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System Design

"Design is not just what it looks like and feels like. Design is how it works." — Steve Jobs

System design is the process of defining the architecture, components, modules, interfaces, and data for a system to satisfy specified requirements.

The Three Pillars

  1. Reliability: The system continues to work correctly (performing the correct function at the desired level of performance) even in the face of adversity (hardware/software faults, human error).
  2. Scalability: The system's ability to cope with increased load (traffic volume, data volume, complexity) by adding resources.
  3. Maintainability: The system should be productive for engineers and operations to work on over time (fixing bugs, keeping it operational, investigating failures, adapting to new use cases).

Core Concepts

CAP Theorem

In a distributed data store, you can only guarantee two of the three:

  • Consistency: Every read receives the most recent write or an error.
  • Availability: Every request receives a (non-error) response, without the guarantee that it contains the most recent write.
  • Partition Tolerance: The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes.

Reality: Partition tolerance is mandatory in distributed systems. You choose between CP (Consistency/Partition Tolerance) and AP (Availability/Partition Tolerance).

ACID vs. BASE

  • ACID (RDBMS): Atomicity, Consistency, Isolation, Durability. Strong consistency.
  • BASE (NoSQL): Basically Available, Soft state, Eventual consistency. High availability.

Back-of-the-Envelope Math

Powers of two for quick estimation:

  • $2^{10} \approx 10^3$ (1 KB)
  • $2^{20} \approx 10^6$ (1 MB)
  • $2^{30} \approx 10^9$ (1 GB)

Latencies (approximate):

  • L1 Cache: 0.5 ns
  • Mutex lock/unlock: 100 ns
  • Main Memory Ref: 100 ns
  • Send 2KB over 1Gbps network: 20,000 ns
  • Read 1MB sequentially from memory: 250,000 ns
  • Round trip within same datacenter: 500,000 ns
  • Disk seek: 10,000,000 ns (10 ms)
  • Send packet CA->Netherlands->CA: 150,000,000 ns (150 ms)

Deep Dives