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REHEARSALS.md

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CI job rehearsals

Pull requests to this repository that contain changes affecting Prow job setup trigger a so-called "rehearsal". Jobs that would be affected by a PR are executed as if run against a target component repository after job changes would be merged. This provides job config authors early feedback about how job config changes impact CI setup for a given repo.

How rehearsal works

All pull requests trigger a ci/prow/pj-rehearse job, which detects the jobs affected by the proposed change. For each job selected for rehearsal (for various reasons, we are not able to rehearse all jobs), new Prowjob is dynamically created, reporting results via a GitHub context named with the ci/rehearse/$org/$repo/$branch/$test pattern. Both the "driver" job (ci/prow/pj-rehearse) and the actual rehearsals are optional, which means that their failures do not block Tide from merging the PR. The driver job waits for individual rehearsals to finish, and its own PASS/FAIL value depends on them: it will fail if at least one of the rehearsals did not succeed.

Which jobs are rehearsed

At the moment, we are not able to rehearse all affected jobs for various reasons. Currently, we only rehearse Presubmit jobs affected by the following changes:

  1. Newly added presubmit jobs
  2. Presubmit jobs whose spec: field was changed
  3. Presubmit jobs whose agent: field was changed to kubernetes from a different value

These jobs are then further filtered to exclude jobs where some factor prevents us from reliably rehearsing them. Decisions about why a job was excluded from rehearsing are logged in the output of the ci/prow/pj-rehearse job. Most importantly, we are currently not rehearsing template-based jobs.

Rehearsal reruns

It is not possible to re-run individual rehearsal jobs (they do not react to any trigger command themselves). Rerunning rehearsals must be always done by re-triggering the "driver" job (ci/prow/pj-rehearse), which then re-triggers each individual rehearsal job, including those that were passing before.

Future work

In the near future, we would like to enlarge the set of jobs that are rehearsed. First, changes to several fields other that spec: should cause a rehearsal. Afterwards, we would like to enable rehearsing template-based jobs. Additionally, we want to consider for rehearsal even jobs that are affected by a change indirectly, e.g., by a change of an underlying ci-operator config file, template or other asset used by the job.