January 8, 2010

Service Type Specifications II

UPDATE: My thinking around this issue has evolved.

Wow, this question was much harder to get straight than I expected:

What constitutes the specification of a service type?

First things first, though! In order to provide a principled answer to that question, another one needs to be answered before:

What is the purpose of a service type specification?

Purpose of Service Type Specifications

1. Service type specifications provide the information client developers need to implement a client for services of the described type. If you hand the service type specification to a developer she should be able to know exactly what to do and what to reasonably expect from any instance of that service type. There should be no further knowledge required (except for following any included references, for example, to media type specifications, of course).

2. Service type specifications provide the information necessary for implementing instances of the specified service type. There should not be any further information required except for implementation specific details behind the service boundary of course.

3. Service type specifications provide the information service owners and maintainers need in order to understand in which way the server can evolve without breaking clients. This is redundant with 2. but mentioning it explicitly emphasizes where exactly the contract is established between client and server owners. Anything that is not specified in the service type specification or referenced material is not part of the contract and constitutes no obligation by either party.

4. Service type specifications enable service discovery by type. Clients that wish to interact with a certain kind of service can use the information provided by the service type specification to detect when they see a service that is of the desired type. The necessary information should be available as a response to the published service URI either by analyzing the set of initial transitions (goals) provided by the service or by looking at the service document's media type (see below).

Having laid out the purposes of a service type specification, we can now make principled decisions about what should be part of a service type specification.

Elements of Service Type Specifications

Here is the short answer (rationale follows below):

Service Type Specifications define the set of hypermedia specifications (media types, link relations, etc.)  used by the service and information about the minimal initial set of available transitions (goals).

The latter can optionally be expressed as a media type, too (a service type specific service document type), which simplifies the definition to 'a set of media types' but leads to the creation of a new media type for a given kind of service.

The essential aspect of the above definition is that the client needs to know what the media types are it needs to understand in order to interact with the service.

So, why is that?

The ideal situation would be to say nothing about the service type at all, just agree on a set of media types that make sense to be understood in general and implement all or any of them in the clients as desired. The problem with this approach is that it does not address purpose 1. above; a client developer would not have any clue what a service is doing or how to interact with it. There would not be any notion of a service type at all; just individual hypermedia semantics (media types, link relations etc.).

But even if the client developer was provided with some means of a service type description (in the form of a dedicated service media type or a set of initial transitions) - see purpose 4. above - there would still be no way for the client developer to have any clue what can be done with the service beyond the initial transitions. Knowing the set of media types provides that clue.

The issue of guiding the service developer (purpose 2.) is addressed because the service type tells the service developer exactly what media types etc. are available to him to solve the given implementation task.

Purpose 3. above is addressed by the fact that it is not possible to remove a hypermedia specification from the specified set without incompatibly changing the semantics of the service type. Service owners are therefore free to evolve the service by adding hypermedia specifications (or extending extensible ones) but may not remove any.

The purpose of discovery (4. above) can be addressed by using a generic service description media type such as Atom Service documents (application/atomsrv+xml) and describing a minimal set of available resources. For example, an ITIL-conforming help desk service type could be specified as providing at least three collections (identified by categories) containing and accepting submissions of Incidents, Problems and Change Requests. Clients (including service registry crawlers) would know they come across an instance of that 'ITIL helpdesk service' when they see an Atom Service with the specified collections.

Alternatively, a new service document media type could be minted (e.g. application/helpdesksrv+xml) to identify the service type. The format of that type would be defined to provide links to the necessary resources. This leads to simpler discovery/registry mechanism but also might cause explosion of media types.

Using a combination of both might actually be best, such as application/atomsrv+xml;profile=helpdesk.

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