A unique identifier for each request is super useful when you want to trace requests of your web application for debugging. For instance if you have a logging system, you can search through a particular request ID to debug logs representing everything that happened throughout the lifetime of a request.

mod_unique_id

The Apache HTTP Server makes this possible with the mod_unique_id server module. Once enabled, httpd starts setting an environment variable called UNIQUE_ID for every incoming request.

If you’re running a PHP web site/application then this is how you can retrieve it:

// Prints `Xq543Ab3gAxREC-cR-KlHAAAAAg`
echo $_SERVER['UNIQUE_ID'];

The documentation page explains how the identifier is as unique as possible in detail. To summarize, encodes the following data points:

1. 32-bit IP Address.
2. 32 bit process ID (pid generated by the operating system).
3. 32 bit timestamp, UTC in seconds (when the request was received).
4. A 16 bit counter in case of timestamp and pid reuse.
5. 32 bit thread index to identify threads (and differ across them).

All the information above is encoded like base64, only difference being in the characters used – [A-Za-z0-9@-] is replaced with [A-Za-z0-9+/].

Logging

If you want the unique request IDs to end up in your access logs then just add %{UNIQUE_ID}e to the LogFormat.

LogFormat "%h %l %u %t \"%r\" %>s %O \"%{Referer}i\" \"%{User-Agent}i\" %{UNIQUE_ID}e" combined

CustomLog /var/log/apache2/access.log combined

Headers

Sometimes it is desired to set a request header with the unique ID for reasons like third party compatibility, legacy code, etc. This can be achieved with the mod_headers module. Once enabled, all you’ve to do is set the header with the environment variable’s value.

# In server config, virtual host, directory, file, location or .htaccess
RequestHeader set "X-Request-Id" "%{UNIQUE_ID}e"

Now with every request, httpd will add a request header called X-Request-Id containing the unique ID generated by mod_unique_id module. Reading it in PHP is super easy:

echo $_SERVER['HTTP_X_REQUEST_ID'];
// or
echo getallheaders()['X-Request-Id'];