- Notification. Nancy is an aircraft mechanic. When she gets to work in the morning, she opens her feeds aggregator to get new and updated content from all of the following sources: airframer, engine manufacturer, component manufacturers, FAA, or airline policies. Essentially, Nancy doesn't want to login into the support sites of all those content providers to find out what is new and updated. She wants content pushed to her instead. This use case is implemented with the Atom syndication format.
- Federated search. While Nancy is repairing the hydraulic tank, she wants to perform a single search against all those content repositories. She wants the results aggregated and returned to her as Atom entries, so that she can subscribe to those items that she is interested in and receive updates via web feeds. This use case is implemented with the OpenSearch specification.
- Airline Originated Changes. Judy is an engineer working on a new engineering order (EO) to be performed on the hydraulic tank. The airline's technical documents are hosted by the aircraft manufacturer. Judy uses an XML editor which is also an AtomPub client to post the EO to the remote content repository (an AtomPub server).
- Distributed Aircraft Manufacturing. Future Composites Inc. is a supplier of composite aircraft structures to X-Aero, a major airframer (systems integrator). Future Composites is also responsible for providing technical content in S1000D to X-Aero on those composite structures. After a failed attempt to connect to X-Aero's repository using their SOAP and WS-* interface, Future Composites and X-Aero mutually agree to go back to the basics and use AtomPub and its simple and generic RESTfull HTTP interface to CRUD (create, retrieve, update, and delete) documents to X-Aero's content repository.
The main argument in favor of this approach is simplicity and scalability. I am glad to see that the software industry is moving in that direction. Having been involved in complex WS-*-based integration projects in the airline industry, I believe this new approach is a breath of fresh air. The RESTful approach is also more amenable to agile software development as opposed to the waterfall approach which is typical when the big up front purchase of a proprietary ESB is involved.
Integration projects are becoming critical to the success of new aircraft projects. Speaking about the repeated postponement of the 787 maiden flight in an internal memo send to Boeing employees on April 21, 2008 (and obtained by the Seattle Times) Boeing CEO Jim McNerney wrote:
I expect we’ll modify our approach somewhat on future programs—possibly drawing the lines in different places with regard to what we ask our partners to do, but also sharpening our tools for overseeing overall supply chain activities.
Why AtomPub specifically? Because too many people have been putting the "REST" label on their unRESTful chef-d'oeuvre (HTTP APIs) lately. AtomPub is a good embodiment of the principles of the REST architectural style and a good place to start.
So, what are the key principles of RESTful design:
- Everything is a URI addressable resource
- Representations (media types such as XHTML, JSON, and Atom) describe resources and use links to describe the relationships between those resources.
- These links drive changes in application state (hence Representational State Transfer or REST).
- The only type that is significant for clients is the representation media type, not any other resource type
- URI templates (a la OpenSearch) as opposed to fixed or hard coded resource names
- Generic HTTP methods (no RPC-style overloaded POST)
- Stalessness (the server keeps no state information)
- Cacheability
Adherence to these principles is what drives massive scalability. Security in a RESTful application can be achieved with any of the following existing solutions:
- XML Signature and Encryption
- OpenID
- HTTP Authentication
- SSL
How does the aviation industry gets started with this new approach? This will require leadership from aviation IT specialists, particularly from original equipment manufacturers (OEMs). I don't think that another Air Transport Association (ATA) standard committee is needed. Such standard committees are plagued by vendor politics. By the time they finish their work, someone may have invented a better solution than AtomPub.
In the Java space, the recently approved Java API for RESTful Web Services (JAX-RS) specification greatly simplifies REST development with simple annotated POJOs. Jersey is the open source reference implementation of JAX-RS. Apache Abdera is an AtomPub implementation with Spring Framework integration. The latest release of Abdera features a collection of pre-bundled Atom Publishing Protocol adapters for JDBC, JCR, and filesystems.
The following is an excellent technical article from InfoQ that explains how REST and AtomPub facilitate integration: "How to Get a Cup of Coffee".
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