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CAP Theorem & Consistency Models.
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01

What is the CAP Theorem and what do the three letters stand for?

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The CAP Theorem, proposed by Eric Brewer, states that in any distributed system, you can guarantee at most two of the following three properties simultaneously:

  • Consistency (C): All nodes see the same data at the same time. Every read receives the most recent write or an error.
  • Availability (A): The system remains operational and responsive, meaning every request receives a response (success or failure).
  • Partition Tolerance (P): The system continues to operate despite network failures that prevent communication between nodes.

The theorem essentially says you must choose between CP (Consistency + Partition Tolerance) or AP (Availability + Partition Tolerance) when network partitions occur, as CA (Consistency + Availability) is impossible in a distributed system with network partitions.

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02

Why is it impossible to achieve all three properties of CAP simultaneously in a distributed system?

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It's impossible because of the fundamental nature of network partitions in distributed systems. When a network partition occurs (nodes cannot communicate), you face a choice:

  1. Choose Consistency: Stop serving requests on partitioned nodes to maintain data consistency. This sacrifices availability.
  2. Choose Availability: Continue serving requests on all nodes, but risk serving stale data, sacrificing consistency.

You cannot have both because:

  • To maintain consistency during a partition, you must either reject requests (losing availability) or wait for network healing (also losing availability)
  • To maintain availability, you must serve requests with potentially stale data (losing consistency)

Partition tolerance is mandatory in distributed systems because network failures are inevitable.

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03

Explain the difference between strong consistency and eventual consistency.

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Strong Consistency:

  • All nodes see the same data simultaneously
  • Reads always return the most recent write
  • Updates are atomic across all nodes
  • Higher latency due to synchronization overhead
  • Example: Traditional RDBMS with ACID properties

Eventual Consistency:

  • Nodes may temporarily have different data
  • The system will eventually reach a consistent state
  • Lower latency as updates can proceed without waiting for all nodes
  • Better availability during network issues
  • Example: DNS, Amazon DynamoDB, Cassandra

Key Difference:
Strong consistency prioritizes correctness over performance, while eventual consistency prioritizes availability and performance over immediate consistency.

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Intermediate 8
04

What are some real-world examples of systems that choose CP vs AP?

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05

What is the BASE model and how does it relate to CAP Theorem?

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06

Explain different consistency models: Strong, Weak, and Eventual consistency.

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07

What are vector clocks and how do they help with consistency in distributed systems?

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08

How does partition tolerance affect system design decisions?

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09

How do NoSQL databases typically handle the CAP theorem trade-offs?

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10

Explain the concept of quorum-based consistency and how it relates to CAP.

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11

What is the difference between ACID and BASE properties?

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Expert 9
12

What is the difference between consistency and consensus in distributed systems?

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13

What are conflict-free replicated data types (CRDTs) and how do they help with consistency?

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14

How does the CAP theorem apply to microservices architecture?

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15

How do consensus algorithms like Raft and Paxos relate to the CAP theorem?

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16

What is causal consistency and how does it differ from other consistency models?

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17

How would you design a distributed cache considering CAP theorem constraints?

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18

What are some strategies for handling network partitions in distributed systems?

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19

How do modern cloud databases handle CAP theorem trade-offs?

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20

What factors should you consider when choosing between CP and AP systems for a specific use case?

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