Tomcat architecture

Here is a sum up on the architecture of tomcat for a quick recap:

Server

• Is the top level component.
• Shutdown port, string are configured here, but the request should be sent from same server on which Tomcat is running
• Only one per jvm
• Any implementation of org.apache.catalina.Server can be used via className attribute, if not specified org.apache.catalina.core.StandardServer is used
• Global naming resources are configured here
• Listeners could be configured here

Listener

• Performs actions when specific events occur, usually Tomcat starting or Tomcat stopping
• May be nested inside a Server, Engine, Host or Context only
• Some Listeners are only intended to be nested inside specific elements only
• APR Lifecycle, Jasper, Server Lifecycle, Global Resources Lifecycle, JMX Remote Lifecycle listeners could be configured only within Server

Container

• Any one of Engine, Host, Context, and Cluster are referred as Container
• Engine contains Host, it contains Context.  Cluster could be inside Engine or Host

Service

• Groups a container (of type Engine) with a set of Connectors
• Cannot define sub-components at level as it is not a Container
• Only components that may be nested inside a Service element are one or more Connector elements, followed by exactly one Engine element
• Can have multiple services under server, name must be unique (probably when we want one per protocol?)
• Any implementation of org.apache.catalina.Service can be used via className attribute, if not specified org.apache.catalina.core.StandardService is used

 Connector

• Is associated with a TCP port to handle communications between the Service and the clients

 Engine

• Highest-level of a container, contains one or more Hosts
• Represents the entire request processing machinery associated with a particular Service
• Receives and processes all requests from one or more Connectors, and returns the completed response to the Connector for ultimate transmission back to the client
• Exactly one Engine element MUST be nested inside a Service element, following all of the corresponding Connector elements associated with this Service

Host

• Receives HTTP requests from the HTTP connector, and direct them to the correct host based on the hostname/IP address in the request header
• Exactly one of the Hosts associated with each Engine MUST have a name matching the defaultHost attribute of that Engine
• Can nest at most one instance of Realm, Valve

Realm

• A Realm is a database of user, password, and role for authentication
• You can define Realm for any container, such as Engine, Host, and Context, and Cluster

Valve

•  A Valve can intercept HTTP requests before forwarding them to the applications, for pre-processing the requests
• A Valve can be defined for any container, such as Engine, Host, and Context, and Cluster