Before we proceed, you should understand the basic
PostgreSQL system architecture.
Understanding how the parts of
PostgreSQL interact will make this
chapter somewhat clearer.
In database jargon, PostgreSQL uses a
client/server model. A PostgreSQL
session consists of the following cooperating processes
A server process, which manages the database files, accepts
connections to the database from client applications, and
performs actions on the database on behalf of the clients. The
database server program is called
The user's client (frontend) application that wants to perform
database operations. Client applications can be very diverse
in nature: a client could be a text-oriented tool, a graphical
application, a web server that accesses the database to
display web pages, or a specialized database maintenance tool.
Some client applications are supplied with the
PostgreSQL distribution, most are
developed by users.
As is typical of client/server applications, the client and the
server can be on different hosts. In that case they communicate
over a TCP/IP network connection. You should keep this in mind,
because the files that can be accessed on a client machine might
not be accessible (or might only be accessible using a different
file name) on the database server machine.
The PostgreSQL server can handle
multiple concurrent connections from clients. For that purpose it
starts ("forks") a new process for each connection.
From that point on, the client and the new server process
communicate without intervention by the original
postmaster process. Thus, the
postmaster is always running, waiting for
client connections, whereas client and associated server processes
come and go. (All of this is of course invisible to the user. We
only mention it here for completeness.)