Auditable architecture · Unit 3 of 5

The process is the log

When every meaningful transition is recorded as an event, state can be reconstructed and a decision can be explained. The snapshot speeds up reads; it doesn't replace the history.

Intermediate to senior35 min

By the end, you'll be able to

Tell an event apart from a snapshot
Reconstruct state by replay

Understand

When every meaningful transition is recorded as an event, state can be reconstructed and a decision can be explained. The snapshot speeds up reads; it doesn't replace the history.

See

Run the replay

Add events, rewind and compare the derived state against the sequence.

Open artifact

Do

Run the practice in your checkout and keep the output as evidence.

Inspect the event store contracts

Verify

Show that you can apply the unit. Progress only advances once you pass the assessment.

Criteria assessed

Derive the displayed state from ordered events.
Identify the evidence lost when persisting only the latest state.

Graded assessment

Solve the 3 scenarios. This unit requires 3 of 3 correct answers.

0 attempts

Question 1 of 3A valid snapshot exists up to event 80 and the stream contains events 81–100. How is the state reconstructed without losing history?

Take the snapshot as the base and apply events 81–100 in order; do a full replay if you need to validate the snapshot.Use only event 100, because the last event implicitly contains the whole state.Edit the snapshot with the fields from event 100 and discard the intermediate events.

Question 2 of 3OrderApproved was recorded by mistake, but the decision must be reversed while keeping an explainable audit trail. Which intervention respects an append-only log?

Delete OrderApproved and renumber the stream so it looks like it never happened.Append a corrective or compensating event with its reason and let the projection derive the new state.Overwrite only the snapshot; the log can keep a history that differs from the state shown.

Question 3 of 3A system stores only status = approved. Months later, someone asks who approved, from which state and why. Which conclusion is valid?

The final state is enough; actor and cause can be inferred unambiguously from approved.A new snapshot can automatically recover transitions that were never persisted.The evidence is already lost: you need to persist meaningful events with their data to reconstruct and explain the decision.

Grading validates answers in this browser; it doesn't certify identity.

Primary sources

Event Store

Content verified: 2026-07-10

By the end, you'll be able to

1Understand

2See

3Do

4Verify

Primary sources

Understand

See

Do

Verify