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From build...@apache.org
Subject svn commit: r829707 - in /websites/staging/ace/trunk/content: ./ user-doc/user-guide.html
Date Wed, 22 Aug 2012 14:18:28 GMT
Author: buildbot
Date: Wed Aug 22 14:18:28 2012
New Revision: 829707

Log:
Staging update by buildbot for ace

Modified:
    websites/staging/ace/trunk/content/   (props changed)
    websites/staging/ace/trunk/content/user-doc/user-guide.html

Propchange: websites/staging/ace/trunk/content/
------------------------------------------------------------------------------
--- cms:source-revision (original)
+++ cms:source-revision Wed Aug 22 14:18:28 2012
@@ -1 +1 @@
-1375979
+1376068

Modified: websites/staging/ace/trunk/content/user-doc/user-guide.html
==============================================================================
--- websites/staging/ace/trunk/content/user-doc/user-guide.html (original)
+++ websites/staging/ace/trunk/content/user-doc/user-guide.html Wed Aug 22 14:18:28 2012
@@ -169,13 +169,12 @@
       <p><a href="/"><i class='icon-home'></i> Home</a>&nbsp;&raquo&nbsp;<a
href="/user-doc/">User-doc</a></p>
       <h1>ACE Users guide</h1>
       <div class="clear"></div>
-      <div id="content"><p>This article describes how to use ACE and should be
a good starting point for new users of Apache ACE. The remainder of this article assumes you've
read and followed the "<a href="/user-doc/getting-started.html">Getting Started</a>"
guide, meaning that you have an ACE server successfully up and running.</p>
-<p>Revision 0.8, last updated: August 22nd, 2012.</p>
+      <div id="content"><p>This article describes how to use ACE and is a good
starting point for new users of Apache ACE. The remainder of this article assumes you've read
and followed the "<a href="/user-doc/getting-started.html">Getting Started</a>"
guide, meaning that you have an ACE server successfully up and running.</p>
 <div class="toc">
 <ul>
 <li><a href="#introduction">Introduction</a></li>
 <li><a href="#workflow">Workflow</a></li>
-<li><a href="#working-with-ace-server">Working with ACE Server</a><ul>
+<li><a href="#working-with-the-ace-server">Working with the ACE Server</a><ul>
 <li><a href="#uploading-artifacts">Uploading artifacts</a></li>
 <li><a href="#creating-a-new-feature-distribution-andor-target">Creating a new
feature, distribution and/or target</a></li>
 <li><a href="#creating-associations">Creating associations</a></li>
@@ -189,64 +188,70 @@
 </div>
 <h2 id="introduction">Introduction</h2>
 <p>Apache ACE is a framework that enables you to provision OSGi software(components)
in a controlled manner. What this means is that you have a central server to which clients,
or "targets" in ACE terminology, connect and fetch their software from. This allows one to
control which target gets which software. </p>
-<p>The software that is deployed to a target, is composed of one or more distributions.
A distribution is roughly similar to a piece of self-contained software. For example, think
of a distribution as C/C++ Development Tooling that you can install on the Eclipse platform.
On its own, distributions consists of one or more features, that provide pieces of functionality
to your software. The difference between a feature and distribution is that the former is
not necessarily fully self-contained: it might need other features in order to work. Each
feature groups one or more artifacts. An artifact is anything from an OSGi bundle, configuration
file or any other kind of artifact that is needed for the software to work. </p>
-<p>The artifacts themselves reside on an <abbr title="OSGi Bundle Repository">OBR</abbr>,
which can be either the default one supplied by ACE, or an external one. Think of an <abbr
title="OSGi Bundle Repository">OBR</abbr> as a kind of Maven repository, storing
read-only versions of artifacts<sup id="fnref:1"><a href="#fn:1" rel="footnote">1</a></sup>.
As the <abbr title="OSGi Bundle Repository">OBR</abbr> is the single source for
all artifacts, and therefore the software that is deployed on a target, ACE is able to calculate
how to upgrade a target from one version to another version. This is possible because all
changes made to (the metadata of) ACE are stored in an internal versioned database. </p>
+<p>The software that is deployed to a target, is composed of one or more distributions.
