Design a Distributed Key-Value Store

1. Problem Statement & Scope

System Mission

Design a distributed, highly available, fault-tolerant Key-Value Store (similar to Amazon DynamoDB or Apache Cassandra) capable of storing petabytes of structured and unstructured key-value data with single-digit millisecond latency at massive global scale.

Functional Requirements

1. put(key, value): Atomically store or update a binary/JSON payload associated with a partition key.
2. get(key): Retrieve the latest value associated with a partition key.
3. delete(key): Remove a key-value record (supporting tombstones and TTL).
4. Tunable Consistency: Support configurable read/write consistency levels per request (EVENTUAL, STRONG, ALL_QUORUM).

Non-Functional Requirements (SLAs/SLOs)

• Availability: $99.999\%$ uptime SLA (AP system under CAP Theorem with multi-AZ quorum).
• Latency: P99 Read latency $< 10\text{ ms}$, P99 Write latency $< 5\text{ ms}$.
• Durability: $99.999999999\%$ (11 9s) zero data loss guarantee via write-ahead logging and 3-AZ quorum replication.
• Scalability: Linear horizontal scalability to $100\text{M+ QPS}$ and petabytes of data.

2. Capacity & Scale Estimation

Traffic Calculations

• Total Registered Keys: 10 Billion Keys ($10^{10}$).
• Average Item Size: $1\text{ KB}$ (Key: $64\text{ Bytes}$, Value: $960\text{ Bytes}$).
• Total Raw Storage: $10\times 10^9 \times 1\text{ KB} = 10\text{ TB}$
• Storage with 3x Multi-AZ Replication: $10\text{ TB} \times 3 = \mathbf{30\text{ TB}}$
• Query Throughput (QPS):
  • Total Average QPS: $100,000\text{ QPS}$ ($80\%$ Reads, $20\%$ Writes).
  • Average Read QPS = $80,000\text{ RPS}$, Average Write QPS = $20,000\text{ WPS}$.
  • Peak QPS ($2.5\times$): $\mathbf{250,000\text{ QPS}}$ (Read Peak: $200,000\text{ RPS}$, Write Peak: $50,000\text{ WPS}$).
• Network Bandwidth:
  • Ingress: $50,000\text{ writes/sec} \times 1\text{ KB} = 50\text{ MB/s} = 400\text{ Mbps}$.
  • Egress: $200,000\text{ reads/sec} \times 1\text{ KB} = 200\text{ MB/s} = 1.6\text{ Gbps}$.

3. AWS-First High-Level Architecture

Interactive Architecture Diagram
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