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Grokking Modern System Design Interview

System Design: Sequencer

Explore the necessity of globally unique identifiers UIDs in large distributed systems for event tracing and primary key assignment. Understand why traditional auto-increment features fail in sharded database environments.

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Motivation

Large distributed systems can process millions of events per second, such as user posts or financial transactions. Each event must be assigned a globally unique identifier. These identifiers typically serve as primary keys in storage systems. Single-node databases often use auto-increment columns, but this approach does not scale in distributed systems where multiple nodes generate identifiers independently. Distributed environments, including horizontally sharded tables, require a coordinated or decentralized strategy for generating a unique ID.

Additionally, unique IDs facilitate debugging and tracing. For example, Facebook’s CanopyAn End-to-End Performance Tracing and Analysis System uses a TraceID to track events across the hundreds of microservices involved in a single user request.

Assigning a unique TraceID to each event
Assigning a unique TraceID to each event

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Why do we prefer time-sortable UIDs?
Explain why we need to design a time-sortable unique ID generator. 

How do we design a sequencer?

We divide the comprehensive design of a sequencer into two lessons:

  1. Design of a unique ID generator: Define system requirements and discuss three generation methods: UUIDs, databases, and range handlers.

  2. Unique IDs with causality: Incorporate time as a factor in ID generation to handle causality.

Designing a unique ID generator for a distributed system is challenging. The next lesson examines the specific requirements for this system.