Step 6: Database Design

An inbox listing and an opened message ask for very different data. The listing reads the sender, subject, date, and read flag of fifty messages, and it runs constantly. Opening a message reads one large body, once. Headers are small and read all the time, while bodies are large and read rarely, so they belong in different stores.

Email storage model

The design uses three stores. Two kinds of data decide the split. Structured data is small facts about users and messages, such as the sender and the date. Unstructured data is large content: bodies and attachments.