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Add TEP-0044: Composing Tasks with Tasks ➰
This TEP describes some difficulty folks have run into, especially when trying to use Tekton Pipelines without PipelineResources: if you want to use the functionality of multiple Tasks, you need to put them together in a Pipeline and share data between them with volumes. It could be a useful abstraction, and more efficient, to make it possible to combine Tasks together without scheudling them on different pods.
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| --- | ||
| status: proposed | ||
| title: Composing Tasks with Tasks | ||
| creation-date: '2021-01-22' | ||
| last-updated: '2021-01-22' | ||
| authors: | ||
| - '@bobcatfish' | ||
| --- | ||
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| # TEP-0044: Composing Tasks with Tasks | ||
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| <!-- toc --> | ||
| - [Summary](#summary) | ||
| - [Motivation](#motivation) | ||
| - [PipelineResources](#pipelineresources) | ||
| - [Goals](#goals) | ||
| - [Non-Goals](#non-goals) | ||
| - [Use Cases (optional)](#use-cases-optional) | ||
| - [Requirements](#requirements) | ||
| - [References (optional)](#references-optional) | ||
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| ## Summary | ||
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| This TEP describes a gap in composability that isn't currently fully solved by Tasks or Pipelines | ||
| (tho PipelineResources do solve some of it!). | ||
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| Ideally by addressing the issues described here we'd make it possible to create more efficient | ||
| Pipelines and make it easier to use a Task easily via a TaskRun, and we'd address some of | ||
| [the problems PipelineResources are trying to solve](https://docs.google.com/document/d/1KpVyWi-etX00J3hIz_9HlbaNNEyuzP6S986Wjhl3ZnA/edit#) | ||
| making it more clear whether or not they are an abstraction we want to keep. | ||
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| [#1673: Pipeline Resources Redesign](https://github.com/tektoncd/pipeline/issues/1673) | ||
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| ## Motivation | ||
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| Currently the only way to combine Tasks together is in a Pipeline. If you combine | ||
| Tasks in a Pipeline and they need to share data (beyond a simple | ||
| [result](https://github.com/tektoncd/pipeline/blob/master/docs/pipelines.md#using-results)), | ||
| you'll need to provision a PVC or do some other similar, cloud specific thing, | ||
| to [make a volume available](https://github.com/tektoncd/pipeline/blob/master/docs/workspaces.md#specifying-volumesources-in-workspaces) | ||
| that can be shared between them. | ||
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| Some problems with this: | ||
| * PVCs add additional overhead, both in speed and in management (somewhat | ||
| mitigated by | ||
| [letting tekton create and delete volumes for you](https://github.com/tektoncd/pipeline/blob/master/docs/workspaces.md#volumeclaimtemplate) | ||
| * Sometimes you just want to run a Task with a teeny bit more functionality, | ||
| e.g. you want to use a task to build images, but you want to clone source | ||
| with git first, and it seems like overkill to have to construct a pipeline | ||
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| ### PipelineResources | ||
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| This kind of compositional functionality was being (somewhat! read on!) provided | ||
| by [PipelineResources](https://github.com/tektoncd/pipeline/blob/master/docs/resources.md). | ||
| [We did not bring these types to beta](https://github.com/tektoncd/pipeline/blob/master/docs/resources.md#why-arent-pipelineresources-in-beta). | ||
| Issues that don't let PipelineResources (as is) solve these problems are: | ||
| * PipelineResource "outputs" do not run when the steps of the Task fail (or if | ||
| a previous PipelienResource "output" fails) (see [unit test use case](#use-cases-optional)) | ||
| * PipelineResources do not use Tasks (which is something we could change), meaning | ||
| you cannot use them to compose Tasks together, you need to build PipelineResources | ||
| * Adding new PipelineResources currently needs to be done in the Tekton Pipelines controller, | ||
| tho there have been several attempts to propose ways to make it extensible: | ||
| * [PipelineResources 2 Uber Design Doc](https://docs.google.com/document/d/1euQ_gDTe_dQcVeX4oypODGIQCAkUaMYQH5h7SaeFs44/edit) | ||
| * [Tekton Pipeline Resource Extensibility](https://docs.google.com/document/d/1rcMG1cIFhhixMSmrT734MBxvH3Ghre9-4S2wbzodPiU/edit) | ||
| * All of this is even though PipelineResources are _very_ similar to Tasks in that they are | ||
| collections of steps (which [get injected before and after a Task's steps](https://github.com/tektoncd/pipeline/issues/1838) | ||
| * Tasks have to declare in advance what PipelineResources they need; you can't decide to use | ||
| a PipelineResource with a Task after it has been written (e.g. in a TaskRun or a Pipeline) | ||
| and you can't mix types of PipelineResource (e.g. if a Task declares it needs a | ||
| [git PipelineResources](https://github.com/tektoncd/pipeline/blob/master/docs/resources.md#git-resource) | ||
