Pablo Castro has a blog post entitled AtomPub support in the ADO.NET Data Services Framework where he talks about the progress they've made in building a framework for using the Atom Publishing Protocol (RFC 5023) as a protocol for communicating with SQL Server and other relational databases. Pablo explains why they've chosen to build on AtomPub in his post which is excerpted below

Why are we looking at AtomPub?

Astoria data services can work with different payload formats and to some level different user-level details of the protocol on top of HTTP. For example, we support a JSON payload format that should make the life of folks writing AJAX applications a bit easier. While we have a couple of these kind of ad-hoc formats, we wanted to support a pre-established format and protocol as our primary interface.

If you look at the underlying data model for Astoria, it boils down to two constructs: resources (addressable using URLs) and links between those resources. The resources are grouped into containers that are also addressable. The mapping to Atom entries, links and feeds is so straightforward that is hard to ignore. Of course, the devil is in the details and we'll get to that later on.

The interaction model in Astoria is just plain HTTP, using the usual methods for creating, updating, deleting and retrieving resources. Furthermore, we use other HTTP constructs such as "ETags" for concurrency checks,  "location" to know where a POSTed resource lives, and so on. All of these also map naturally to AtomPub.

From our (Microsoft) perspective, you could imagine a world where our own consumer and infrastructure services in Windows Live could speak AtomPub with the same idioms as Astoria services, and thus could both have a standards-based interface and also use the same development tools and runtime components that work with any Astoria-based server. This would mean less clients/development tools for us to create and more opportunity for our partners in the libraries and tools ecosystem out there.

Although I'm not responsible for any public APIs at Microsoft these days, I've found myself drawn into the various internal discussions on RESTful protocols and AtomPub due to the fact that I'm a busy body. :)

Early on in the Atom effort, I felt that the real value wasn't in defining yet another XML syndication format but instead in the editing protocol. Still I underestimated how much mind share and traction AtomPub would eventually end up getting in the industry. I'm glad to see Microsoft making a huge bet on standards based, RESTful protocols especially given our recent history where we foisted Snakes On A Plane on the industry.

However since AtomPub is intended to be an extensible protocol, Astoria has added certain extensions to make the service work for their scenarios while staying within the letter and spirit of the spec. Pablo talks about some of their design decisions when he writes

We are simply mapping whatever we can to regular AtomPub elements. Sometimes that is trivial, sometimes we need to use extensions and sometimes we leave AtomPub alone and build an application-level feature on top. Here is an initial list of aspects we are dealing with in one way or the other. We’ll also post elaborations of each one of these to the appropriate Atom syntax|protocol mailing lists.
...
c) Using AtomPub constructs and extensibility mechanisms to enable Astoria features:

• Inline expansion of links (“GET a given entry and all the entries related through this named link”, how we represent a request and the answer to such a request in Atom?).
• Properties for entries that are media link entries and thus cannot carry any more structured data in the <content> element
• HTTP methods acting on bindings between resources (links) in addition to resources themselves
• Optimistic concurrency over HTTP, use of ETags and in general guaranteeing consistency when required
• Request batching (e.g. how does a client send a set of PUT/POST/DELETE operations to the server in a single go?)

d) Astoria design patterns that are not AtomPub format/protocol concepts or extensions:

• Astoria gives semantics to URLs and has a specific syntax to construct them
• How metadata that describes the structure of a service end points is exposed. This goes from being to find out entry points (e.g. collections in service documents) to having a way of discovering the structure of entries that contain structured data

Pablo will be posting more about the Astoria design decisions on atom-syntax and atom-protocol in the coming weeks. It'll be interesting to see the feedback on the approaches they've taken with regards to following the protocol guidelines and extending it where necessary.

It looks like I'll have to renew my subscription to both mailing lists.

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