In this module9 lessons
Data and databases
Know where a product's data lives, read a simple SQL query, and answer "SQL or NoSQL for this?" with a clear, defensible call.
6 min readFull kit
Where data lives
A product's data rarely sits in one place. Before any database question, it helps to know the four common homes.
| Home | What it holds | Example |
|---|---|---|
| Production database | The live records the app reads and writes | Users, orders, messages |
| Cache | Copies of frequently read data, kept in memory for speed | A popular product page, a user's session |
| Object storage | Files: images, videos, documents, backups | Profile photos, uploaded PDFs |
| Analytics warehouse | Copies of data reshaped for analysis | Every order from the last five years, for dashboards |
The production database is built to handle many small reads and writes quickly and safely. The warehouse is built to scan huge amounts of data for questions like "revenue by region by month". Data is usually copied from one to the other on a schedule, often called a pipeline or ETL (extract, transform, load). That is why a dashboard can lag behind reality by hours, a fact worth knowing before you diagnose a metric drop that might just be a late pipeline.
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