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Web Workers · 21 Separating MessageChannels
Series contents · Technology map
Stage: Queues and Error Recovery · Editable example · Storage ID: web-workers-step-21
Goals and learning outcomes
- Learning outcome: explain the role of Ports. Input cases: two simultaneous tasks.
- Deliverable: messages for each channel and close records. Explain the rationale for handling Disposal, linking it to the code.
Prerequisites
Prepare the experiment results for Worker failure and Retry from Unit 20, “Failure Recovery”.
Key concepts and explanations
Ports
MessageChannel creates an independent messaging path. A port can be passed in the transfer list.
Disposal
Close a port that is no longer used. Request completion and channel lifetime are separate.
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-21.js in a separate practice project, pass a port to the Worker to separate progress and result channels. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: two simultaneous tasks. Baseline: Single Worker channel. Run under the baseline conditions, then record observations and errors in notes/day-21.md.
- Variable: Channel; changed value: Per-request port. Run again through the same input path. Deliverable to save: messages for each channel and close records. Hypothesis to check: Messages from different tasks are distinguished. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Channel | Single Worker channel | Per-request port | Messages from different tasks are distinguished. |
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: messages for each channel and close records. 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: Ports and Disposal. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Ports affect in this exercise? Input cases: two simultaneous tasks.
- Variable: Channel. Hypothesis to check: Messages from different tasks are distinguished. Did the actual results agree with the hypothesis? Explain the evidence in terms of Disposal.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Separating MessageChannels 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: Failure Recovery · Next: Splitting and Merging Data