Alfresco architecture

Many of Alfresco's competitors (particularly in the closed-source space) have sprawling footprints composed of multiple, sometimes competing, technologies that have been acquired and integrated over time. Some have undergone massive infrastructure overhauls over the years, resulting in bizarre vestigial tails of sorts. Luckily, Alfresco doesn't suffer from these traits. On the contrary, Alfresco's architecture shows the following characteristics:

• It is relatively straightforward
• It is built with state-of-the-art frameworks and open source components
• It supports several important content management and related standards

Let's look at each of these characteristics, starting with a high-level look at the Alfresco architecture.

High-level architecture

The following diagram shows Alfresco's high-level architecture. By the time you finish this book, you'll be intimately familiar with just about every box in the diagram:

The important takeaways at this point are as follows:

• There are many ways to get content into or out of the repository, whether that's via the protocols (for example CIFS on the diagram) or the APIs on the left.
• Alfresco runs as a web application within a servlet container. From Alfresco 5.x, this web application doesn't provide anymore a user interface for end users. It includes only an administration console. If you need a user interface, you'll need to deploy the Alfresco Share web application (in the same container or not) and an extension package in the Alfresco web application to provide all web scripts required by Alfresco Share. You can as well implement your own user interface on top of your Alfresco repository.
• Customizations and extensions run as part of the Alfresco repository or Share web application. An extension mechanism separates customizations from the core product to keep the path clear for future upgrades.
• Metadata resides in a relational database. In a typical Alfresco installation, content files are usually stored in a different server than Alfresco itself (as compared to filesystem). Solr indexes needs to be located in the system as the Solr web application that can reside in a dedicated server too.

Add-ons

The add-ons are pieces of functionality not found in the core Alfresco distribution. If you are working with the binary distribution, it means you'll have additional files to download and install on top of the base Alfresco or Share installation.

Add-ons are provided by Alfresco, third-party software vendors, and members of the Alfresco community such as partners and customers. Alfresco makes several add-on modules available such as Records Management, Google Docs, Office Services or Kofax integration. Members of the Alfresco community create and share add-on modules via the Alfresco add-Ons (https://addons.alfresco.com/), a website set up by Alfresco for that purpose. At the time of writing, this website contains 444 different add-ons compatible with Alfresco Enterprise and/or Community.

Open source components

One of the reasons Alfresco has been able to create a viable offering so quickly is because it didn't start from scratch. The Alfresco engineers assembled the product from many finer-grained open source components. Why does this matter? First, instead of reinventing the wheel, they used proven components. This saved them time, but it also resulted in a more robust, more standard-based product. Second, it eases the transition for people new to the platform. If a developer already knows Spring, for example, many of the customization concepts are going to be familiar. Alfresco uses Surf, a Spring framework extension for building or extending MVC applications. And besides, as a developer, wouldn't you rather invest your time and effort in learning standard development frameworks rather than proprietary development kits?

The following table lists some of the major open source components used to build Alfresco:

Open source component	Use in Alfresco
Apache Solr (http://lucene.apache.org/solr/)	Full-text and metadata search
Hibernate (http://www.hibernate.org/)	Database persistence
FreeMarker (http://freemarker.org/)	Web script framework views, custom views in the web client, web client dashlets, email templates
Mozilla Rhino JavaScript Engine (http://www.mozilla.org/rhino/)	Web script framework controllers, server-side JavaScript, actions
OpenSymphony Quartz (http://www.quartz-scheduler.org/)	Scheduling of asynchronous processes
Spring ACEGI (http://projects.spring.io/spring-security/)	Security (authorization), roles, and permissions
Apache Axis (http://ws.apache.org/axis/)	Web services
LibreOffice (http://www.libreoffice.org/)	Conversion of office documents into PDF
Apache FOP (http://xmlgraphics.apache.org/fop/)	Transformation of XSL:FO into PDF
Apache POI (http://poi.apache.org/)	Metadata extraction from Microsoft Office files
Activiti (http://www.activiti.org/)	Advanced workflow
ImageMagick (http://www.imagemagick.org/)	Image file manipulation
GhostScript (http://www.ghostscript.com/)	Image file manipulation

Does this mean you have to be an expert in all open source components used to build Alfresco to successfully implement and customize the product? Not at all! Developers looking to contribute significant product enhancements to Alfresco or those making major, deep customizations to the product may require experience with a particular component, depending on exactly what they are trying to do. Everyone else will be able to customize and extend Alfresco using basic Java and web application development skills.

Major standards and protocols supported

Software vendors love buzz words. As new acronyms climb the hype cycle, vendors scramble to figure out how they can at least appear to support the standard or protocol so that the prospective clients can check that box on the Request for proposal (RFP). Commercial open source vendors are still software vendors and thus are no less guilty of this practice. But because open source software is developed in the open by a community of developers, its compliance to standards tends to be more genuine. It makes more sense for an open source project to implement a standard than to go off in some new direction because it saves time. It promotes interoperability with other open source projects, and stays true to what open source is all about--freedom and choice.

Here, are the significant standards and protocols Alfresco supports:

Standard/protocol	Comment
HTTP	The main protocol used to access Alfresco content repository via for example the Alfresco REST APIs.
CMIS	CMIS is a standard allowing information sharing between different content management systems. Alfresco supports the version 1.0. and 1.1 of the CMIS standard.
FTP	Content can be contributed to the repository via FTP.
WebDAV	WebDAV is an HTTP-based protocol commonly supported by content management vendors. It is one way to make the repository look like a file system.
CIFS	CIFS allows the repository to be mounted as a shared drive by other machines. As opposed to WebDAV, systems (and people) can't tell the difference between an Alfresco repository mounted as a shared drive through CIFS and a traditional file server.
IMAP	IMAP protocol is used by any modern email clients. Directly from your client, you can connect to your Alfresco repository.
SMTP	It is possible to email content into the repository (InboundSMTP). A folder can be dedicated as an email target.
SPP	Enables Alfresco to act as a Microsoft SharePoint Server. Allows Microsoft Office users to access documents within the Alfresco repository.
Alfresco Office Services	Using Alfresco Office Services (AOS), you can access your documents directly via all Microsoft Office software. AOS replaces and improves the Microsoft SharePoint protocol available in the previous versions.
SOAP	The Alfresco Web Services API uses SOAP-based web services.
OpenSearch (http://www.opensearch.org)	Alfresco repositories can be configured as an OpenSearch data source, which allows Alfresco to participate in federated search queries.
XSLT, XSL:FO	Web form data can be transformed using XSL 1.0.
LDAP	Alfresco can authenticate against an LDAP directory or a Microsoft Active Directory server.