Dienstag, 3. September 2013

Developing with CoreMedia

A while ago I had the chance to attend a training on web development with CoreMedia. It's a quite enterprisey commercial Content Management System that powers large corporate websites like telekom.com as well as news sites like Bild.de (well, you can't hold CoreMedia responsible for the kind of "content" people put into their system). As I have been working with different Java based Content Management Systems over the years I was really looking forward to learn about the system I heard really good things about. In this post I'll describe the basic structure of the system as well how it feels like to develop with it.

System Architecture

As CoreMedia is built to scale to really large sites the architecture is also built around redundant and distributed components. The part of the system the editors are working on is seperated from the parts that serve the content to the internet audience. A publication process copies the content from the editorial system to the live system.

The heart of CoreMedia is the Content Server. It stores all the content in a database and makes it retrievable. You rarely access it directly but only via other applications that then talk to it in the background via CORBA. Editors used to work with CoreMedia using a Java client (used to be called the Editor, now known as the Site Manager), starting with CoreMedia 7 there is also the web based Studio that is used to create and edit content. A preview application can be used to see how the site looks before being published. Workflows, that are managed using the Workflow Server, can be used to control the processes around editing as well as publication.

The live system consists of several components that are mostly laid out in a redundant way. There is one Master Live Server as well as 0 to n Replication Live Servers that are used for distributing the load as well as fault tolerance. The Content Management Servers are accessed from the Content Application Engine (CAE) that contains all the delivery and additional logic for your website. One or more Solr instances are used to provide the search services for your application.

Document Model

The document model for your application describes the content types that are available in the system. CoreMedia provides a blueprint application that contains a generic document model that can be used as a basis for your application but you are also free to build something completely different. The document model is used throughout the whole system as it describes the way your content is stored. The model is object oriented in nature with documents that consist of attributes. There are 6 attribute types like String (fixed length Strings), XML (variable length Strings) and Blob (binary data) available that form the basis of all your types. An XML configuration file is used to describe your specific document model. This is an example of an article that contains a title, the text and a list of related articles.

<DocType Name="Article">
  <StringProperty Name="title"/>
  <XmlProperty Grammar="coremedia-richtext-1.0" Name="text"/>
  <LinkListProperty LinkType="Article" Name="related"/>
</DocType>

Content Application Engine

Most of the code you will be writing is the delivery code that is part of the Content Application Engine, either for preview or for the live site. This is a standard Java webapp that is assembled from different Maven based modules. CAE code is heavily based on Spring MVC with the CoreMedia specific View Dispatcher that takes care of the rendering of different documents. The document model is made available using the so called Contentbeans that can be generated from the document model. Contentbeans access the content on demand and can contain additional business logic. So those are no POJOs but more active objects similar to Active Record entities in the Rails world.

Our example above would translate to a Contentbean with getters for the title (a java.lang.String), the text (a com.coremedia.xml.Markup) and a getter for a java.util.List that is typed to de.fhopf.Article.

Rendering of the Contentbeans happens in JSPs that are named according to classes or interfaces with a specific logic to determine which JSP should be used. An object Article that resides in the package de.fhopf would then be found in the path de/fhopf/Article.jsp, if you want to add a special rendering mechanism for List this would be in java/util/List.jsp. Different rendering of objects can be done by using a view name. An Article that is rendered as a link would then be in de/fhopf/Artilcle.link.jsp.

This is done using one of the custom Spring components of CoreMedia, the View Dispatcher, a View Resolver that determines the correct view to be invoked for a certain model based on the content element in the Model. The JSP that is used can then contain further includes on other elements of the content, be it documents in the sense of CoreMedia or one of the attributes that are available. Those includes are again routed through the View Dispatcher.

Let's see an example for rendering the list of related articles for an article. Say you call the CAE with a certain content id, that is an Article. The standard mechanism routes this request to the Article.jsp described above. It might contain the following fragment to include the related articles:

<cm:include self="${self.related}"/>

Note that we do not tell which JSP to include. CoreMedia automatically figures out that we are including a List, for example a java.util.ArrayList. As there is no JSP available at java/util/ArrayList.jsp Coremedia will automatically look for any interfaces that are implemented by that class, in this case it will find java/util/List.jsp. This could then contain the following fragment:

<ul>
<c:forEach items="${self}" var="item">
  <li><cm:include self="${item}" view="link"></li>
</c:forEach>
</ul>

As the List in our case contains Article implementations this will then hit the Article.link.jsp that would finally render the link. This is a very flexible approach with a high degree of reusability for the fragments. The List.jsp we are seeing above has no connection to the Article. You can use it for any objects that should be rendered in a List structure, the View Dispatcher of CoreMedia takes care of which JSP to include for a certain type.

To minimize the load on the Content Server you can also add caching via configuration settings. Data Views, that are a layer on top of the Contentbeans, are then held in memory and contain prefilled beans that don't need to access the Content Management Server anymore. This object cache approach is different to the html fragment caching a lot of other systems are doing.

Summary

Though this is only a very short introduction you should have seen that CoreMedia really is a nice system to work with. The distributed nature not only makes it scalable but this also has implications when developing for it: When you are working on the CAE you are only changing code in this component. You can start the more heavyweight Contentserver only once and afterwards work with the lightweight CAE that can be run using the Maven jetty plugin. Restarts don't take a long time so you have short turnaround times. The JSPs are very cleanly structured and don't need to include scriptlets (I heard that this has been different for earlier versions). As most of the application is build around Spring MVC you can use a lot of knowledge that is around already.

About Florian Hopf

I am working as a freelance software developer and consultant in Karlsruhe, Germany. If you liked this post you can follow me on Twitter or subscribe to my feed to get notified of new posts. If you think I could help you and your company and you'd like to work with me please contact me directly.

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