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The three major components are the frontend (running on the client) and the postmaster and backend (running on the server). The postmaster and backend have different roles but may be implemented by the same executable.
A frontend sends a startup packet to the postmaster. This includes the names of the user and the database the user wants to connect to. The postmaster then uses this, and the information in the pg_hba.conf(5) file to determine what further authentication information it requires the frontend to send (if any) and responds to the frontend accordingly.
The frontend then sends any required authentication information. Once the postmaster validates this it responds to the frontend that it is authenticated and hands over to a backend.
Subsequent communications are query and result packets exchanged between the frontend and the backend. The postmaster takes no further part in the communication.
When the frontend wishes to disconnect it sends an appropriate packet and closes the connection without waiting for a response for the backend.
Packets are sent as a data stream. The first byte determines what should be expected in the rest of the packet. The exception is packets send from a frontend to the postmaster, which comprise a packet length then the packet itself. The difference is historical.
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