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WebAssembly · 12 Capacity Limits and Failure
Series contents · Technology map
Stage: Memory and Array Boundaries · External example · preview instructions · Storage ID: webassembly-step-12
Goals and learning outcomes
- Learning outcome: explain the role of Maximum. Input cases: lengths on either side of a page boundary.
- Deliverable: a table of required·allocated pages and errors. Explain the rationale for handling Capacity calculation, linking it to the code.
Prerequisites
Prepare the experiment results for Grow and Recreation from Unit 11, “Memory Grow and View Lifetime”.
Key concepts and explanations
Maximum
Memory can limit growth with maximum. Handle failures when a request exceeds the limit.
Capacity calculation
Round required bytes up to pages. Allocating one page too few leaves the final element out of bounds.
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 iframe preview is an experiment with WebAssembly.Memory and JS reference computations; do not present it as execution of an actual Wasm module. Practice with actual modules in a separate Node 22+ environment or a controlled local web project. Use kernel.wat·kernel.wasm·benchmark.mjs in the existing src/labs/webassembly-02·03 folders, and rebuild with the official wat2wasm tool when modifying WAT. Preserve CSP·isolation, and record 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 day-12.mjs in a separate practice project, add required-page calculation and grow exception handling to the wrapper. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: lengths on either side of a page boundary. Baseline: within the limit. Run under the baseline conditions, then record observations and errors in notes/day-12.md.
- Variable: requested capacity; changed value: over the limit. Run again through the same input path. Deliverable to save: a table of required·allocated pages and errors. Hypothesis to check: An excessive request ends with an error, and the previous data is preserved. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| requested capacity | within the limit | over the limit | An excessive request ends with an error, and the previous data is preserved. |
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 required·allocated pages and errors. 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: Maximum and Capacity calculation. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Maximum affect in this exercise? Input cases: lengths on either side of a page boundary.
- Variable: requested capacity. Hypothesis to check: An excessive request ends with an error, and the previous data is preserved. Did the actual results agree with the hypothesis? Explain the evidence in terms of Capacity calculation.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Standalone WAT + Node assertions; iframe CSP blocks Wasm compilation.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Memory Grow and View Lifetime · Next: Scalar and Batch Calls