I just spent the past week in a developer training course for Adobe Communiqué 5.4 - a content management system on steroids. I thought I’d jot down some of my thoughts while they’re fresh in my mind.
CQ5 is a Java based CMS that is built around the JCR-283 (Java Content Repository) spec which essentially defines a sophisticated object database that is indexed by Lucene for easy searching and cross-referencing of objects. CQ5’s JCR implementation is called CRX, but there is also an open source reference implementation named Apache Jackrabbit if you have an allergy to commercial software.
It is not entirely correct to call the JCR an object database as it isn’t used to store Java objects directly - but the fact that it defines a tree of nodes and that all content is stored and accessed in a hierarchical fashion makes its use very similar to that of an object database. As such, it is natural to draw comparisons with Zope and its object database, the ZODB.
JCR vs ZODB
Zope, a python-based application framework, is radically different than the traditional relationship database model of web application development. The ability to store Python objects directly in the database and have them indexed solved many development problems, but it also created a few problems that would make maintenance of an ever-changing web application more difficult. Namely:
1. When you make changes to a class, it can break all of the existing objects of that class in the database (you need to run a migration).
2. If you try to load an object whose class definition can’t be found the system barfs.
This problem of class versions, managing upgrades of content types etc.. , was the single biggest problem with devleoping on Zope - and while I’m sure that there are best practices to work around this problem, I believe that the JCR solution of of storing content nodes but not actual objects is a much cleaner way of handling content.
The JCR stores a tree of content nodes, each of which have properties and their own child nodes. These structures translate well to different formats like XML (so you can dump entire branches of the repository as XML) and JSON - not so with a pure object database like the ZODB whose structures can be far more complex and include dependencies to classes. Data in the JCR can always be browsed independent of the component libraries which may be loaded into the system. You can browse the repository using WebDAV, the web-based content explorer that is built into CRX (the JCR implementation that is packaged with CQ5), or using CRXDE (the Eclipse-based development environment that is freely available to developers).
You can still define custom node types for your repository but this would merely dictate the name of the node type and perhaps which properties are required.
So, at first glance, this seems like a very stable base upon which to build web applications.
The CQ5 stack looks like this:
- Sling - HTTP server that makes it easy to read and write from the repository using HTTP requests. Very slick (this part is open source).
- CRX - The content repository itself. Handles all permissions, storage, replication, etc… This part is proprietary. It performs the same function as Apache Jackrabbit, but includes a number of enterprise level improvements including a more powerful security model (I am told).
Author & Publish Deployment Instances
The recommended deployment is to have separate author and publish environments each running their own stack, and use the built-in replication feature to propagate authors’ changes to the publish instance whenever a piece of content is activated. This functionality, luckily, has been streamlined to hide most of the complexity. Workflow is built-in to allow you to activate each piece of content individually. Activation automatically triggers replication to the publish instance(s). This model seems to be very well suited to websites with few authors and many public viewers. It is scalable also, as you can add as many publish instances as you want to share the load.
This standard flow control (replicating changes from the author instance to the publish instances) leads me to wonder about cases where you do want the public to be able to interact with your site (e.g. through comments). We didn’t get into this scenario very much in the training, but, as I understand it, any content posted to the publish instance will go into an “outbox” for that instance that will be replicated to the author instances and await approval. They will then be re-replicated back to the publish instances once approved.
The security model is quite different than that of most systems. Rather than having security attached to content types (because there are no content types) like with a relational database, or defining a large set of permissions corresponding to each possible action in the system as Zope does, security is 100% attached to the nodes themselves. Each node in the JCR includes an ACL (access control list) which maps only a small set of permissions to each user. There are only a few possible permissions that can be assigned or denied on each node. Basically it boils down to permission to read, write, delete, create new, set permissions, and get permissions on a node level. If there are no permissions assigned to a user on a particular node, then it will use permissions from the node’s parent.
One implication of this security model is that you must pay attention to the content hierarchy when developing applications. You cannot treat this like a relational database!
This is important. I suspect that many developers coming from a relational database background will be tempted to try merge the best of both worlds and try to create pseudo-content types in the system. After-all, all properties in the JCR are indexed, so you could easily just add a property called ‘contentType’ to your nodes to identify them as a particular content type, then build functionality that allows users to add instances of this content type. You could then create view templates that aggregate these content types to treat them as a table. You could do this, *but* you must be aware that you don’t have the same level of control that you have in a relational database system over what a user can do with your content types.
If you are querying the repository solely based on a property on a node - and not based on the path, then you may be surprised by the results that you obtain. At the very least, the JCR security model, despite appearing to be simple, is actually far more difficult to implement than its relational cousin - when trying to imitate the functionality of a relational database. You cannot control what properties are added to every node in the repository so querying based on property values may produce undesirable results. Instead you have to fully embrace the hierarchical model of data step very carefully when you try to import concepts from other paradigms as they could cause you to inadvertently introduce holes.
Custom Content Types (Sort of)
While CQ doesn’t have custom content types, it does allow you to map content nodes to a set of rendering scripts which produces something very much likc a content type. By setting the “sling:resourceType” property on a node to the path to a “component” that you develop, you can dictate where CQ looks for scripts that are used to render the node when requests are made. Components can be either “page” components, which represent an entire page, or regular components, which are included inside a page.
You can register page components to show up in the list of types of pages that can be added by authors when they add a new page to the system. Similarly you can register your regular components to show up in the “sidekick” (i.e. component palette) for authors when they are editing a page, so that it can be dragged onto a page. You can define which types of components are allowed to be parents or children of other components, and you can define which parts of site are allowed to have a particular component types added.
The Component Hierarchy
You can also define a “resourceSuperType” for components to allow them to inherit from other components in the system. This is handy for code reuse as there are hundreds or thousands of existing components that can be overridden or extended. We ran through several exercises creating and extending components. I’m satisfied that this process is not difficult and quite powerful.
A component without a dialog is really a lame duck. Users (especially authors) need to be able to interact with your components. E.g. if you create a photo album component, you need to allow your user to add photos to it. Adding dialogs is not difficult but I suspect that the development process is slated for improvements and more automation for future releases. The dialog forms are created entirely by creating appropriately named subtrees under your component’s node. E.g. you would create a child node of a particular type named “dialog”, which contains a child node named “items”, which contains a subnode named “tabs”, etc… 6 or 7 layers deep.
Each tab, each widget, each panel, is represented by a node in the repository. This is clever but somewhat tedious. It is like building a UI using only the UI hierarchy tree in the left panel of the IDE without the visual editor. I suspect that future versions will probably include a proper WYSIWYG UI editor for developing these dialogs but for now this manual system will have to do.
Despite the tediousness of the process, in the scheme of things it is still quite efficient. In only a few minutes you can produce a multi-tab, multi-field UI with rich widgets that allows your users to add and edit a myriad of content types on your site.