Define crash-recovery evidence — Build the smallest useful slice

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

Build a recovery checkpoint fixture

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: Disposable record r starts at version1,value A; intended update is version2,value B.

Record baseline, interruption point and expected recovered versions. Under an atomic update contract, recovered state should be whole v1/A or whole v2/B, never v2/A. A version/content mismatch is an integrity failure even if the record exists.

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 — Build a recovery checkpoint fixture
// Manual worked artifact; comments below are not executable source.
// INPUT / CONTRACT: Supplied fixture: Disposable record r starts at version1,value A; intended update is
// version2,value B.
// DERIVATION / EXPECTED ARTIFACT: Record baseline, interruption point and expected recovered versions.
// Under an atomic update contract, recovered state should be whole v1/A or whole v2/B, never v2/A. A
// version/content mismatch is an integrity failure even if the record exists.

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.

Prepare v4/X→v5/Y fixtures with precommit and postcommit interruption points. 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

Prepare v4/X→v5/Y fixtures with precommit and postcommit interruption points. 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.

• Build a recovery checkpoint fixture
• Disposable record r starts at version1,value A; intended update is version2,value B.
• 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.