A distribution is roughly similar to a piece of self-contained software. For example, it could
be a plugin or even a full application. On their own, distributions consist of one or more
features, that provide pieces of functionality to your software. The difference between a
feature and distribution is that the former is not necessarily fully self-contained: it might
need other features in order to work. Each feature groups one or more artifacts. An artifact
is anything from an OSGi bundle to a configuration file or any other kind of artifact that
is needed for the software to work.</p>
+<p>The artifacts themselves reside in an <abbr title="OSGi Bundle Repository">OBR</abbr>,
which can be either the default one supplied by ACE, or an external one. Think of an <abbr
title="OSGi Bundle Repository">OBR</abbr> as a repository, like the Maven repository
or a content repository, storing immutable versions of artifacts<sup id="fnref:1"><a
href="#fn:1" rel="footnote">1</a></sup>. As the <abbr title="OSGi Bundle
Repository">OBR</abbr> is the single source for all artifacts, and therefore the
software that is deployed on a target, ACE is able to calculate how to upgrade a target from
one version to another version. This is possible because all changes made to (the metadata
of) ACE are stored in an internal versioned database. In other words, we always keep a full
history and audit trail.</p>
 <h2 id="workflow">Workflow</h2>
-<p>The typical use case for using ACE is where you want to control and manage which
software runs on what target. So, how does one use ACE in practice? To explain the typical
workflow of ACE, let's take the following example.<br />
-Assume you are working on a large OSGi-based system that provides some kind of service to
your customers (the exact details on what it does isn't relevant for this example). Since
you're trying to make real money out of it, and take development seriously, you want to set
up a <abbr title="Development, Testing, Acceptance and Production">DTAP</abbr>
environment. Your developers are working on their development machines, using the bleeding
edge software. They are smart guys that can set up their own environments, so ACE won't be
of much help here.<br />
-</p>
-<p>There is also have a tester on your team, responsible for testing all the new features
and bug fixes made by your developers. This tester does not want to have to set up its own
environment and fiddle around with configuration files and such. Instead, he simply wants
a working test environment that runs a recent version of your software. This is were ACE can
help you out. All you need to do is create a testing target, on which a recent software version
gets deployed. That "recent version", can, for example, be a nightly build that comes out
of your <abbr title="Continuous Integration">CI</abbr> server that automatically
gets pushed to ACE after the nightly build is finished. Alternatively, the tester also can
deploy an older version to its testing target through the ACE server UI, for example, to do
some regression testing.</p>
-<p>Before a new version of your software is put into production, your customers want
to do some acceptance testing to see whether this new version does not break anything. The
acceptance environment should only be updated when a new version is released and after that
remains as-is. In practice, this is most often not the case, as the acceptance environment
is, when not used by any customer, confiscated by your tester for its daily testing purposes,
simply, because ACE allows him to easily deploy other versions to the acceptance environment.
When a customer asks for an acceptance test, he simply deploys the latest greatest official
release to the acceptance environment, and lets the customer do its testing. After that, he
can repurpose it again.</p>
+<p>The typical use case for using ACE is where you want to control and manage which
software runs on what target. So, how does one use ACE in practice? To explain the typical
workflow of ACE, let's take the following example.</p>
+<p>Assume you are working on a large OSGi-based system that provides some kind of service
to your users (the exact details on what it does isn't relevant for this example).</p>
+<p>Since you're a professional and take development seriously, you want to set up a
<abbr title="Development, Testing, Acceptance and Production">DTAP</abbr> environment.

+Your developers are working on their development machines, using the bleeding edge software.
