Design a Dynamo-style key-value store

Advanced55 minBuilds Distributed Key-Value StoreRead first: 16.2 Quorums and repair, 24.1 The hash ring in depth, 27.2 Anti-entropy and content addressing

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The brief

You are designing a key-value store for a shopping cart, in the spirit of Amazon's Dynamo. The business rule is blunt: a customer must always be able to add to their cart, even while servers are failing. A briefly stale read is acceptable; a rejected write is lost money.

How big it is

Reads: 6,000 / second
Writes: 3,000 / second
Value size: about 1 KB
Replication factor N: 3
p99 goal: under 200 ms

What makes it tricky

No single machine holds everything. Keys must be spread over many nodes, and nodes join and leave.
Machines die all the time. At this scale some node is always down or slow.
Replicas disagree. With several copies and no leader, two writes can land on different replicas, and someone has to reconcile them.

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