In Tomcat, a Realm element represents a "database" of usernames, passwords, and roles (similar to Unix groups) assigned to those users.

This allows Catalina, nominally a Java Servlet or JSP container, to also act as a plain web server that serves local files as HTTP documents.

A high-availability feature has been added to facilitate the scheduling of system upgrades (e.g. new releases, change requests) without affecting the live environment.

[6] Tomcat started off as a servlet reference implementation by James Duncan Davidson, a software architect at Sun Microsystems.

He later helped make the project open-source and played a key role in its donation by Sun Microsystems to the Apache Software Foundation.

[7] The Apache Ant software build automation tool was developed as a side-effect of the creation of Tomcat as an open source project.

[32] In October 2011, the project obtained certification by Oracle Corporation as a compatible implementation of the Java EE 6 Web Profile.