+Even though they are smart guys that know the software inside out and can set up their own
environments, they still might want to use ACE on their local machines to validate that changes
they make indeed work when deployed. In this cases it makes sense to use the same deployment
method that is used in production lateron as well.</p>
+<p>There is also a tester on your team, responsible for testing all the new features
and bug fixes made by your developers. This tester does not want to have to set up his own
environment and fiddle around with configuration files and such. Instead, he simply wants
a working test environment that runs a recent version of your software. This is were ACE can
help you out. All you need to do is create targets for testing, on which a recent software
version gets deployed. That "recent version", can, for example, be a nightly build that comes
out of your <abbr title="Continuous Integration">CI</abbr> server that automatically
gets pushed to ACE after the nightly build is finished. Alternatively, the tester also can
deploy an older version to his testing target through the ACE server UI, for example, to do
some regression testing. A third scenario might be to duplicate the exact configuration and
version that is currently deployed to a specific user's target.</p>
+<p>Before a new version of your software is put into production, your users want to
do acceptance testing to see whether this new version does not break anything. The acceptance
environment should only be updated when a new version is released and after that remains as-is.
In practice, this is most often not the case, as the acceptance environment is, when not used
by any user, confiscated by your tester for its daily testing purposes, simply, because ACE
allows him to easily deploy other versions to the acceptance environment. When a user asks
for an acceptance test, he simply deploys the latest greatest official release to the acceptance
environment, and lets the user do its testing. After that, he can repurpose it again. Another
scenario obviously is that users setup dedicated targets for the purpose of acceptance testing.</p>
 <p>When all acceptance tests are successful, the new version of your software needs
to be deployed on several production environments, which is done by you, the release manager.
As most production environments only differ in a few details, such as IP addresses and database
credentials, you use the template engine of ACE to make specific configuration files for each
production target. This way, you can easily scale up your production environment by defining
new targets and provide them with the necessary configuration values.</p>
-<h2 id="working-with-ace-server">Working with ACE Server</h2>
+<h2 id="working-with-the-ace-server">Working with the ACE Server</h2>
 <p>The server UI might look a bit daunting at first, but once you become more familiar
with it, you'll see that it is rather easy to work with.<br />
 </p>
 <p><a href="ace_server_ui.png" target="_blank"><img src="ace_server_ui.png"
width="640px" title="Figure 1: The server UI of ACE, showing the control area at the top,
and the resource area below that." /></a><br />
 <strong>Figure 1</strong>: The server UI of ACE after logging in, showing the
control area at the top, and the resource area below that (click on image to see full size).</p>
 <p>After logging in, the main window consists of two main areas:</p>
 <ol>
-<li>The control area at the top of the screen, where you can perform actions like,
retrieving the latest repository changes, revert the changes you've made locally, add new
artifacts, and so on;</li>
-<li>The resource area, consisting of (up to) four columns showing the current artifacts,
features, distributions and targets that are known to ACE.</li>
+<li>The control area at the top of the screen, where you can perform actions like retrieving
the latest repository changes, revert the changes you've made locally, add new artifacts,
and so on;</li>
+<li>The resource area, consisting of (up to) four columns showing the current artifacts,
features, distributions and targets that are known to ACE. When you select an entity here,
the associated entities in other columns will automatically be highlighted, giving you an
instant overview of the links within the system.</li>
 </ol>
 <h3 id="uploading-artifacts">Uploading artifacts</h3>
-<p>To upload one or more artifacts, you click on the "Add artifact…" button. An
"Add artifact" dialogs opened, showing both the artifacts currently in the <abbr title="OSGi
Bundle Repository">OBR</abbr> and a list of uploaded artifacts. There are two possibilities
to upload a file:</p>
+<p>To upload one or more artifacts, you click on the "Add artifact…" button. An
"Add artifact" dialog opens, showing both the artifacts currently in the <abbr title="OSGi
Bundle Repository">OBR</abbr> but not in the artifact list and a list of uploaded
artifacts. There are two possibilities to upload a file:</p>
 <ol>
 <li>Upload the individual artifacts by pressing the "Upload" button and selecting the
artifact from the file chooser dialog, or;</li>
 <li>by using drag-and-drop: select all artifacts in an Explorer or Finder and drag
them onto the "Upload artifact" area. This way, you can upload multiple artifacts in one go.</li>
 </ol>
-<p>Once artifacts are uploaded, they appear in the Artifacts listing. For each artifact,
you can edit its properties by double clicking on it. In addition, you can unlink an artifact
from a feature, which will be discussed later on, and remove an artifact. <strong>Note</strong>:
removing an artifact will only remove it from the server's metadata, <em>not</em>
from the <abbr title="OSGi Bundle Repository">OBR</abbr>. </p>
-<p>If you try to upload an artifact that is not recognized by ACE, an error is displayed
noting that that particular artifact is not uploaded. Adding support for particular artifacts
in ACE is discussed in <a href="/dev-doc/adding-artifact-recognizers.html">this article</a>.</p>
+<p>Once artifacts are uploaded, they appear in the Artifacts column. For each artifact,
you can edit its properties by double clicking on it. In addition, you can unlink an artifact
from a feature, which will be discussed later on, and remove an artifact. <strong>Note</strong>:
removing an artifact will only remove it from the server's metadata, <em>not</em>
from the <abbr title="OSGi Bundle Repository">OBR</abbr>. </p>
+<p>If you try to upload an artifact that is not recognized by ACE, an error is displayed
noting that that particular artifact is not uploaded. Adding support for new types of artifacts
in ACE is discussed in <a href="/dev-doc/adding-artifact-recognizers.html">this article</a>.</p>
 <p><strong>NOTE</strong>: All changes made through the Web UI need to be
stored explicitly by pressing the "Store" button, otherwise they will not be visible to the
ACE server. In case you want to revert all changes, you can use the "Revert" button. This
will ignore all changes and retrieve the latest state from the server. In case there are no
local changes, you can still retrieve the latest state from the server using the "Retrieve"
button.</p>
 <h3 id="creating-a-new-feature-distribution-andor-target">Creating a new feature, distribution
and/or target</h3>
-<p>Adding features and distributions are very similar. You either click the "Add Feature…"
or "Add Distribution…" buttons. In both cases, you are presented with a dialog that allows
you to enter the (mandatory) name of feature (or distribution) and an optional description.
<br />
-There are two ways of adding a target to ACE: either you click the "Add Target…" button
and enter the name of the new target, or let a running target register itself to ACE. The
details on this will be discussed later on.</p>
+<p>Adding features and distributions are very similar. You either click the "Add Feature…"
or "Add Distribution…" buttons. In both cases, you are presented with a dialog that allows
you to enter the (mandatory) name of the feature or distribution and an optional description.</p>
+<p>There are two ways of adding a target to ACE:</p>
+<ol>
+<li>You can pre-register a target by clicking the "Add Target…" button and entering
the name of the new target. This allows you to associate software to this target even before
it has ever been started or seen by the server.</li>
+<li>You can register a target that is already running and has already tried to fetch
software from the ACE server. The details on this will be discussed later on.</li>
+</ol>
 <p>After a feature, distribution or target is created, you can edit its properties
by double clicking it. For features and distributions, this means you can alter their description,
while for targets, there are more possibilities to view and alter:</p>
 <ul>
 <li>On the Management tab, you can change whether or not the target should be automatically
updated to the latest software. If selected, all updates will automatically be approved and
distributed to that target. If not, any update must be explicitly approved prior to being
distributed to the target;</li>
 <li>On the Info tab, you can view the current state of the target, such as the currently
installed version, or the latest available software version;</li>
 <li>On the LogViewer tab, you can view the installation log of the target. This allows
you to review the installation of updates on that target;</li>
-<li>The Tag Editor tab allows you to define tags and their replacement values that
will be replaced in any artifacts that contain those tags. This is useful, for example, to
have a single configuration file that can be used for multiple targets. </li>
+<li>The Tag Editor tab allows you to define tags and their values. Tags can be used
for many things, but one of their more interesting use cases is that they can be used to replace
variables in template artifacts that contain those tags. This is useful, for example, to have
a single configuration file that can be used for multiple targets, because it is largely the
same for all of them.</li>
 </ul>
 <h3 id="creating-associations">Creating associations</h3>
-<p>To link artifacts to features, you simply select the artifact and drag it on top
of the feature to which it should be associated. The same principle also apply to associate
features to distributions and distributions to targets.<br />
-To delete an association once is created, you can click either the left- or the right side
of the association (viz. either the artifact or the feature), and click the "-" on the opposite
side of the association. For example, to delete an association between an feature and distribution,
you can select the feature first, and hit the "-" on the distribution. Alternatively, you
can select the distribution first and hit the "-" on the feature to delete the association.</p>
-<p>Associations can be parameterized, allowing it to be dynamic in what it matches
on left-side and/or right-side of the association. For example, by default an association
between a bundle artifact and a feature will be made to match the <em>latest</em>
version of the bundle. This way, if you upload a new version of a bundle, the feature will
automatically link to that version. While this is certainly handy in many situations, there
are also situations in which you do not always want to link to the latest greatest version
of a bundle. An example might be the bundles that should run on your production environment,
which should only get an update in controlled upgrades, not when you upload a new artifact
to ACE. To disable the "dynamic" associations between, uncheck the "Dynamic Links" option
in the UI <em>before</em> you create the association. This will create an association
that is explicitly bound to the symbolic name and version of a bundle.</p>
+<p>To link artifacts to features, you simply select the artifact and drag it on top
of the feature to which it should be associated. The same principle also applies if you want
to associate features to distributions and distributions to targets.
