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0015-pending-pipeline.md

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title authors creation-date last-updated status
pending-pipeline-run
@jbarrick-mesosphere
2020-09-10
2020-09-10
implemented

TEP-0015: Support Pending PipelineRuns

Summary

This TEP proposes a method for pausing PipelineRuns that are then ignored by the Tekton reconciler. These pending resources can be used by platform implementers to control when a PipelineRun can create new TaskRuns, for example, in cases where the cluster is under heavy load and not ready to process new TaskRuns.

PipelineRuns can be created in a pending state so that they are not started at all.

Motivation

The primary motivation for this is to support operators who need to control the start up of PipelineRuns in clusters that are under heavy load. By allowing PipelineRuns to be created without actually starting them, one could implement a custom controller to implement any PipelineRun scheduling policy that is transparent to the PipelineRun controller and PipelineRun creator (e.g., Tekton triggers).

With this feature, platforms can control when PipelineRuns are started without introducing a bespoke system for storing PipelineRuns - potentially adding new failure scenarios or UI complexity.

Goals

  • Provide a pending mechanism for PipelineRuns that operators can use to control whether or not a PipelineRun is running (e.g., "creating new TaskRuns").

Non-Goals

  • Support pausing of a running TaskRun as that would requiring pausing the Kubernetes pods themselves.

Requirements

  • Provide a mechanism to operators that can be used to mark a PipelineRun in a pending state.
  • PipelineRuns that are created with the pending setting enabled are not started until the setting is toggled off.

Proposal

The PipelineRunSpec supports a field called Status of the type PipelineRunSpecStatus. Currently the field only supports a single setting, PipelineRunCancelled. This field will be expanded to support an additional setting, PipelineRunPending.

When the Status field is set to PipelineRunPending, the Tekton reconciler will set the Succeeded Condition Reason in the PipelineRun Status to PipelineRunPending:

status:
  conditions:
  - lastTransitionTime: "2020-09-15T18:15:29Z"
    message: 'PipelineRun is pending.'
    reason: PipelineRunPending
    status: "Unknown"
    type: Succeeded

To start a PipelineRun that was created in a pending state, the operator should set the Status field to empty.

We will introduce a check to the beginning of the PipelineRun reconciler that exits the reconciler method if the Status is Pending. If the Status is cleared, then the reconciler will see that update and run the reconciler method without exiting it early.

As the startTime status field is used to determine if a PipelineRun has timed out and not the PipelineRun creation time, this feature does not affect timeout behavior.

If the PipelineRunPending setting is set after a PipelineRun has been created, the validating webhook should reject the update.

User Stories

Platform Implementor

As a platform implementor, I want to be able to control when a PipelineRun is able to run without introducing bespoke machinery.

Alternatives

  • Instead of adding this setting directly to the PipelineRun object, users could introduce a new CRD, e.g., a QueuedPipelineRun, which has all the same fields as a PipelineRun but is inert. This has a number of drawbacks in that users would need to be aware of it, the UI would need to be aware of it, and the added complexity of maintaining a new CRD or abstraction.
  • PipelineRuns that are not ready to be started could be stored in a non-Kubernetes-native queue, such as Redis, and only submitted once they are ready to run. This adds complexity in that a new data store must be run and it is not easy to provide users visibility into the queue.

Implementation Pull request(s)

  1. API Changes, docs and e2e tests