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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
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 48. 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.