More on Web services and distributed objects

I wanted to respond to some of the detail in Werner’s article, in addition to ranting about how document exchange was state transfer. So here goes …

The first statement that really gave me pause was this;

The goal of web services is to enable machine-to-machine communication at the same scale and using the same style of protocols as human interface centered World Wide Web.

I don’t believe that’s the case, and it’s certainly not been accomplished IMO. I think that if you asked anybody involved since the early days, that they’d say the goal is just the first part; to enable machine-to-machine communication over the Web (or perhaps Internet). “using the same style of protocols” has never been a requirement or goal of this community that I’ve seen.

Consider what can be done with a Web services identifier versus a Web identifier. Both are URIs, but because Web architecture uses late binding, I know what methods I can invoke when I see a URI (the URI scheme, specifically) and what they mean (because there’s a path back to RFC 2616 from the URI scheme). With an identifier for a Web service, I don’t have sufficient information to know what the interface is and what it means, because Web services are early/statically bound (creating centralization dependencies, ala UDDI). I don’t consider changing the architecture from late/dynamic binding to early/static binding to be “using the same style of protocols”.

I suppose I also take issue with the implicit definition of “distributed objects” as part of Misconception #1, when it says;

An important aspect at the core of distributed object technology is the notion of the object life cycle: objects are instantiated by a factory upon request, a number of operations are performed on the object instance, and sometime later the instance will be released or garbage collected.

I’ll present my definition first; distributed objects are identifiable things encapsulating state and behaviour, which present an interface upon which operations can be invoked remotely. Obviously software objects, like all software, do have a lifecycle. But it’s primarily an implementation detail, and not exposed through the object’s interface. Some systems chose to tie the identifier to some particular in-memory instantiation of an object (rather than to an abstraction for which an object could be instantiated to proxy for, in effect), which created a real mess, but I don’t consider that key to the definition.

Misconception #2 also seems prima facie incorrect to me, at least by my definition of “RPC”; an architectural style where the developer of each component is provided the latitude to define the interface for that component. More concretely, I believe the statement “there are no predefined semantics associated with the content of the XML document sent to the service” to be incorrect because, as I mentioned in my last post, if there is a method name in the document, then that is an explicit request for “predefined semantics”.

I agree with the titles of Misconceptions #3 and #4; Web services don’t need HTTP or Web servers. But I disagree with the explanation provided. That Web services are transport agnostic is fine, but that does not at all imply that they should be application protocol agnostic, although most people use them this way. The core of my disagreement is with this statement in the last paragraph of #4;

The REST principles are relevant for the HTTP binding, and for the web server parsing of resource names, but are useless in the context of TCP or message queue bindings where the HTTP verbs do not apply.

That is incorrect. REST is protocol independent, and has applicability when used with other protocols, be they application or transport. REST is an architectural style, and following its constraints guides the style of use of all technologies you might choose to work with. For example, if you were using FTP RESTfully, then you would do things like identify files with URIs (rather than server/directory/file-name tuples), and interact with them via a single “retrieve” semantic (an abstract “GET”), rather than the chatty and stateful user/pass/binary/chdir/retr process (not-coincidentally, this is how browsers deal with FTP). In essence (though drastically oversimplifying), what REST says is “exchange documents in self-descriptive messages, and interpret them as representations of the state of the resource identified by the associated URI”. That philosophy can be applied to pretty much any protocol you might want to name, especially other transfer protocols (as the Web constitutes an “uber architecture”, of sorts, for data transfer).

That’s most of the big issues as I see them. Anyhow, that was an enjoyable and informative read. Thanks!

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