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>44.2. How Connections are Established</A
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><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is implemented using a
simple <SPAN
CLASS="QUOTE"
>"process per user"</SPAN
> client/server model. In this model
there is one <I
CLASS="FIRSTTERM"
>client process</I
> connected to
exactly one <I
CLASS="FIRSTTERM"
>server process</I
>. As we do not
know ahead of time how many connections will be made, we have to
use a <I
CLASS="FIRSTTERM"
>master process</I
> that spawns a new
server process every time a connection is requested. This master
process is called <TT
CLASS="LITERAL"
>postgres</TT
> and listens at a
specified TCP/IP port for incoming connections. Whenever a request
for a connection is detected the <TT
CLASS="LITERAL"
>postgres</TT
>
process spawns a new server process. The server tasks
communicate with each other using <I
CLASS="FIRSTTERM"
>semaphores</I
> and
<I
CLASS="FIRSTTERM"
>shared memory</I
> to ensure data integrity
throughout concurrent data access.
</P
><P
> The client process can be any program that understands the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> protocol described in
<A
HREF="protocol.html"
>Chapter 46</A
>. Many clients are based on the
C-language library <SPAN
CLASS="APPLICATION"
>libpq</SPAN
>, but several independent
implementations of the protocol exist, such as the Java
<SPAN
CLASS="APPLICATION"
>JDBC</SPAN
> driver.
</P
><P
> Once a connection is established the client process can send a query
to the <I
CLASS="FIRSTTERM"
>backend</I
> (server). The query is transmitted using plain text,
i.e., there is no parsing done in the <I
CLASS="FIRSTTERM"
>frontend</I
> (client). The
server parses the query, creates an <I
CLASS="FIRSTTERM"
>execution plan</I
>,
executes the plan and returns the retrieved rows to the client
by transmitting them over the established connection.
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