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 44. 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.
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