50.2. How Connections Are Established#
PostgreSQL implements a“process per user” client/server model. In this model, everyclient process connects to exactly onebackend process. As we do not know ahead of time how many connections will be made, we have to use a“supervisor process” that spawns a new backend process every time a connection is requested. This supervisor process is calledpostmaster and listens at a specified TCP/IP port for incoming connections. Whenever it detects a request for a connection, it spawns a new backend process. Those backend processes communicate with each other and with other processes of theinstance usingsemaphores andshared memory to ensure data integrity throughout concurrent data access.
The client process can be any program that understands thePostgreSQL protocol described inChapter 53. Many clients are based on the C-language librarylibpq, but several independent implementations of the protocol exist, such as the JavaJDBC driver.
Once a connection is established, the client process can send a query to the backend process it's connected to. The query is transmitted using plain text, i.e., there is no parsing done in the client. The backend process parses the query, creates anexecution plan, executes the plan, and returns the retrieved rows to the client by transmitting them over the established connection.