The scheme used for PURLs is generally HTTP. All resources are stored under persistent URLs (so-called PURLs). Most PURLs are so-called "simple PURLs", which provide a redirection to the desired resource. Active PURLs have been implemented in PURLz 2.0 and The Callimachus Project. Active PURLs act similar to a stored procedure in relational databases. Email blasts and PURLS allow a company to find out information about their consumer. PURLs redirect HTTP clients using HTTP status codes. PURLs implement one form of persistent identifier for virtual resources. The Callimachus Project implemented PURLs as of its 1.0 release in 2012. PURLs allow third party control over both URL resolution and resource metadata provision. PURLs are themselves valid URLs, so their components must map to the URL specification.