Fundamental Architecture of PostgreSQL

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Page: https://www.postgresql.org/docs/16/tutorial-arch.html
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PostgreSQL uses a client/server architecture. 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. A PostgreSQL
database, in a broader sense, consists of the following cooperating
processes (programs):
• A PostgreSQL instance process that manages a collection of databases also
known as Postgres Cluster. A Postgres Cluster is stored at a common file
system location (the “data area”). More than one PostgreSQL instance can be
initiated on a single system as long as they use different data areas and
different communication ports.
• A Client Process initiates a connection (or session) for the PostgreSQL
instance. Client applications can be very diverse in nature. A Postgres
Client could be a text-oriented tool, a graphical application, a web server
that accesses the PostgreSQL as a backend database, or a specialized
database maintenance tool.
• A Server Process initiated by the PostgreSQL instance as soon as a
Postgres Client sends a connection request. The PostgreSQL instance can
handle multiple such connection requests concurrently, originating from
various Postgres Client processes that are running on a single or multiple
hosts. To achieve this, the PostgreSQL instance initiates (forks) a new
Server Process for every new connection request coming from a Postgres
Client. The Server Process accepts the connection request and after
authentication, performs database actions on behalf of the corresponding
Postgres Client while interacting with the PostgreSQL instance. In other
words, after establishing the connection with one of the Server Processes,
the Postgres Client sends requests to the Server Process while the Server
Process listens to these requests and returns responses accordingly. Once a
connection is established, the Postgres Client and the Server Process can
communicate with each other without directly intervening with the PostgreSQL
instance. Note that, the PostgreSQL instance is always running and waiting
to accept any new client connection requests coming from Postgres Clients,
whereas the Postgres Client and associated Server Process come and go.