What I personally call "Natural Requirements" (NR) are requirements not coming from end users, analysts or marketing people. Natural Requirements are often more operational than functional or technical, let me call the latter "static requirements", and I mean the one we usually capture in the early phases of the requirements gathering process. NR instead derive from the natural and time-dependant evolution of a system well after the systems has been built and put into production. NR are naturally imposed by the environment, the context and by the fact that systems are always evolving and requires a lot of maintenance, and there is nothing we can do to eliminate them: instead we need to deal with them from a strategic standpoint, looking at the long run.
A SOA is made of a collection of interacting business services, each one with its own history and evolution. Starting from domain and data models, services defines an atomic and well identified piece of data, algorithmic result or a combination of both. Services creates a public API, they expose internal technical artefacts as integrated business, higher-level information. We (almost) all know what the SOA objectives are, so it is not interesting to repeat them here.
Natural Requirements for Service Virtualization
Services anyway pose a maintenance problem: they evolve, need to be monitored and policies have to be enforced. A public API, once published, can't be changed easily, as it is a public contract and we cannot control how many clients and how they make use of the API. When we change a public API we know that we are going to break contracts with clients, and clients need to be changed to adapt to the new interfaces.
A first Natural Requirement is that services have to be versioned to reach a level of backward compatibility. For this purpose, modern SOA Governance solutions offer not only a Registry but also a Repository - ideally a combination of both - where service interfaces and messages can be versioned to avoid breaking contracts when things change. Each client can point to the proper version of the same service and related messages.
Services implements business logic atomic features, but in their lifecycle they also deal with other orthogonal concerns, which are not functional in their nature: Policies, End point definitions, messages structure manipulation and message formats are all non-business aspects.
So Separation of Concerns is one of the main and most frequent Natural Requirement we could ever encounter in our daily routine as SOA architects. Orthogonal concerns must be consistently keep divided from business logic and other aspects. For example, one of the most common orthogonal concerns we must split from business logic is Security. For the sake of this discussion, we could identify a couple of concrete technical ways to implement services: SOAP messages over HTTP and REST Web Services. The first are usually secured by using some complex WS-Security implementation, the latter by simpler HTTP usual methods, such as basic authentication + SSL. Policy enforcement and core services must be kept separated, business logic developers shouldn't have to know that such a thing like WS-Security even exists.
"Composite applications must have the business logic defined separately from each application's infrastructure. For example, instead of baking security semantics into the code, use an external, general-purpose security framework. This will be your template for enforcing authentication and authorization rules based on declarative policies that specify the required security semantics for the service within the context of a specific application."
[ Anne Thomas Manes - The Elephant Has Left The Building ]
Services Virtualization is usually defined as a virtual view (much like a database view, at least conceptually) of a real serviced managed through a policy. A Policy is a declarative (not-programmatic) set of administrative tasks. For example you can have authentication, authorization and generally speaking access management policies. Or you can define a set of minimal performances level policies in terms of SLAs. Or then you can have billing policies for your services (think at Amazon's web services) based on volumes or number of calls.
Natural Requirements for PLM
SOA implies that business services and Composite Applications (CA) have to be handled like real products, so PLM (Product Lifecycle Management) issues and practices apply. Many vendors now offers (almost) comprehensive governance solutions which embrace policies + registry + artifacts repository and all the related versioning, but that's not enough. Governance solutions are platforms, but PLM is about the product and the process. Services needs to be created as the building blocks for CA, so they are part of the product lifecycle: the final product is the CA, while services are the components, which can be re-assembled and mixed to create other CA.
The main steps are:
Phase 1: Conceive
Imagine, Specify, Plan, Innovate
Phase 2: Design
Describe, Define, Develop, Test, Analyse and Validate
Phase 3: Realize
Manufacture, Make, Build, Procure, Produce, Sell and Deliver
Phase 4: Service
Use, Operate, Maintain, Support, Sustain, Phase-out, Retire, Recycle and Disposal
Why we should talk about PLM and not about ALM (Application lifecycle management)? ALM is just about software, while PLM is about manufacturing, so the first acronym sounds more natural. Because applications are monolithic, they are not composite in the same terms we use when building a product in manufacture. The roads to assemble a Composite Application are somehow different from Application ones, actually they are closer to manufacturing processes. The reason is that we want to create a predictable and repeatable assembly chain for services. ALM is really a subset of PLM: the Use, Operate, Maintain, Support, Sustain, Phase-out, Retire, Recycle and Disposal activities of Phase 4 needs to be operated at the level of each single Service and not at the Application level only. When we talk about SOA, services (the building blocks) are way more crucial than the resulting Composite Applications, like reliable foundations are for a house.
When we think again about the manufacturing metaphor, and specifically about car manufactures, we could assimilate the car as the final product of an assembly of other products. Let's say car wheels and tires has a clearly defined interface and the car maker just buys the product from another supplier. Tires producer knows its product can be assembled by a number of different clients, so it cannot make too many assumptions on the external world: they just have to respect the contract.
In conclusions, to be effective Services construction, assembly and maintenance require us to build our own assembly chain, following best PLM practices. But still best practices in this field are in their infancy.
The description of SOA I enjoy most is still Dan North's A Classic Introduction to SOA, which is additionally a very easy reading.