Open code, practice, and notes →
Download source with English comments · for reading
OffscreenCanvas · 21 Recovering from Worker Failure
Series contents · Technology map
Stage: Rendering Lifecycle and Recovery · Editable example · Storage ID: offscreen-canvas-step-21
Goals and learning outcomes
- Learning outcome: explain the role of Distinguishing errors. Input cases: Errors during initialization and rendering.
- Deliverable: A table of error guidance, selected mode, and scene preservation. Explain the rationale for handling Fallback, linking it to the code.
Prerequisites
Prepare the experiment results for Retransfer limits and Restart state from Unit 20, “Restarting with a New Canvas”.
Key concepts and explanations
Distinguishing errors
Rendering computation errors and Worker loading errors have different causes. The main thread is responsible for restoring state.
Fallback
After an error, you can switch to a main-thread renderer on a new Canvas. This does not reclaim the already transferred context.
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 offscreen-canvas-01·02·03 are starting examples for supported browsers. Implement the new units in main-thread and Worker files in a separate local HTTP project. Check API and context support, and provide a main-thread fallback on a new Canvas if transfer fails. Preserve CSP and isolation, and record performance measurements 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, reinitialize the fallback from worker.onerror and error messages. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Errors during initialization and rendering. Baseline: Initialization. Run under the baseline conditions, then record observations and errors in notes/day-21.md.
- Variable: Failure timing; changed value: During rendering. Run again through the same input path. Deliverable to save: A table of error guidance, selected mode, and scene preservation. Hypothesis to check: Both paths restore guidance and interaction. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Failure timing | Initialization | During rendering | Both paths restore guidance and interaction. |
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 error guidance, selected mode, and scene preservation. 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: Distinguishing errors and Fallback. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Distinguishing errors affect in this exercise? Input cases: Errors during initialization and rendering.
- Variable: Failure timing. Hypothesis to check: Both paths restore guidance and interaction. Did the actual results agree with the hypothesis? Explain the evidence in terms of Fallback.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Recovering from Worker Failure 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: Restarting with a New Canvas · Next: Transferring ImageBitmap Frames