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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.
   </P
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