Jakarta Tomcat of JServ from Apache.
Some more information:
What is mod_jk?
mod_jk is a replacement to the elderly mod_jserv. It is a completely
new Tomcat-Apache plugin that handles the communication between
Tomcat and Apache
Why mod_jk?
Several reasons:
mod_jserv was too complex. Because it was ported from Apache/JServ,
it brought with it lots of JServ specific bits that aren't needed
by Apache.
mod_jserv supported only Apache. Tomcat supports many web servers
through a compatibility layer named the jk library. Supporting two
different modes of work became problematic in terms of support,
documentation and bug fixes. mod_jk should fix that.
The layered approach provided by the jk library makes it easier to
support both Apache1.3.x and Apache2.xx.
Better support for SSL. mod_jserv couldn't reliably identify whether
a request was made via HTTP or HTTPS. mod_jk can, using the newer
Ajpv13 protocol.
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