Over the years we have heard a lot about the benefits of REST for Web-based developments, particularly in the context of Web Services and more recently in terms of its relevancy to SOA. Therefore with the increase in use of Cloud, even if only at the early adopter stages, it was no surprise to see REST being adopted by various implementations.
Back in 2009, William Vambenepe considered the role of REST in Cloud and concluded that at that point in time Sun and Rackspace had APIs that were more RESTful than others. Well not quite two years later, with more development experience, more users and more choice, William looks at probably the most successful Cloud provider to date, Amazon, and asks the question (we paraphrase) "If Amazon doesn't use REST is it really necessary for successful Cloud?" As he says:
Every time a new Cloud API is announced, its “RESTfulness” is heralded as if it was a MUST HAVE feature. And yet, the most successful of all Cloud APIs, the AWS API set, is not RESTful.
Now you may disagree with William's assessment that we are far enough into using Cloud to assume this is not a coincidence, but it is an interesting situation to consider. Furthermore, William isn't suggesting that REST is not important, only that at least as far as Cloud management is concerned it simply is not important and does not offer any appreciable benefits over, say, RPC.
AWS mostly uses RPC over HTTP. You send HTTP GET requests, with instructions like ?Action=CreateKeyPair added in the URL. Or DeleteKeyPair. Same for any other resource (volume, snapshot, security group…). Amazon doesn’t pretend it’s RESTful, they just call it “Query API” (except for the DevPay API, where they call it “REST-Query” for unclear reasons).
As he points out, the lack of RESTful APIs has not stopped many people using it, nor has the scalability of deployed systems been limited or affected adversely. Neither has it impacted security or restricted the types of applications and languages that can make use of it.
Here’s a rule of thumb. If most invocations of your API come via libraries for object-oriented languages that more or less map each HTTP request to a method call, it probably doesn’t matter very much how RESTful your API is.
William points out that although an earlier article from the engineers at Rackspace comparing and contrasting the RESTfulness of their API with AWS is accurate, it doesn't seem to make a difference where it counts: with developers and users. In conclusion, he suggests that ultimately the RESTfulness of Cloud (at least Cloud management) does not matter as much as the simplicity.
The AWS API being an example of the latter without the former. As I wrote in my review of the Sun Cloud API, “it’s not REST that matters, it’s the rest”. One and a half years later, I think the case is closed.
His article has attracted many comments, most of whom disagree. For instance, one commenter states:
What you say isn’t false to my mind, although the success of EC2 has very little to do with their API, as John (comment #3) points out too. More generally though I’m not sure I like the sound of all of this. Would the Web be here today if Tim Berners-Lee had designed an interface – specific to physicists – allowing them to share results once they had installed some specific “collaboration software for physicists” on their machines ?
Another adds:
Your questions are great but they totally miss the mark: these details (like RESTful APIs) primarily affect library developers, and good libraries can abstract away any kind of API into something more resource-oriented. That requires someone able to map concepts effectively and consistently. Once good libraries are available, there’s not a great barrier for adoption and excessive misuse and waste are completely coincidental. It always comes down to consistency: as long as you’re able to provide a consistent interface, even if it’s not consistent with previously established concepts and mappings, an API will likely succeed given the product is worth the (extra) effort.
With more a more Cloud implementations being developed by vendors and open source efforts, their RESTfulness is usually mentioned as an important feature. But William's question remains: if the most successful Cloud vendor to date does not use REST, does it really matter?