+To delete an association once is created, you can click either the left- or the right hand
side of the association (viz. either the artifact or the feature), and click the "-" on the
opposite side of the association. For example, to delete an association between an feature
and distribution, you can select the feature first, and hit the "-" on the distribution. Alternatively,
you can select the distribution first and hit the "-" on the feature to delete the association.</p>
+<p>Associations can be parameterized, allowing them to be dynamic in what they match
on the left hand side and/or the right hand side of the association. For example, by default
an association between a bundle artifact and a feature will be made to match the <em>latest</em>
version of the bundle. This way, if you upload a new version of a bundle, the feature will
automatically link to that version. While this is certainly handy in many situations, there
are also situations in which you do not always want to link to the latest greatest version
of a bundle. An example might be the bundles that should run on your production environment,
which should only get an update in controlled upgrades, not when you upload a new artifact
to ACE. To disable the "dynamic" associations between, uncheck the "Dynamic Links" option
in the UI <em>before</em> you create the association. This will create an association
that is explicitly bound to the symbolic name and version of a bundle.</p>
 <h2 id="running-a-target">Running a target</h2>
-<p>As mentioned, a target represents a client on which software can be deployed by
ACE. Actually, a target consists of a small OSGi runtime that runs an management agent that
periodically checks with the ACE server whether or not it is has new software for it. In case
new software is available for a target, it will automatically download it and install it.
</p>
-<p>ACE provides a fully self-contained management agent, that can be run as plain Java
JAR, named <tt>org.apache.ace.launcher.jar</tt>. This management agent wrapper
accepts the following command line arguments:</p>
+<p>As mentioned, a target represents a client on which software can be deployed by
ACE. Actually, a target consists of an OSGi runtime that runs a management agent that periodically
checks with the ACE server whether or not it has new software for it. In case new software
is available for a target, it can automatically download and install it.</p>
+<p>ACE provides a fully self-contained target that includes a management agent and
can be run as plain Java JAR, named <tt>org.apache.ace.launcher.jar</tt>. This
target accepts the following command line arguments:</p>
 <dl>
 <dt><code>agents</code></dt>
-<dd>configures the target to act as multiple management agents: <code>agent-id,identification,discovery[;agent-id,identification,discovery]*</code>;</dd>
+<dd>Configures the target to have multiple management agents: <code>agent-id,identification,discovery[;agent-id,identification,discovery]*</code>.
If you specify this option, the identification and discovery arguments below are ignored.
