Quorums, Leases and Changing the Membership
A majority is one point on a curve: any read set and write set that intersect will do, and the choice moves cost between reads and writes. A lease buys a leader a bounded window of authority, and changing the member set safely needs a protocol of its own.
What a learner can do afterwards
- Choose read and write quorum sizes for a stated read-heavy workload and justify them
- Say what a lease assumes about clocks and what breaks when that assumption fails
- Explain why adding and removing members in one step can split a cluster
1 · Read
A majority is one point on a curve. Any read set and write set that intersect will do, and the choice moves cost between reads and writes. For five nodes with a read quorum of 2, the write quorum must exceed 3, so 4 is the smallest that still overlaps every read.
A read-heavy shop picks read quorum 2 and write quorum 4 on five nodes. Every 2-node read meets every 4-node write in at least one node, so reads stay cheap while writes pay the price. Check the maths the same way every time: read plus write must exceed the node count.
A lease buys the leader a bounded window of authority without asking anyone, which quorums alone cannot give. It assumes clocks stay roughly in sync. When clocks skew, two leaders can believe themselves in charge at once.
Never add and remove members in one step. Old and new majorities can then disagree and elect rival leaders, so change the set in careful stages.
Intersecting quorums trade read cost for write cost, leases rent authority on clock trust, and membership changes go stepwise.
2 · Watch
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