Using a database adapter
An Adapter in Auth.js connects your application to whatever database or backend system you want to use to store data for users, their accounts, sessions, etc. Adapters are optional, unless you need to persist user information in your own database, or you want to implement certain flows. The Email Provider requires an adapter to be able to save Verification Tokens.
When using a database, you can still use JWT for session handling for fast access. See the session.strategy option. Read about the trade-offs of JWT in the FAQ.
We have a list of official adapters that are distributed as their own packages under the @next-auth/{name}-adapter namespace. Their source code is available in their various adapters package directories at nextauthjs/next-auth: