The Architecture of Tomcat


Tomcat 4 is a complete rewrite of its ancestors. At the core
of this rewrite is the Catalina servlet engine, which acts
as the top-level container for all Tomcat instances.

 
With this rewrite of Tomcat comes an entirely new
architecture composed of a grouping of application
containers, each with a specific role. The sum of all of
these containers makes up an instance of a Catalina engine.

 
The following code snippet provides an XML representation of
the relationships between the different Tomcat containers:

 
<Server>
<Service>
<Connector />
<Engine>
<Host>
<Context>
</Context>
</Host>
</Engine>
</Service>
</Server>

 
This instance can be broken down into a set of containers
including a server, a service, a connector, an engine, a
host, and a context. By default, each of these containers is
configured using the server.xml file, which we describe
later in more detail.

The Server

The first container element referenced in this snippet is
the <Server> element. It represents the entire Catalina
servlet engine and is used as a top-level element for a
single Tomcat instance. The <Server> element may contain one
or more <Service> containers.

The Service

The next container element is the <Service> element, which
holds a collection of one or more <Connector> elements that
share a single <Engine> element. N-number of <Service>
elements may be nested inside a single <Server> element.
 
The Connector
 
The next type of element is the <Connector> element, which
defines the class that does the actual handling requests and
responses to and from a calling client application.

The Engine
 
The third container element is the <Engine> element. Each
defined <Service> can have only one <Engine> element, and
this single <Engine> component handles all requests received
by all of the defined <Connector> components defined by a
parent service.

The Host

The <Host> element defines the virtual hosts that are
contained in each instance of a Catalina <Engine>. Each
<Host> can be a parent to one or more Web applications, with
each being represented by a <Context> component.

 The Context
 
The <Context> element is the most commonly used container in
a Tomcat instance. Each <Context> element represents an
individual Web application that is running within a defined
<Host>. There is no limit to the number of contexts that can
be defined within a <Host>.