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Overlay WebApp Deployer

Overview
Overlays
JTrac Overlay Example
Configuring Jetty for Overlays
Installing the WebApp
Installing a Template Overlay
Installing an Instance Overlay
Things to Know and Notice

Note

This feature is reintroduced in Jetty 9.0.4

The Jetty Overlay Deployer allows you to overlay multiple WAR files so that you can customise, configure, and deploy a web application without unpacking, modifying and repacking the WAR file. This has the following benefits:

This tutorial describes how to configure Jetty to use the Overlay deployer, and how to deploy multiple instances of a web application, using the JTrac application in the example.

Overview

Customising, configuring and deploying a web application bundled as a WAR file frequently includes some or all of these steps:

  • Editing the WEB-INF/web.xml file to set init parameters, add filters/servlets or to configure JNDI resources.

  • Editing other application specific configuration files under WEB-INF/.

  • Editing container specific configuration files under WEB-INF/ (for example, jetty-web.xml or jboss-web.xml).

  • Adding/modifying static content such as images and CSS to create a style or themes for the web application.

  • Adding Jars to the container classpath for Datasource and other resources.

  • Modifying the container configuration to provide JNDI resources.

The result is that the customisations and configurations blend into both the container and the WAR file. If you upgrade either the container or the base WAR file to a new version, it can be a very difficult and error prone task to identify all the changes that you have made, and to reapply them to a new version.

Overlays

To solve the problems highlighted above, Jetty 7.4 introduces WAR overlays (a concept borrowed from the Maven WAR plugin). An overlay is basically just another WAR file, whose contents merge on top of the original WAR so that you can add or replace files. Jetty overlays also allow you to mix in fragments of web.xml, which means you can modify the configuration without replacing it.

JTrac Overlay Example

The JTrac issue tracking web application is a good example of a typical web application, as it uses the usual suspects of libs: spring, hibernate, dom4j, commons-*, wicket, etc. The files for this demonstration are available in overlays-demo.tar.gz. You can expand it on top of the jetty distribution; this tutorial expands it to /tmp and installs the components step-by-step:


$ cd /tmp
$ wget http://webtide.intalio.com/wp-content/uploads/2011/05/overlays-demo.tar.gz
$ tar xfvz overlays-demo.tar.gz
$ export OVERLAYS=/tmp/overlays

    

Configuring Jetty for Overlays

Overlays support is included in jetty distributions from 7.4.1-SNAPSHOT onwards, so you can download a distribution from oss.sonatype.org or maven central and unpack into a directory. You need to edit the start.ini file so that it includes the overlay option and configuration file. The resulting file should look like:

The smarts of this are in etc/jetty-deploy.xml, which installs the OverlayedAppProvider into the DeploymentManager. You can then start Jetty normally:


$ java -jar start.jar

    

Jetty is now listening on port 8080, but with no webapp deployed.

Important

You should conduct the rest of the tutorial in another window with the JETTY_HOME environment set to the jetty distribution directory.

Installing the WebApp

You can download and deploy the WAR file for this demo using the following commands, which essentially downloads and extracts the WAR file to the $JETTY_HOME/overlays/webapps directory.


$ cd /tmp
$ wget -O jtrac.zip http://sourceforge.net/projects/j-trac/files/jtrac/2.1.0/jtrac-2.1.0.zip/download
$ jar xfv jtrac.zip jtrac/jtrac.war
$ mv jtrac/jtrac.war $JETTY_HOME/overlays/webapps

    

When you have run these commands (or equivalent), you see in the Jetty server window a message saying that the OverlayedAppProvider has extracted and loaded the WAR file:

Unlike the normal webapps dir, loading a WAR file from the overlays/webapp dir does not deploy the web application. It simply makes it available to use as the basis for templates and overlays.

Installing a Template Overlay

A template overlay is a WAR structured directory/archive that contains just the files that you have added or modified to customize/configure the web application for all instances you plan to deploy.

