Persist It: A Database Behind Your API

• ✓Explain what a database gives a service that variables held in memory cannot
• ✓Model tables, keys and relationships as SQLAlchemy classes, and use the engine and the session
• ✓Change a live database with Alembic migrations, and prove each one against the data and in reverse
• ✓Store money as whole cents instead of decimal floats
• ✓Count the queries a piece of code sends, and find and fix the N+1 problem
• ✓Read a query plan, add an index with a migration, and avoid the function-on-a-column mistake
• ✓Make a group of changes all-or-nothing with a transaction
• ✓Add constraints that the database itself enforces, and check that they really are enforced
• ✓Reproduce a lost-update race with threads, and fix it with one atomic statement

Put a service on a real database, change its shape without losing data, find and fix its slow queries, and keep its data correct when things fail and when many users act at once.