The Sling Launchpad

This tries to explain how exactly the Sling Launchpad works, what constitutes the Sling Launchpad and how you can use the Sling Launchpad to custom create you Sling launchers.

Sling Home

Since Sling requires some space on the filesystem to store various files Sling has to know where this filesystem space is located.

The following is a list of uses for the Sling Home directory:

• sling.properties – The main configuration file used by Sling to launch the framework. It mainly contains OSGi framework configuration and initial configuration for some bundles. This file is read on each startup of Launcher JAR. That is, changes to this file require a restart of the Launcher JAR.
• org.apache.sling.launchpad.base.jar – This is the Launcher JAR file used by the Standalone Application or the Web Application to start the OSGi Framework. This file is initially placed when first starting Sling and may later be updated by updating the system bundle with a new Launcher JAR.
• felix – The directory into which the Apache Felix Framework places the bundles, which have been installed into the system. This does generally not need to be touched.
• config – The directory into which the Apache Felix Configuration Admin Service stores the configurationes. Do not change any files in this directory, since changes will not generally be picked up.
• jackrabbit – The directory in which the Apache Jackrabbit JCR repository is started. Amongst other things this also contains the Jackrabbit configuration file repository.xml. You may modify this file, but don't forget to restart the Embedded Jackrabbit Repository bundle after doing this.
• logs – Contains the log files generated by Sling. By default this contains the error.log and its rotated generations.

Command Line Options

The Java Standalone Application supports a number of command line options, which influence the operation of the launch process.

The Sling Standalone application looks for a definition of the sling.home setting in the following locations in order of precendence:

1. The -c command line option
2. The sling.home system property
3. The SLING_HOME environment variable
4. If none of the above resolves to a non-null value, the default value of sling is assumed

Security Implication

Note that using a control connection for the Sling Standalone Application presents a potential security issue. For this reason the following defaults apply:

• The server side socket for a running Sling Standalone Application is only created if the application is started with the start command line option. If this option is omitted – the default – the server side socket is not created and the Sling Standalone Application instance cannot be remotely controlled.
• The default host name for the socket is localhost/127.0.0.1 meaning that the socket is only accessible from the same system as the Sling Standalone Application is running on. If the socket is accessible from remote systems additional must be taken to prevent malicious attackers to stop the system. The server side control connection implementation implements no security precaution (except from supporting to not create such a connection at all).

Servlet Parameters

The Web Application does not require specific servlet parameters. Those which are specified are used to overwrite any properties with the same name from the sling.properties file. One exception to this rule is the sling.home parameter, which is used to set the value of the sling.home property. If no parameter with this name is defined the Sling home directory is derived from the context path at which the Sling Web Application is registered.

The sling.home folders for Sling Web Applications without the sling.home servlet parameter are all located in the sling folder in the current working directory as reported by the user.dir system property. The name of the actual directory is derived from the Web Application Context Path by replacing all slash characters / by underscore characters _. For the root context a single underscore character _ is used.

Examples:

Starting with Launchpad Base 2.2.2 the fixed prefix sling is configurable with the sling.home.prefix system property. If this property is set the value used as the prefix.

Examples: Assume the sling.home.prefix system property is set to /var/sling

sling.properties

The sling.properties file contains the initial setup of the Sling Application and the OSGi framework. Some of the parameters are required and should not be modified without a very good reason. Some parameters may be freely modified to your needs. Please see the inlined comment in the sling.properties file installed when Sling is first started.

One thing to note is, that the sling.properties file is a simple Java Properties file with support for property references. That is, the value of properties may refer other property values by means of the well known ${name} notation. Such property references may even be cascaded as in

java.packages=${jre-${java.specification.version}}

Components

The Sling Launchapd consists of Launchbad Base project and three additional projects which ultimately create a Standalone Java Application and a Web Appliction with standard parts of Sling.

Launchpad Base

The Launchpad Base projects creates the following artifacts, which are required in actual setups to get a Sling application:

• Launcher JAR – The primary artifact of the Base project contains the actual support to launch the Apache Felix OSGi Framework and install bundles, which are packaged with the application. It also contains the Apache Felix Framework together with the OSGi R4.1 Core and Compendium libraries as well as the Equinox HttpService bridge and the Servlet API.

• App JAR – The secondary artifact with classifier app is a minimal Standalone Java Application which may be started by simply typing

$ java -jar org.apache.sling.launchpad.base-app.jar

• Web App Archive – The secondary artifact with classifier webapp is a minimal Web Application, which may simply be deployed into your favirourite servlet container, provided it supports at least Servlet API 2.4.

• Source JAR – The secondary artifact with the classifier sources is simple the source of the Launchpad Base project.

To build a very basic Sling launcher, the Launchpad Base is actually all you need. But to really glue this together and get a usable system, some more work is required. Lets see how the additionaly projects Launchpad Bundles, Launchpad App, and Launchpad WebApp get to that.

Launchpad Bundles

The second project we want to look at is the Launchpad Bundles project. This is a very simple project in that it only collects together a number of OSGi bundles, which will make up the final application. The bundles are stored in a (big) Java Archive in folders whose path is formed with resources/9 where 9 is a start level which is assigned to the bundles as they are installed. The special start level 0 instructs the installer to not actually assign a specific start level to the bundles contained in that folder.

Currently the following start level assignement is used by the Launchpad Bundles project:

Launchpad App and Launchpad WebApp

The Launchpad App and Launchpad WebApp bundles are actually projects which just glue together artifacts from the Launchpad projects. There is nothing special about them. Here's what is done:

• Take the appropriate secondary artifact from the Launchpad Base project: app for the Standalone Java Application or webapp for the Web Application and unpack
• Take the Launchpad Base primary artifact and place it under the name org.apache.sling.launchpad.base.jar into the resources folder
• Take the Launchpad Bundles artifact and unpack it
• Finally pack all together into a single big JAR or WAR file

That's it. The resulting artifact may be directly used to launch the Standalone Java Application or may directly be deployed into any Servlet API 2.4 (or later) compliant servlet container.