Define crash-recovery evidence — Change one requirement

PART I

Warm-up

Reconnect with a useful idea before starting define crash-recovery evidence. In one or two sentences, describe the input, output or host boundary from the previous exercise that this lesson builds on. If you are arriving directly, skim the worked example first; there is no locked prerequisite.

A rough note is enough. You can revise it after the example.

PART II

Instruction & guided example

Adapt recovery to multiple records

Recoverability requires a declared invariant, a disposable initial fixture, an interruption or conflict schedule and authoritative final evidence. Atomic updates, durability, backups, code rollback, data migration and unchanged-client compatibility are different guarantees. The source query example supplies none of these operational APIs by itself.

Supplied fixture: An order and its reservation must move together from pending/unreserved to confirmed/reserved.

Recovery checks the relationship, not merely whether both rows are readable. confirmed/unreserved and pending/reserved violate the promised pair invariant. Define a recovered-state matrix before running future host experiments.

The source anchor below is illustrative. WillowScript, its object/anonymous-function extensions and native libraries remain proposed/unimplemented here. Application and data symbols are exercise-host inputs. State missing signatures or recovery guarantees as requirements; do not invent transaction, cancellation or restore methods and do not claim these paper traces executed.

Worked reasoning and supplied artifact

// PAPER FIXTURE — Adapt recovery to multiple records
// Manual worked artifact; comments below are not executable source.
// INPUT / CONTRACT: Supplied fixture: An order and its reservation must move together from
// pending/unreserved to confirmed/reserved.
// DERIVATION / EXPECTED ARTIFACT: Recovery checks the relationship, not merely whether both rows are
// readable. confirmed/unreserved and pending/reserved violate the promised pair invariant. Define a
// recovered-state matrix before running future host experiments.

Illustrative example from the proposed WillowScript design. The language and WUIX/WMATH/WDB/WAUDIO/WTMP services are not implemented in the supplied source. This site has no interpreter, VM or native host and does not execute or certify the snippet.

Try the guided change

Guided work: reproduce the fixture as a state/operation ledger and identify the exact evidence boundary supporting its conclusion.

• Write the supplied inputs and initial state before following the example.
• Reconstruct each intermediate calculation or host decision, then compare with the worked artifact.
• Explain why the contrasting alternative fails before attempting the changed inputs in Part IV.

PART III

Take a breather

Step back after the guided example. Look away from the screen, stretch, or reread your prediction. This is an optional learning pause, not a timer or an assessment gate.

When you return, try to name the lesson’s key idea without copying its wording. If that feels difficult, return to Part II and trace one line rather than restarting the entire lesson.

PART IV

Independent practice

Project context: Recovery rehearsal plan. Build a small, understandable change around define crash-recovery evidence.

Create a matrix for invoice/payment state pairs, with allowed original and committed combinations. Use different disposable record/resource identities from the worked fixture. Supply intended invariant, step-by-step normal trace, contrasting failed trace, final state and unresolved host requirements.

What to produce

Create a matrix for invoice/payment state pairs, with allowed original and committed combinations. Include explicit inputs, intermediate states, correct final artifacts and acceptance criteria.

• State the intended behavior in a sentence before editing.
• Use only source-backed syntax and the host services whose contract you have been given.
• Record a normal case and a boundary or unavailable-resource case.
• Label hand predictions separately from anything actually measured in a supplied toolchain.

Syntax colors are a reading aid, not a compiler. The line you are editing stays uncolored until you move to a different line; capitalization is never changed. Drafts stay on this browser and code is not executed.

import wdb;
domain Inventory {
    const lowStock = wdb.query({
        from: products,
        select: [id, name, stock],
        where: [[ stock < 10 ]],
        order: { by: stock, direction: ascending }
    });
}

PART V

Self-check

Use this check to notice what to revisit. It does not award a private course grade, block another page or claim to verify compiled code. Every answer has a learning explanation.

You may keep exploring even if you are unsure. Use the hint, compare the trace, and revise your explanation.

PART VI

Recap & small task

Bring the idea back to the mod feature you are building. A useful recap connects the mechanism to a decision you can make in your own code.

• Adapt recovery to multiple records
• An order and its reservation must move together from pending/unreserved to confirmed/reserved.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Change one timing, permission, version or ownership condition in the supplied fixture. State which final artifact changes and which invariant must remain preserved.

This marks your personal practice record only. It is not a certification or an official workspace assessment.