Failure Models and Replication · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Crashed or just slow

Computing · Networks & Security · ages 20-21
Name ______________________   Date ____________

  1. What is replication an answer to?

    • Slow networks needing longer cables
    • Clocks drifting between machines
    • Single copies failing, so truth lives on several machines

  2. You call a node and hear nothing. Which two situations look exactly this way from outside?

    • A crash and mere slowness
    • A crash and a reboot
    • Slowness and a full disk

  3. A replica that rejoins with stale state may serve reads immediately.

    Circle one:   True   False

  4. Why do systems use timeouts if a timeout cannot prove a crash?

    • Timeouts let the system act despite never knowing for sure
    • Timeouts prove the node crashed
    • Timeouts restart crashed disks

  5. A node misses three writes while down, then rejoins. What are its correct steps?

    • Serve reads at once, then copy the writes quietly
    • Copy the three missed writes, then serve reads
    • Delete its copy and stay silent forever

  6. Clients read from a replica and get an order the primary never sent. What promise did the replica break?

    • Answering requests quickly
    • Storing data on disk
    • Serving only what it has caught up with

  7. Two clients read different values for the same key seconds apart, and both replicas are healthy. What is the most likely story?

    • One replica is stale and has not yet caught up
    • The network cables are too short
    • Both clients asked the wrong key

  8. A designer treats every silent node as crashed and deletes its data. One slow node returns to find its copy wiped. What was the design error?

    • Timeouts were set too generously
    • Silence was read as proof of crash, which it never is
    • Replication was used where a single copy sufficed
LightMySky · lightmysky.comW1-mt_aYK210-NIS-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Crashed or just slow W1-mt_aYK210-NIS-s1

  1. Single copies failing, so truth lives on several machines · Copies survive failures that would kill one machine's truth.
  2. A crash and mere slowness · Both produce silence, so no outside observer can separate them.
  3. False · It must copy missed changes first, or clients read old truth.
  4. Timeouts let the system act despite never knowing for sure · Waiting forever is worse, so a timeout bounds the silence.
  5. Copy the three missed writes, then serve reads · Catching up first keeps every served read fresh.
  6. Serving only what it has caught up with · Replicas may serve their prefix of the primary's order, nothing beyond it.
  7. One replica is stale and has not yet caught up · Copies converge over time, so a lagging replica briefly disagrees.
  8. Silence was read as proof of crash, which it never is · Slow and crashed look alike, so deletion needs stronger proof.
Worksheet · LightMySky