVED]>> | ||
| ``` | ||
| When editing TEPS, aim for tightly-scoped, single-topic PRs to keep discussions | ||
| focused. If you disagree with what is already in a document, open a new PR | ||
| with suggested changes. | ||
| 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. | ||
| --> | ||
|
|
||
| <!-- | ||
| 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 | ||
| template. | ||
| Ensure the TOC is wrapped with | ||
| <code><!-- toc --&rt;<!-- /toc --&rt;</code> | ||
| tags, and then generate with `hack/update-toc.sh`. | ||
| --> | ||
|
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||
| <!-- toc --> | ||
| - [Summary](#summary) | ||
| - [Motivation](#motivation) | ||
| - [Goals](#goals) | ||
| - [Non-Goals](#non-goals) | ||
| - [Use Cases](#use-cases) | ||
| - [Requirements](#requirements) | ||
| - [References](#references) | ||
| <!-- /toc --> | ||
|
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| ## Summary | ||
|
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||
| <!-- | ||
| 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 | ||
| --> | ||
|
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| `Tasks` can have either resource dependencies or ordering dependencies between them. Resource dependencies are based on | ||
| `Results` and `Workspaces`, while ordering dependencies are defined using `runAfter` to sequence `Tasks`. | ||
|
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| Today, users cannot specify resource dependencies based on `Workspaces`, that is, a `Task` should execute and use a | ||
| given `Workspace` before another `Task` executes and uses the same `Workspace`. We need to provide a way for users to | ||
| specify resource dependencies based on `Workspaces` to ensure that failure and skipping strategies for common CI/CD | ||
| use cases work and users don't get unexpected `Pipeline` failures when we roll out those features. | ||
|
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||
| ## Motivation | ||
|
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||
| <!-- | ||
| 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 | ||
| --> | ||
|
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| In Tekton, `Pipelines` are `Directed Acyclic Graphs` where: | ||
| - The `Nodes` are `Tasks` | ||
| - The `Edges` between the `Nodes` are: | ||
| - **resource dependencies**: specified using resources that need to be passed from one `Task` to another | ||
| `Task`, through `Results` or `Workspaces` | ||
| - **ordering dependencies**: specified using `runAfter`, which are used to define sequencing between `Tasks` | ||
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| ``` | ||
| task-1 | ||
| / \ | ||
| task-2 (result) | ||
| / \ | ||
| (workspace file) task-4 | ||
| / | ||
| task-3 | ||
| ``` | ||
|
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| In the example above: | ||
| - `task-4` is resource-dependent on `task-1` based on a `Result` (a string is passed between them) | ||
| - `task-3` is resource-dependent on `task-2` based on a `Workspace` (a file is passed between them) | ||
| - `task-2` is ordering-dependent on `task-1` based on `runAfter` (no resource is passed between them) | ||
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| Today, users cannot specify resource dependencies based on `Workspaces`; they can only use `runAfter` as a workaround. | ||
| However, it has become critical that we solve this problem and provide a way to explicitly specify `Workspace` | ||
| dependencies between `Tasks`. | ||
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| We are designing failure and skipping strategies to solve common CI/CD use cases in | ||
| [TEP-0050 Ignore Task Failures](https://github.com/tektoncd/community/blob/main/teps/0050-ignore-task-failures.md) and | ||
| [TEP-0059 Skip Guarded Task Only](https://github.com/tektoncd/community/blob/main/teps/0059-skip-guarded-task-only.md). | ||
| These TEPs will unblock the execution of dependent `Tasks` when a parent `Task` fails or is skipped. In those scenarios, | ||
| ordering-dependent `Tasks` should execute successfully while resource-dependent `Tasks` should be skipped because of | ||
| missing resources. | ||
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| However, because users cannot specify resource dependencies based on `Workspaces`, resource-dependent | ||
| `Tasks` based on `Workspaces` will be executed. The resource-dependent `Tasks` won't be able to resolve missing | ||
| resources that they expect in the `Workspaces`, so they will fail and the whole `Pipeline` would fail. Taking the | ||
| example above, if `task-2` is skipped or fails, and we unblock the execution of `task-3`, then `task-3` can't resolve | ||
| the `workspace file` so `task-3` would fail causing the `Pipeline` to fail. | ||
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| We need to enable users to specify resource dependencies based on `Workspaces` before we roll out the skipping and | ||
| failure strategies for common CI/CD use cases to ensure we avoid unexpected failures. | ||
|
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| ### Goals | ||
|
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||
| <!-- | ||
| List the specific goals of the TEP. What is it trying to achieve? How will we | ||
| know that this has succeeded? | ||
| --> | ||
|
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| The main goal of this TEP is to enable users to specify resource dependencies based on `Workspaces`. | ||
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| ### Non-Goals | ||
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| <!-- | ||