Configuring multiple management agents is a very specific use case that should be avoided
unless you know exactly what you're doing. It was added so a target can fetch different, non-overlapping
parts of the software from different servers. In general though, it is preferable and more
convenient to channel all software updates through a single server.</dd>
 <dt><code>auth</code></dt>
 <dd>point to the properties file containing the authentication credentials for a certain
subsystem. Can be either a directory, file or URL;</dd>
 <dt><code>discovery</code></dt>
-<dd>sets the ACE server to connect to, should be an URL. Defaults to <code>http://localhost:8080</code>;</dd>
+<dd>Sets the ACE server to connect to, should be an URL. Defaults to <code>http://localhost:8080</code>.</dd>
 <dt><code>id</code> or <code>identification</code></dt>
-<dd>defines the name to identify the target on in the ACE server. Defaults to <code>defaultTargetID</code>;</dd>
+<dd>Defines the name to identify the target on in the ACE server. Defaults to <code>defaultTargetID</code>.</dd>
 <dt><code>bundle</code></dt>
-<dd>adds an additional bundle to be started with this management agent: <code>bundle=my.fully.qualified.BundleActivator</code>;</dd>
+<dd>Adds an additional bundle to be started with this management agent. The bundle
itself has to be on the Java classpath. <code>bundle=my.fully.qualified.BundleActivator</code>;</dd>
 <dt><code>fwOption</code></dt>
-<dd>sets framework options for the OSGi framework to be created. This argument may
be repeated. For example: <code>fwOption=org.osgi.framework.system.packages.extra=sun.misc,com.sun.management</code>.</dd>
+<dd>Sets framework options for the OSGi framework to be created. This argument may
be repeated. For example: <code>fwOption=org.osgi.framework.system.packages.extra=sun.misc,com.sun.management</code>.</dd>
 </dl>
 <p>An example on how to run the launcher is:</p>
 <div class="codehilite"><pre><span class="nv">$ </span>java -jar
org.apache.ace.launcher.jar <span class="nv">id</span><span class="o">=</span>MyTarget
<span class="nv">discovery</span><span class="o">=</span>http://192.168.1.1:8080
@@ -261,8 +266,8 @@ Started management agent.
 
 <p>After the management agent is started, a new target should appear in the ACE server
after you "Retrieve" the latest changes or "Revert" the current changes. If a target is added
this way to the ACE server (instead of adding it through the "Add target…" button), it
initially will be <em>unregistered</em>. This means that no metadata is present
in the ACE server yet and will not be created. To register a target, you can double click
the target to edit its properties. On the "Management" tab, you can select the "Registered?"
(and optionally the "Auto approve?" option as well) and close the dialog by pressing "Ok"<sup
id="fnref:2"><a href="#fn:2" rel="footnote">2</a></sup>. </p>
 <h3 id="using-the-template-engine-for-targets">Using the template engine for targets</h3>
-<p>If you want to provision software to multiple targets, those targets probably need
to have their own specific configuration. For example, the IP address on which it should listen
for web requests, or the credentials to access a database. One could create specific configuration
files for each target, however, this can become quite tedious is you have lots of targets.
Aside that, ACE requires that each artifact has an unique name, so you need to create unique
file names for your configuration files for each change you make. Fortunately, ACE provides
an easier way to solve this problem: an template engine.</p>
-<p>All configuration files<sup id="fnref:3"><a href="#fn:3" rel="footnote">3</a></sup>
can be regarded as templates, in which variables are replaced with values supplied by ACE.
In fact, the values are definable per target, distribution, feature or artifact. To define
variables and their replacement values (or "tags") for, for example, a distribution, open
up its properties dialog by double clicking on it, and selecting the "Tag Editor" tab. Each
line in this editor represents a tag. The key of a tag defines that (part of) the variable
name to be replaced in configuration files, and the value of a tag the actual replacement
value. </p>
+<p>If you want to provision software to multiple targets, those targets probably need
to have their own specific configuration. For example, the IP address on which it should listen
for web requests, or the credentials to access a database. One could create specific configuration
files for each target, however, this can become quite tedious if you have lots of targets.
Besides that, ACE requires that each artifact has a unique name, so you need to create unique
file names for your configuration files for each change you make. Fortunately, ACE provides
an easier way to solve this problem: a template engine.</p>
+<p>All configuration files<sup id="fnref:3"><a href="#fn:3" rel="footnote">3</a></sup>
can be regarded as templates, in which variables are replaced with values supplied by ACE.
