How was PostgreSQL sharding implemented — what served as the router?

In PostgreSQL sharding implementation, an external router or proxy is often used to direct requests to the appropriate sharded database instance. The router can be:

• A specialized proxy server (e.g., Pgpool-II, Citus coordinator) that analyzes requests and redirects them.
• The application's own logic, which selects the required database instance based on the sharding key.

In some cases, middleware or a service that accepts requests and distributes them across shards, providing transparency for the client, is used.

Thus, a "router" is a component that knows how to map a sharding key to a specific PostgreSQL server and direct the request there.