mod_proxy*.xml: improve connection reuse documentation

      <p>The two default workers have a fixed configuration

      and will be used if no other worker matches the request.

      They do not use HTTP Keep-Alive or connection pooling.

      They do not use HTTP Keep-Alive or connection reuse.

      The TCP connections to the origin server will instead be

      opened and closed for each request.</p>

      <note><title>DNS resolution for origin domains</title>

      <p>DNS resolution happens when the socket to

        the origin domain is created for the first time.

        When connection pooling is used, each backend domain is resolved 

        only once per child process, and reused for all further connections 

        When connection reuse is enabled, each backend domain is resolved 

        only once per child process, and cached for all further connections 

        until the child is recycled. This information should to be considered 

        while planning DNS maintenance tasks involving backend domains. 

        Please also check <directive module="mod_proxy">ProxyPass</directive>

    a local virtual path; <var>url</var> is a partial URL for the

    remote server and cannot include a query string.</p>

    <note><strong>Note: </strong>This directive is not supported within

    <note>It is strongly suggested to review the concept of a

    <a href="#workers">Worker</a> before proceeding any further

    with this section.</note>

    <note>This directive is not supported within

    <directive type="section" module="core">Directory</directive> and

    <directive type="section" module="core">Files</directive> containers.</note>

    using  <code>key=value</code> parameters, described in the tables

    below.</p>

    <note type="warning"><title>Maximum connections to the backend</title>

    <p>By default, mod_proxy will allow and retain the maximum number of

    connections that could be used simultaneously by that web server child

    process.  Use the <code>max</code> parameter to reduce the number from

    the default.  Use the <code>ttl</code> parameter to set an optional

    the default. The pool of connections is maintained per web server child

    process, and <code>max</code> and other settings are not coordinated

    among all child processes, except when only one child process is allowed

    by configuration or MPM design.</p>

    </note>

    <p>Use the <code>ttl</code> parameter to set an optional

    time to live; connections which have been unused for at least

    <code>ttl</code> seconds will be closed.  <code>ttl</code> can be used

    to avoid using a connection which is subject to closing because of the

    backend server's keep-alive timeout.</p>

    <p>The pool of connections is maintained per web server child

    process, and <code>max</code> and other settings are not coordinated

    among all child processes, except when only one child process is allowed

    by configuration or MPM design.</p>

    <example><title>Example</title>

        <highlight language="config">

        ProxyPass "/example" "http://backend.example.com" max=20 ttl=120 retry=300

        </highlight>

    </example>

    <table border="2"><tr><th>BalancerMember parameters</th></tr></table>

    <table border="2"><tr><th>Worker|BalancerMember parameters</th></tr></table>

    <table>

    <tr><th>Parameter</th>

        <th>Default</th>

    httpd and

    the backend server (regardless of protocol) tends to silently

    drop connections or when backends themselves may be under round-

    robin DNS. To disable connection pooling reuse,

    robin DNS.

    When connection reuse is enabled each backend domain is resolved

    (with a DNS query) only once per child process and cached for all further

    connections until the child is recycled. To disable connection reuse,

    set this property value to <code>On</code>.

    </td></tr>

    <tr><td>enablereuse</td>

    <p> The following example passes the request URI as a filesystem

    path for the PHP-FPM daemon to run. The request URL is implicitly added

    to the 2nd parameter. The hostname and port following fcgi:// are where

    PHP-FPM is listening.  Connection pooling is enabled.</p>

    PHP-FPM is listening.  Connection pooling/reuse is enabled.</p>

    <example><title>PHP-FPM</title>

    <highlight language="config">

ProxyPassMatch "^/myapp/.*\.php(/.*)?$" "fcgi://localhost:9000/var/www/" enablereuse=on

<p>Using this flag triggers the use of <module>mod_proxy</module>, without handling of persistent connections. This

means the performance of your proxy will be better if you set it up with <directive module="mod_proxy">ProxyPass</directive> or

<directive module="mod_proxy">ProxyPassMatch</directive></p>

<p>This is because this flag triggers the use of the default worker, which does not handle connection pooling.</p>

<p>This is because this flag triggers the use of the default worker, which does not handle connection pooling/reuse.</p>

<p>Avoid using this flag and prefer those directives, whenever you can.</p>

</note>