The db-sync node is written in a highly modular fashion to allow it to be as flexible as possible.

The cardano-db-sync node connects to a locally running cardano-node (ie one connected to other nodes in the Cardano network over the internet with TCP/IP) using a Unix domain socket, retrieves blocks and stores parts of each block in a local PostgreSQL database. The database does not store things like cryptographic signatures but does store enough information to follow the chain of blocks and look at the transactions within blocks.

The PostgreSQL database is designed to be accessed in a read-only fashion from other applications. The database schema is highly normalised which helps prevent data inconsistencies (specifically with the use of foreign keys from one table to another). More user friendly database queries can be implemented using Postgres Views to implement joins between tables.