You can install the demo template from the downloaded files with the command:


$ mv $OVERLAYS/jtracTemplate\=jtrac-2.1.0 $JETTY_HOME/overlays/templates/

    

In the Jetty server window, you should see the template loaded with a message like:

The contents of the loaded template are as follows:

name of the template directory (or WAR)

Uses the ‘=’ character in jtracTemplate=jtrac-2.1.0 to separate the name of the template from the name of the WAR file in webapps that it applies to. If = is a problem, then you can instead use --.

WEB-INF/classes/jtrac-init.properties

Replaces the JTrac properties file with an empty file, as the properties it contains are configured elsewhere.

WEB-INF/log4j.properties

Configures the logging for all instances of the template.

WEB-INF/overlay.xml

A Jetty XML formatted IoC file that injects/configures the ContextHandler for each instance. In this case it just sets up the context path:

WEB-INF/template.xml

a Jetty XML formatted IoC file that injects/configures the resource cache and classloader that all instances of the template share. It runs only once per load of the template:

WEB-INF/web-overlay.xml

a web.xml fragment that Jetty overlays on top of the web.xml from the base WAR file; it can set init parameters and add/modify filters and servlets. In this example it sets the application home and springs rootKey:

Notice the parameterisation of values such as ${overlays.instance.classifier}, as this allows the configuration to be in the template, and not customised for each instance.

Without the Overlay Deployer, you would still need to have configured all of the above, but rather than being in a single clear structure the configuration elements would have been either in the server's common directory, the server's webdefaults.xml (aka server.xml), or baked into the WAR file of each application instance using copied/modified files from the original. The Overlay Deployer allows you to make all these changes in one structure; moreover it allows you to parameterise some of the configuration, which facilitates easy multi-tenant deployment.

Installing an Instance Overlay

Now that you have installed a template, you can install one or more instance overlays to deploy the actual web applications:


$ mv /tmp/overlays/instances/jtracTemplate\=blue $JETTY_HOME/overlays/instances/
$ mv /tmp/overlays/instances/jtracTemplate\=red $JETTY_HOME/overlays/instances/
$ mv /tmp/overlays/instances/jtracTemplate\=blue $JETTY_HOME/overlays/instances/

    

As each instance moves into place, you see the Jetty server window react and deploy that instance. Within each instance, there is the structure:

WEB-INF/overlay.xml

a Jetty XML format IoC file that injects/configures the context for the instance. In this case it sets up a virtual host for the instance:

favicon.ico

Replaces the icon in the base WAR with one that has a theme for the instance, in this case red, blue, or green.

resources/jtrac.css

Replaces the style sheet from the base WAR with one that his a theme for the instance.

You can now view the deployed instances by pointing your browser at http://127.0.0.1:8080, http://127.0.0.2:8080 and http://127.0.0.3:8080. The default username/password for JTrac is admin/admin.

Things to Know and Notice

  • Each instance has themes with images and style sheets from the instance overlay.

  • Each instance is running with its own application directory (that is, /tmp/jtrac-red), set in templates web-overlay.xml.

  • A virtual host set in the instance overlay.xml distinguishes the instances.

  • All instances share static content from the base WAR and template. Specifically there is a shared ResourceCache so only a single instance of each static content is loaded into memory.

  • All instances share the classloader at the base WAR and template level, so that only a single instance of common classes is loaded into memory. You can configure classes with non shared statics to load in the instances classloader.

  • Jetty hot deploys all overlays and tracks dependencies.

    • If an XML changes in an instance, Jetty redeploys it.

    • If an XML changes in a template, then Jetty redeploys all instances using it.

    • If a WAR file changes, then Jetty redeploys all templates and all instances dependant on it.

  • You can esaily deploy new versions. For example, when JTrac-2.2.0.war becomes available, you can just drop it into overlays/webapps and then rename jtracTemplate\=jtrac-2.1.0 to jtracTemplate\=jtrac-2.2.0

  • There is a fuller version of this demo in overlays-demo-jndi.tar.gz, that uses JNDI (needs options=jndi, annotations and jetty-plus.xml in start.ini) and shows how you can add extra JARs in the overlays.

See an error or something missing? Contribute to this documentation at Github!(Generated: 2014-08-01T01:00:20-07:00)