hen an attribute is not
available. For example, <code>response.code</code> may not be available when a request
ends abruptly. When attribute generation fails, it will not populate the
attribute.</p><p>If the generated attribute is used by an authz plugin, it should account
for the possibility that the attribute may be missing. Use
<code>has(attribute_name)</code> function to check for presence of an attribute before
using its value, and provide appropriate defaults. For example the
following is a safe use o