Figure 2-1. How a connection is established
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Before we begin, you should understand the basic Postgres system architecture. Understanding how the parts of Postgres interact will make the next chapter somewhat clearer. In database jargon, Postgres uses a simple "process per-user" client/server model. A Postgres session consists of the following cooperating UNIX processes (programs):
A supervisory daemon process (postmaster),
the user's frontend application (e.g., the psql program), and
the one or more backend database servers (the postgres process itself).
A single postmaster manages a given collection of databases on a single host. Such a collection of databases is called an installation or site. Frontend applications that wish to access a given database within an installation make calls to the library. The library sends user requests over the network to the postmaster (How a connection is established), which in turn starts a new backend server process
and connects the frontend process to the new server. From that point on, the frontend process and the backend server communicate without intervention by the postmaster. Hence, the postmaster is always running, waiting for requests, whereas frontend and backend processes come and go.The libpq library allows a single frontend to make multiple connections to backend processes. However, the frontend application is still a single-threaded process. Multithreaded frontend/backend connections are not currently supported in libpq. One implication of this architecture is that the postmaster and the backend always run on the same machine (the database server), while the frontend application may run anywhere. You should keep this in mind, because the files that can be accessed on a client machine may not be accessible (or may only be accessed using a different filename) on the database server machine.
You should also be aware that the postmaster and postgres servers run with the user-id of the Postgres "superuser." Note that the Postgres superuser does not have to be a special user (e.g., a user named "postgres"). Furthermore, the Postgres superuser should definitely not be the UNIX superuser ("root")! In any case, all files relating to a database should belong to this Postgres superuser.
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