Step 5: High-Level Design

An email service has three jobs: take in mail that strangers send to our users, send our users' mail out to other providers, and let users read and search what they have. We follow one example through all three. Bob, at sender.org, emails Alice at abc@gmail.com. Alice reads it, searches for it later, and sends a reply.

Each job scales differently, so each runs as its own program with its own servers. A design built this way is a set of microservices: separate programs, each with one job, that talk to each other over defined APIs