Add deferred task resume flow to task lifecycle diagram

[yuseok89]

closes: #28580

The deferred branch of the task lifecycle diagram asked "A triggerer task?", a question about task type, at exactly the point where the reader needs the event timeline. Waiting, the trigger firing, and the return to scheduled were all collapsed behind that one branch, so the path out of deferred existed but could not be followed.

Diagram, deferred region

The branch now reads as a sequence of events.

From	Branch
deferred → "Trigger fired?"	NO → back to deferred, the wait loop
	YES → "Task completes in the triggerer?"
"Task completes in the triggerer?"	NO → Scheduler, an ordinary deferrable task resumes on a worker
	YES → the existing task-end handling, which is the end_from_trigger path

"Task completes in the triggerer?" is not a rename for its own sake. It names the BaseTaskEndEvent overload of handle_event_submit, whose user-facing subclasses are TaskSuccessEvent, TaskFailedEvent and TaskSkippedEvent.

A task that raises a defer signal now transitions straight to deferred rather than to the Triggerer node. The worker writes DEFERRED and the trigger row in a single Execution API call, and a triggerer claims the trigger afterwards, so the old edge drew a synchronous hand-off that does not happen. The edge carries a trigger registered label so the triggerer is still part of the story.

Diagram, upstream failure check

Separate from the deferred work, Required upstream task(s) failed? had its branches backwards. NO pointed at upstream_failed and YES continued toward scheduled. Its two sibling diamonds are oriented correctly, so this was an authoring slip rather than a house convention. It sits on the single path every task takes out of the Scheduler box, and the deferred resume path this PR documents runs straight through it, so it is fixed here. Happy to split it into its own PR if reviewers prefer.

Prose

deferring.rst credited the scheduler with the deferred to scheduled write. The triggerer performs it, and the scheduler's own deferred to scheduled write is the trigger-timeout failure path. The same sentence promised a return to a worker unconditionally, which is not true for end_from_trigger. Both are corrected, and the paragraph that restated the bullets above it is reduced to the cross-reference it was carrying.

Screenshots

Deferred region

Upstream failure check

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  • Opus 5

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[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[potiuk]

Hello @yuseok89 - thank you for your contributions to Apache Airflow!

The Airflow community has introduced a limit of 5 open pull requests at a time for contributors without write access to the repository. You currently have 11 open pull requests, so - as a one-time step of introducing the limit - we closed the ones where maintainers have not engaged yet:

• Add deferred task resume flow to task lifecycle diagram #65323 - Add deferred task resume flow to task lifecycle diagram
• Add TaskGroup-level retries to retry a whole group as a unit #68139 - Add TaskGroup-level retries to retry a whole group as a unit
• Dedent task docstrings used as doc_md #71108 - Dedent task docstrings used as doc_md
• Fix task duration increasing forever after Dag run timeout skips it #71549 - Fix task duration increasing forever after Dag run timeout skips it
• Ignore task start_date and end_date on manual runs #71758 - Ignore task start_date and end_date on manual runs
• Make HttpSensor defer when response_check is provided #71922 - Make HttpSensor defer when response_check is provided
• Flag ui-field-behaviour drift between hooks and provider.yaml in static checks #72089 - Flag ui-field-behaviour drift between hooks and provider.yaml in static checks
• Skip GCS folder-marker keys in GCSToSFTPOperator #72638 - Skip GCS folder-marker keys in GCSToSFTPOperator
• i18n(ko): Add missing translations (Sep25) #73677 - i18n(ko): Add missing translations (Sep25)

These pull requests stay open because maintainers are already engaged in them - they count towards your limit:

• Show recent task states on the Dags list page #69619 - Show task state counts for the latest Dag run on the Dags list page
• UI: Add a time-window scope to the Dags list run state filter #70470 - UI: Add a time-window scope to the Dags list run state filter

This is not a judgement of you or of your changes. We never told contributors before that opening many pull requests at once was a problem, so there is nothing to feel bad about - and nothing is lost: your branches, commits and the review history stay where they are.

What we ask you to do is to make your first prioritization decision: choose which of the pull requests above matter most to you, and reopen them (up to 5 open at a time, including the ones still open) with the "Reopen pull request" button or gh pr reopen <PR_NUMBER> --repo apache/airflow. Reopen the ones you are ready to follow through - keep them rebased, respond to review comments and fix failing checks.

While your pull requests are waiting for review, the most valuable thing you can do is help in other ways - reviewing other contributors' pull requests, helping with issues, and taking part in the discussions on the devlist and Slack.

Why we introduced the limit, what it means for you and how to reopen or restore a pull request is explained in https://github.com/apache/airflow/blob/main/contributing-docs/32_open_pull_request_limit.rst.

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Drafted-by: Claude Code (Opus 5); reviewed by @potiuk before posting