System Overview

Almost every system you build needs to assign IDs to things — users, orders, messages, transactions. The simplest approach is a database auto-increment (1, 2, 3...). That works fine on a single database, but the moment you have multiple database servers (which every large system does), two servers can both assign ID 1001 to different records at the same time. Collision.

A Unique ID Generator is a dedicated service that generates IDs which are:

• Globally unique — no two IDs are ever the same, across any server, anywhere in the world
• Sortable by time — newer records get higher IDs, so you can sort by ID and get chronological order for free
• Fast — systems like Twitter, Instagram, and Uber need millions of IDs per second

The interesting design challenge is: how do you generate IDs that are unique and time-ordered and fast at massive scale — without a single bottleneck that every write in your entire system has to wait on?