Server - Service - Engine - Host, Tomcat
A Tomcat Server
can run one or more Service
s. Each Service comprises a number of Connector
s and an Engine
, which contains one or more Host
You connect to a Tomcat Server via an appropriate connector, which defines the port number, the protocol, and various other connection parameters, and within the Tomcat server information is generated for the particular host (domain) that you're serving.
Q: Why would you want multiple services?
A: So that you can use a single copy of Tomcat to serve different hosts in different ways.
The top diagram shows you a server with five connectors and a single service, accessing 3 hosts. Any of the five connectors can access any of the five hosts, so you've a total of fifteen different routes to the data.
The lower diagram shows, again, 5 connectors and 3 hosts - but this time two of the connectors access a service who's engine looks after just one host, and the other three connectors access a second service that looks after two hosts. Result? Just ( 2 x 1 + 3 x 2) = 8 different routes to data.
Why might this be useful? Let's say that the single host with two connectors provided internet-accessible http and https services, and the other two hosts were accessible via ajp and http proxies, together wish a second http port for testing. And that's all you wanted - no crossovers, but also no need to run two complete Tomcats! (written 2008-12-14, updated 2008-12-15)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesA651 - Web Application Deployment - Tomcat Overview 
Servlet v JSP (Java Server Page). What is the difference? - (2013-02-06) 
You do not just fly - that is just part of the journey - (2010-05-06) 
An FAQ on the Apache httpd and Apache Tomcat web servers, and on using them together - (2009-05-17) 
Why put Apache httpd in front of Apache Tomcat - (2009-03-12) 
Apache httpd and Apache Tomcat miscellany - (2009-01-30) 
More HowTo diagrams - MySQL, Tomcat and Java - (2008-08-24) 
Apache, Tomcat, Jakarta, httpd, web server - what are they? - (2007-07-13) 
Web Application Components - (2006-03-28) 
Why run two different web servers - (2006-01-25) 
Why is Tomcat called Tomcat? - (2005-11-17) 
Business is the predominant user of Tomcat, Perl and Tcl - (2004-09-15)
Some other Articles
Summary - Apache httpd build on LinuxForwarding session and cookie requests from httpd to TomcatPort and GlassesChristmas scenes and eventsServer - Service - Engine - Host, TomcatURL rewriting with front and back serversmod_proxy_ajp and mod_proxy_balancer examplesPredictive Load Balancing - PHP and / or JavaGetting hold of the wrong end of the stickQuick Summary - PHP installation
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