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Web Workers · 16 A Bounded Task Queue
Series contents · Technology map
Stage: Queues and Error Recovery · Editable example · Storage ID: web-workers-step-16
Goals and learning outcomes
- Learning outcome: explain the role of Backpressure. Input cases: 8 simultaneous requests.
- Deliverable: a table of accepted, rejected, and completed IDs. Explain the rationale for handling State transitions, linking it to the code.
Prerequisites
Prepare the experiment results for Reporting frequency and Monotonic progress from Unit 15, “Progress Reporting”.
Key concepts and explanations
Backpressure
The queue grows when incoming work exceeds production capacity. A length limit and rejection policy are required.
State transitions
Separate queued, running, and done states. Define which states allow cancellation.
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-16.js in a separate practice project, create a FIFO queue with an upper limit of 5 and a rejection notice. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: 8 simultaneous requests. Baseline: 5 requests. Run under the baseline conditions, then record observations and errors in notes/day-16.md.
- Variable: Queue limit; changed value: 2 requests. Run again through the same input path. Deliverable to save: a table of accepted, rejected, and completed IDs. Hypothesis to check: The number accepted is limited, and memory does not grow without bound. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Queue limit | 5 requests | 2 requests | The number accepted is limited, and memory does not grow without bound. |
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 accepted, rejected, and completed IDs. 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: Backpressure and State transitions. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Backpressure affect in this exercise? Input cases: 8 simultaneous requests.
- Variable: Queue limit. Hypothesis to check: The number accepted is limited, and memory does not grow without bound. Did the actual results agree with the hypothesis? Explain the evidence in terms of State transitions.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive bounded task queue experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.