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Web Workers · 18 Restoring Result Order
Series contents · Technology map
Stage: Queues and Error Recovery · Editable example · Storage ID: web-workers-step-18
Goals and learning outcomes
- Learning outcome: explain the role of Completion order. Input cases: 4 arrays of different sizes.
- Deliverable: a table of submission, completion, and display order. Explain the rationale for handling Index, linking it to the code.
Prerequisites
Prepare the experiment results for Parallelism and Idle assignment from Unit 17, “Worker Pool”.
Key concepts and explanations
Completion order
Short tasks may finish first. A screen that requires submission order needs a separate assembly step.
Index
Use the task index to choose a result slot. Choosing positions by completion count mixes up results.
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-18.js in a separate practice project, include an index in every request and collect Promise results into their slots. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: 4 arrays of different sizes. Baseline: Arrival order. Run under the baseline conditions, then record observations and errors in notes/day-18.md.
- Variable: Display policy; changed value: Submission order. Run again through the same input path. Deliverable to save: a table of submission, completion, and display order. Hypothesis to check: The output array stays in a stable order even when computations finish out of order. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Display policy | Arrival order | Submission order | The output array stays in a stable order even when computations finish out of order. |
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: a table of submission, completion, and display order. 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: Completion order and Index. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Completion order affect in this exercise? Input cases: 4 arrays of different sizes.
- Variable: Display policy. Hypothesis to check: The output array stays in a stable order even when computations finish out of order. Did the actual results agree with the hypothesis? Explain the evidence in terms of Index.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Restoring Result Order 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: Worker Pool · Next: Timeouts and Duplicate Responses