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freedev
Asked: 2013-04-18 19:46:25 +0800 CST

Apache as reverse proxy more resources

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I'm moving a big directory of images from an HTTP server to another and I cannot change any configuration.

I would like my user don't even suppose that a moving in progress. So I thought to add an Apache httpd server configured as reverse proxy in front of both the servers.

This reverse proxy when a resource is not found in the destination should try to get it in the origin server.

Is there any way to do this using mod_rewrite or some standard Apache httpd module?

The resources are not on the front-end server, but on two back-end servers.

apache-2.2

2 Answers

  1. To check for remote resources in RewriteCond you can use the following:

    '-U' (is existing URL, via subrequest) Checks whether or not TestString is a valid URL, accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care - it can impact your server's performance!

    I never personally used it. But it works like testing for a file existence and the like.

    You can then have a RewriteRule that proxies the request (the [P] option).

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  2. Best Answer

    I have implemented a reverse proxy with Apache httpd 2.2, but it is not possible do it using only apache directives. So I had to use a simple PHP script in order to check remote url existence.

    When a new request comes, it check the existence of current path on Server A and, if successfully found, it reverse proxy the request on same server. On the other hand, if the requested path is not found on Server A, it implements a rewrite rule reverse proxy on serverB.

    I have used a RewriteMap with MapType prg: and a PHP script. There are the Apache directives:

    # Please pay attention to RewriteLock
# this directive must be defined in server config context
RewriteLock /tmp/if_url_exists.lock

RewriteEngine On

ProxyPreserveHost Off
ProxyRequests Off

RewriteMap  url_exists "prg:/usr/bin/php /opt/local/scripts/url_exists.php"
RewriteCond ${url_exists:http://serverA%{REQUEST_URI}}   >0
RewriteRule .            http://serverA%{REQUEST_URI} [P,L]

RewriteRule .            http://serverB%{REQUEST_URI} [P,L]

    Here comes the interesting and tricky part. This is the url_exists.php script, executed by Apache. It is waiting on the standard input stream and write into standard output. This scripts return 1 if the resource is found and readable, otherwise 0. Actually it is quite simple because it implements only an HTTP request using the HEAD method.

    <?php

function check_if_url_exists($line) {
    $curl_inst = curl_init($line);

    curl_setopt( $curl_inst, CURLOPT_CONNECTTIMEOUT, 30);
    curl_setopt( $curl_inst, CURLOPT_LOW_SPEED_LIMIT, 1);
    curl_setopt( $curl_inst, CURLOPT_LOW_SPEED_TIME, 180);
    curl_setopt( $curl_inst, CURLOPT_HEADER, true);
    curl_setopt( $curl_inst, CURLOPT_FAILONERROR, true);

    // Exclude the body from the output and request method is set to HEAD.
    curl_setopt( $curl_inst, CURLOPT_NOBODY, true);
    curl_setopt( $curl_inst, CURLOPT_FOLLOWLOCATION, true);
    curl_setopt( $curl_inst, CURLOPT_RETURNTRANSFER, true);

    $raw = curl_exec($curl_inst);

    curl_close($curl_inst);

    return ($raw != false) ? true : false;

}

set_time_limit(0);
$keyboard = fopen("php://stdin","r");

while (true) {
    $line = trim(fgets($keyboard));
    if (!empty($line)) {
        $str = (check_if_url_exists($line)) ? "1" : "0";
        error_log($line. " ".$str);
        echo $str."\n";
    } else {
        error_log('empty line!');
    }
}
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