| What is out of scope for this TEP? Listing non-goals helps to focus discussion | ||
| and make progress. | ||
| --> | ||
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| Specifying the files that a `Task` produces to or expects from a `Workspace` is out of scope for this TEP and is | ||
| addressed in [TEP-0030 Workspace Paths](https://github.com/tektoncd/community/blob/main/teps/0030-workspace-paths.md). | ||
|
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| ### Use Cases | ||
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| <!-- | ||
| 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?). | ||
| --> | ||
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| Let's take the manual approval use case described in further detail in [TEP-0059 Skip Guarded Task Only](https://github.com/tektoncd/community/blob/main/teps/0059-skip-guarded-task-only.md). | ||
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| A user needs to design a `Pipeline` with a _manual approval_ `Task` that is executed when merging a pull request only. | ||
| The execution of the _manual approval_ `Task` is guarded using `WhenExpressions`. To reuse the same `Pipeline` when | ||
| merging and not merging, the user needs the dependent `Tasks` to execute when the guarded _manual approval_ `Task` is | ||
| skipped. | ||
|
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| ``` | ||
| lint unit-tests | ||
| | | | ||
| v v | ||
| report-linter-output integration-tests | ||
| | | ||
| v | ||
| manual-approval | ||
| | | | ||
| v (approver data) | ||
| build-image | | ||
| | v | ||
| v slack-msg | ||
| deploy-image | ||
| ``` | ||
|
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| In this use case, `manual-approval` produces information about the approver as a `Result` or a `Workspace` file; | ||
| `slack-msg` consumes the approver data and send a Slack notification about the manual approval. As such, `build-image` | ||
| is ordering-dependent to `manual-approval` while `slack-msg` is resource-dependent to `manual-approval` where the | ||
| resource is information about the approver. If `slack-msg` needs the name of the approver only, then the approver name | ||
| can be passed as a `Result`. If `slack-msg` needs detailed information about the approver beyond the name, | ||
| then `manual-approval` would need to write to a file in a `Workspace` that `slack-msg` would read from. | ||
|
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| If `WhenExpressions` in `manual-approval` evaluate to `True`, then `manual-approval` is executed and: | ||
| - if `manual-approval` succeeds, then `build-image`, `deploy-image` and `slack-msg` are executed | ||
| - if `manual-approval` fails, then `build-image`, `deploy-image` and `slack-msg` are not executed because the `Pipeline` | ||
| fails | ||
|
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| Today, if the `WhenExpressions` in `manual-approval` evaluate to `False`, then `manual-approval`, `build-image`, | ||
| `deploy-image` and `slack-msg` are all skipped. In [TEP-0059 Skip Guarded Task Only](https://github.com/tektoncd/community/blob/main/teps/0059-skip-guarded-task-only.md), | ||
| we'll provide the flexibility to unblock the execution of `build-image`, `deploy-image` and `slack-msg` when | ||
| `manual-approval` is skipped. This would allow the user to reuse the `Pipeline` in both scenarios (merging and not merging). | ||
|
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| If the approver resource is a `Result`, we can identify it as a resource dependency and skip `slack-msg`. However, if the | ||
| approver resource is passed through a `Workspace`, we can't identify that as a resource dependency. So we'll execute | ||
| `slack-msg` which would fail because it can't resolve missing resources. When `slack-msg` fails, the whole `Pipeline` | ||
| would fail. | ||
|
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| This TEP would enable the user to specify the resource dependency based on `Workspace` between `manual-approval` and | ||
| `slack-msg`. Then `slack-msg` would be skipped because of missing resources when `manual-approval` is skipped, and | ||
| `build-image` and `deploy-image` would execute successfully. As such, the failure and skipping strategies in the | ||
| `Pipeline` would work as expected when the resource dependencies are based on either `Results` or `Workspaces`. | ||
|
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| ## 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. | ||
| --> | ||
|
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| Users should be able to specify resource dependency based on `Workspaces`, that is, a `Task` should execute and use a | ||
| given `Workspace` before another `Task` executes and uses the same `Workspace`. | ||
|
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| ## 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. | ||
| --> | ||
|
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||
| - [Allow Tasks in Pipelines to express a dependency on workspaces 'from' previous Tasks #3109](https://github.com/tektoncd/pipeline/issues/3109) | ||
| - [TEP-0059: Skip Guarded Task Only](https://github.com/tektoncd/community/blob/main/teps/0059-skip-guarded-task-only.md) | ||
| - [TEP-0050: Ignore Task Failures](https://github.com/tektoncd/community/blob/main/teps/0050-ignore-task-failures.md) | ||
| - [TEP-0030: Workspace Paths](https://github.com/tektoncd/community/blob/main/teps/0030-workspace-paths.md) | ||
| - [TEP-0007: Conditions Beta](https://github.com/tektoncd/community/blob/main/teps/0007-conditions-beta.md) | ||
|
|
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