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Web Workers · 11 Rejecting Stale Results
Series contents · Technology map
Stage: Ownership and Cancellation · Editable example · Storage ID: web-workers-step-11
Goals and learning outcomes
- Learning outcome: explain the role of Latest request. Input cases: a large task followed by a small task.
- Deliverable: response order and the ID accepted for display. Explain the rationale for handling Distinguishing computation cancellation, linking it to the code.
Prerequisites
Prepare the experiment results for Round-trip ownership and Pool states from Unit 10, “Returning Buffers”.
Key concepts and explanations
Latest request
A UI such as search only needs the result for the latest input. A task started earlier may finish later.
Distinguishing computation cancellation
Ignoring a stale result may leave its computation running. Distinguish reducing cost from keeping the screen consistent.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. The existing web-workers-01·02·03 are starting examples. Implement the new assignments in main.js and worker.js in a separate local HTTP project. Preserve CSP and iframe isolation. For SharedArrayBuffer, check COOP and COEP requirements on an external server you control; use the transfer path if they are not met. Measure performance yourself.
The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.
Practice tasks
- In src/day-11.js in a separate practice project, compare latestRequestId with response IDs and count discarded responses. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: a large task followed by a small task. Baseline: After completion. Run under the baseline conditions, then record observations and errors in notes/day-11.md.
- Variable: Request interval; changed value: Rapid succession. Run again through the same input path. Deliverable to save: response order and the ID accepted for display. Hypothesis to check: A stale result does not overwrite the latest screen state. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Request interval | After completion | Rapid succession | A stale result does not overwrite the latest screen state. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Required deliverable: response order and the ID accepted for display. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
- The explanation of differences between the two conditions connects both concepts: Latest request and Distinguishing computation cancellation. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Latest request affect in this exercise? Input cases: a large task followed by a small task.
- Variable: Request interval. Hypothesis to check: A stale result does not overwrite the latest screen state. Did the actual results agree with the hypothesis? Explain the evidence in terms of Distinguishing computation cancellation.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Rejecting Stale Results experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Returning Buffers · Next: Cooperative Cancellation