In fact, the values are definable per target, distribution, feature or artifact and ACE will
collect all tags of entities that are associated with a specific target. To define variables
and their replacement values (or "tags") for, for example, a distribution, open up its properties
dialog by double clicking on it, and selecting the "Tag Editor" tab. Each line in this editor
represents a tag. The key of a tag defines that (part of) the variable name to be replaced
in configuration files, and the value of a tag the actual replacement value. </p>
 <p>For example, consider the following configuration file:</p>
 <div class="codehilite"><pre><span class="nt">&lt;properties&gt;</span>
   <span class="nt">&lt;key&gt;</span>ipAddress<span class="nt">&lt;/key&gt;</span>
@@ -272,13 +277,14 @@ Started management agent.
 
 
 <p>The <tt>${context.address}</tt> represent the variable that will be
replaced. The "context." part is mandatory, and everything after that is user definable. Suppose
we want to deploy this configuration file to two targets, "Target1", which is supposed to
listen on address 192.168.2.1 and "Target2", which is supposed to listen on address 192.168.2.2.
To make the configuration file specific for both targets, we simply need to define a tag on
"Target1", like: <code>address</code> -&gt; <code>192.168.2.1</code>,
and a similar tag on "Target2", like <code>address</code> -&gt; <code>192.168.2.2</code>.</p>
-<p><strong>NOTE</strong>: In case a configuration file consists of a variable
that cannot be resolved, it will simply not be replaced, but left as-is. This way, you can
still include Velocity-like templates in your distribution without having them garbled by
ACE.</p>
+<p>Under the covers, ACE uses Velocity<sup id="fnref:4"><a href="#fn:4" rel="footnote">4</a></sup>
to parse the template. This means that, apart from variable substitution, you can also use
other Velocity macros and create more complex configurations that might contain conditional
sections, loops and other features Velocity provides.</p>
+<p><strong>NOTE</strong>: In case a configuration file consists of a variable
that cannot be resolved, it will simply not be replaced, but left as-is.</p>
 <p>ACE will scan all configuration files and replace all known variables as soon as
a new deployment is created. This means that for our example, both "Target1" and "Target2"
will get their own copy of the configuration file with their specific content. ACE also automatically
versions these generated files, to aid downgrading software.</p>
 <div class="footnote">
 <hr />
 <ol>
 <li id="fn:1">
-<p>Once an artifact is uploaded to the <abbr title="OSGi Bundle Repository"><abbr
title="OSGi Bundle Repository"><abbr title="OSGi Bundle Repository">OBR</abbr></abbr></abbr>,
it cannot be modified anymore. This is necessary in order to allow both software upgrades
as downgrades possible.&#160;<a href="#fnref:1" rev="footnote" title="Jump back to
footnote 1 in the text">&#8617;</a></p>
+<p>Once an artifact is uploaded to the <abbr title="OSGi Bundle Repository"><abbr
title="OSGi Bundle Repository"><abbr title="OSGi Bundle Repository">OBR</abbr></abbr></abbr>,
it cannot be modified anymore. This is necessary in order to allow both software upgrades
as downgrades and to ensure that everything you do is reproducible. One thing to note is that
this is not compatible with the way that Maven handles snapshot versions. A snapshot can contain
anything. In stead we usually use the version qualifier to append a timestamp in such scenarios.&#160;<a
href="#fnref:1" rev="footnote" title="Jump back to footnote 1 in the text">&#8617;</a></p>
 </li>
 <li id="fn:2">
 <p>Do not forget to store your changes!&#160;<a href="#fnref:2" rev="footnote"
title="Jump back to footnote 2 in the text">&#8617;</a></p>
@@ -286,6 +292,9 @@ Started management agent.
 <li id="fn:3">
 <p>In fact any artifact can be considered as an template, but by default ACE only considers
configuration files. &#160;<a href="#fnref:3" rev="footnote" title="Jump back to footnote
3 in the text">&#8617;</a></p>
 </li>
+<li id="fn:4">
+<p>Apache Velocity is an engine that can generate documents by combining a template
with a context that contains variables. To learn more about it, visit the <a href="http://velocity.apache.org/">Velocity
website</a>.&#160;<a href="#fnref:4" rev="footnote" title="Jump back to footnote
4 in the text">&#8617;</a></p>
+</li>
 </ol>
 </div></div>
       <hr>



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