| it can't use a different kind of PipelineResource to provide those files (tho you can avoid | ||
| doing the clone multiple times using [from](https://github.com/tektoncd/pipeline/blob/master/docs/pipelines.md#using-the-from-parameter)) | ||
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| ### Goals | ||
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| - Make it possible to combine Tasks together so that you can run multiple | ||
| Tasks as "one unit" (see [Requirements](#requirements): | ||
| - At runtime in a TaskRun | ||
| - At Pipeline authoring time (which would ultimately be realized through TaskRuns) | ||
| - Add some of [the features we don't have without PipelineResources](https://docs.google.com/document/d/1KpVyWi-etX00J3hIz_9HlbaNNEyuzP6S986Wjhl3ZnA/edit#) | ||
| to Tekton Pipelines (without requiring use of PipelineResources) | ||
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| ### Non-Goals | ||
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| - Composing Tasks within Tasks at [Task authoring time](https://github.com/tektoncd/community/blob/master/design-principles.md#reusability). | ||
| We could decide to include this if we have some use cases that need it; for now avoiding | ||
| this allows us to avoid many layers of nesting (i.e. Task1 uses Task2 uses Task3, etc.) | ||
| or even worse, recursion (Task 1 uses Task 2 uses Task 1...) | ||
| - One way to support this later could be via | ||
| - Supporting this composition at runtime in a PipelineRun (not quite sure what that | ||
| would look like, I guess you could refer to tasks in the Pipeline and specify | ||
| Tasks to run with them) | ||
| - Completely replacing PipelineResources: we could decide to solve this by improving PipelineResources, | ||
| or we could add a new feature via this TEP and still continue to support PipelineResources | ||
| (since they provide [more functionality than just composition](https://docs.google.com/document/d/1KpVyWi-etX00J3hIz_9HlbaNNEyuzP6S986Wjhl3ZnA/edit#)) | ||
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| ### Use Cases (optional) | ||
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| - A user wants to use catalog Tasks to checkout code, run unit tests and upload results, | ||
| and does not want to incur the additional overhead (and performance impact) of creating | ||
| volume based workspaces to share data between them in a Pipeline. e.g. specifically | ||
| cloning with [git-clone](https://github.com/tektoncd/catalog/tree/master/task/git-clone/0.2), | ||
| running tests with [golang-test](https://github.com/tektoncd/catalog/tree/master/task/golang-test/0.1) | ||
| and uploading results with [gcs-upload](https://github.com/tektoncd/catalog/tree/master/task/gcs-upload/0.1). | ||
| - A user wants to use a Task from the catalog with a git repo and doesn't want the extra | ||
| overhead of using a Pipeline, they just want to make a TaskRun, | ||
| e.g. [@mattmoor's feedback on PipelineResources and the Pipeline beta](https://twitter.com/mattomata/status/1251378751515922432)) | ||
| where he wants to checkout some code and [use the kaniko task](https://github.com/tektoncd/catalog/tree/master/task/kaniko/0.1) | ||
| without having to fiddle with volumes | ||
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| ## Requirements | ||
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| - Tasks can be composed together run as "one unit": | ||
| - Must be able to share data without requiring | ||
| an external volume (i.e. probably the containers that make them up are run within | ||
| the same pod) | ||
| - All Tasks should run even if one fails | ||
| - This is to support use cases such as uploading test results, even if the test | ||
| Task failed | ||
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| ## References (optional) | ||
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| * [Tekton PipelineResources Beta Notes](https://docs.google.com/document/d/1Et10YdBXBe3o2x6lCfTindFnuBKOxuUGESLb__t11xk/edit) | ||
| * [Why aren't PipelineResources in beta?](https://github.com/tektoncd/pipeline/blob/master/docs/resources.md#why-arent-pipelineresources-in-beta) | ||
| * [@mattmoor's feedback on PipelineResources and the Pipeline beta](https://twitter.com/mattomata/status/1251378751515922432)) | ||
| * [PipelineResources 2 Uber Design Doc](https://docs.google.com/document/d/1euQ_gDTe_dQcVeX4oypODGIQCAkUaMYQH5h7SaeFs44/edit) | ||
| * Task specialization: | ||
| * [Specializing Tasks - Vision & Goals](https://docs.google.com/document/d/1G2QbpiMUHSs4LOqcNaIRswcdvoy8n7XuhTV8tXdcE7A/edit) | ||
| * [Task specialization - most appealing options?](https://docs.google.com/presentation/d/12QPKFTHBZKMFbgpOoX6o1--HyGqjjNJ7own6KqM-s68/edit#slide=id.p) | ||
| * Issues: | ||
| * [Pipeline Resources Redesign](https://github.com/tektoncd/pipeline/issues/1673) | ||
| * [#1838 Extract Pre/Post Steps from PipelineResources 2 design into Tasks](https://github.com/tektoncd/pipeline/issues/1838) | ||
| * [Abstract task and nested tasks](https://github.com/tektoncd/pipeline/issues/1796) | ||
| * Oldies but goodies: | ||
| * [Link inputs and outputs without using volumes](https://github.com/tektoncd/pipeline/issues/617) | ||
| * [Design PipelineResource extensibility](https://github.com/tektoncd/pipeline/issues/238) | ||
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