# 50.2. How Connections Are Established

PostgreSQL is implemented using a simple “process per user” client/server model. In this model there is one *client process* connected to exactly one *server process*. As we do not know ahead of time how many connections will be made, we have to use a *master process* that spawns a new server process every time a connection is requested. This master process is called `postgres` and listens at a specified TCP/IP port for incoming connections. Whenever a request for a connection is detected the `postgres` process spawns a new server process. The server tasks communicate with each other using *semaphores* and *shared memory* to ensure data integrity throughout concurrent data access.

The client process can be any program that understands the PostgreSQL protocol described in [Chapter 52](https://www.postgresql.org/docs/12/protocol.html). Many clients are based on the C-language library libpq, but several independent implementations of the protocol exist, such as the Java JDBC driver.

Once a connection is established the client process can send a query to the *backend* (server). The query is transmitted using plain text, i.e., there is no parsing done in the *frontend* (client). The server parses the query, creates an *execution plan*, executes the plan and returns the retrieved rows to the client by transmitting them over the established connection.