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<span class="productname">PostgreSQL</span> is implemented using a
simple <span class="quote">“<span class="quote">process per user</span>”</span> client/server model. In this model
there is one <em class="firstterm">client process</em> connected to
exactly one <em class="firstterm">server process</em>. As we do not
know ahead of time how many connections will be made, we have to
use a <em class="firstterm">master process</em> that spawns a new
server process every time a connection is requested. This master
process is called <code class="literal">postgres</code> and listens at a
specified TCP/IP port for incoming connections. Whenever a request
for a connection is detected the <code class="literal">postgres</code>
process spawns a new server process. The server tasks
communicate with each other using <em class="firstterm">semaphores</em> and
<em class="firstterm">shared memory</em> 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 class="xref" href="protocol.html" title="Chapter 53. Frontend/Backend Protocol">Chapter 53</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 <em class="firstterm">backend</em> (server). The query is transmitted using plain text,
i.e., there is no parsing done in the <em class="firstterm">frontend</em> (client). The
server parses the query, creates an <em class="firstterm">execution plan</em>,
executes the plan and returns the retrieved rows to the client
by transmitting them over the established connection.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="query-path.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="overview.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="parser-stage.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">51.1. The Path of a Query </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 51.3. The Parser Stage</td></tr></table></div></body></html>
