Quorums, Leases and Changing the Membership · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Quorums, leases and new members

Computing · Networks & Security · ages 22-23
Name ______________________   Date ____________

  1. What does a lease give a leader?

    • A bounded window of authority without asking
    • A faster disk so writes finish before reads start
    • A new network address that other nodes must obey

  2. What must every read set and write set do?

    • Avoid sharing nodes so reads never wait for writes
    • Intersect in at least one shared node
    • Contain every node in the cluster, always

  3. Switching a cluster from its old member set to a new one in a single step is safe.

    Circle one:   True   False

  4. Seven nodes run read quorum 3. What is the smallest write quorum that still overlaps every read?

    Answer: ______________

  5. A read-heavy photo site runs on six nodes and wants cheap reads that still overlap every write. Which pairing fits?

    • Read 5 with write 2
    • Read 2 with write 4
    • Read 2 with write 5

  6. Clocks skew badly across the cluster. What threatens the leader's lease?

    • A rival leader that also believes it is in charge
    • Disk writes that finish faster than the lease allows
    • Log entries that grow too long to send in time

  7. A cluster must move from members A, B and C to members A, B, C, D and E. Which plan avoids two leaders?

    • Let each node switch to the new set when it hears the news
    • Use a joint stage needing a majority of old and of new
    • Switch every node at once, then hold the election afterwards

  8. Read 1 plus write 1 on five nodes is proposed for speed. What is the flaw?

    • Nothing, it is the fastest pair that stays correct
    • One-node sets cannot be used on a cluster of five nodes
    • The sets need not meet, so a read can miss the write
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Answer key

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

Quorums, leases and new members W1-mt_2uRmRqFutD-s1

  1. A bounded window of authority without asking · The lease rents authority for a fixed time.
  2. Intersect in at least one shared node · Overlap is what keeps reads fresh.
  3. False · One-step changes let old and new majorities disagree.
  4. 5 · Read plus write must exceed 7, so 3 plus 5 is the smallest fit.
  5. Read 2 with write 5 · 2 plus 5 is 7, above 6, so every read meets every write, and reads stay small.
  6. A rival leader that also believes it is in charge · Leases trust clocks, and skewed clocks can leave two leaders in charge at once.
  7. Use a joint stage needing a majority of old and of new · A joint stage needs a majority of the old set and of the new set, so two rival majorities cannot form.
  8. The sets need not meet, so a read can miss the write · One plus one never exceeds five, so reads can miss writes.
Worksheet · LightMySky