Even in the thick of perplexing debates, enterprise architects often agree on the meaning of processes and services, the former set from a business perspective, the latter from a system one. Considering the rarity of such a consensus, it could be used to rally the different approaches around a common understanding of some of EA’s objectives.
A Governing Dilemma
Systems have long been of three different species that communicated but didn’t interbred: information ones were calmly processing business records, industrial ones were tensely controlling physical devices, and embedded ones lived their whole life stowed away within devices. Lastly, and contrary to the natural law of evolution, those three species have started to merge into a versatile and powerful new breed keen to colonize the whole of enterprise ecosystem.
When faced with those pervading systems, enterprises usually waver between two policies, containment or integration, the former struggling to weld and confine all systems within technology boundaries, the latter trying to fragment them and share out the pieces between whichever business units ready to take charge.
While each approach may provide acceptable compromises in some contexts, both suffer critical flaws:
- Centralized solutions constrict business opportunities and innovation by putting all concerns under a single unwieldy lid of technical constraints.
- Federated solutions rely on integration mechanisms whose increasing size and complexity put the whole of systems integrity and adaptability on the line.
Service oriented architectures may provide a way out of this dilemma by introducing a functional bridge between enterprise governance and systems architectures.
Separation of Concerns
Since governance is meant to be driven by concerns, one should first consider the respective rationales behind business processes and system functionalities, the former driven by contexts and opportunities, and the latter by functional requirements and platforms implementation.
While business processes usually involve various degrees of collaboration between enterprises, their primary objective is to fulfill each one’s very specific agenda, namely to beat the others and be the first to take advantage of market opportunities. That put systems at the cross of a dual perspective: from a business point of view they are designed to provide a competitive edge, but from an engineering standpoint they aim at standards and open infrastructures. Clearly, there is no reason to assume that those perspectives should coincide, one being driven by changes in competitive environments, the other by continuity and interoperability of systems platforms. That’s where Service Oriented Architectures should help: by introducing a level of indirection between business processes and system functionalities, services naturally allow for the mapping of requirements with architecture capabilities.
Along that reasoning (and the corresponding requirements taxonomy), the design of services would be assessed in terms of optimization under constraints: given enterprise organization and objectives (business requirements), the problem is to maximize the business value of supporting systems (functional requirements) within the limits set by implementation platforms (non functional requirements).
Services & Capabilities
Architectures and processes are orthogonal descriptions respectively for enterprise assets and activities. Looking for the footprint of supporting systems, the first step is to consider how business processes should refer to architecture capabilities :
- From a business perspective, i.e disregarding supporting systems and platforms, processes can be defined in terms of symbolic objects, business logic, and the roles of agents, devices, and systems.
- The functional perspective looks at the role of supporting systems; as such, it is governed by business objectives and subject to technical constraints.
- From a technical perspective, i.e disregarding the symbolic contents of interactions between systems and contexts, operational processes are characterized by the nature of interfaces (human, devices, or other systems), locations (centralized or distributed), and synchronization constraints.
Service oriented architectures typify the functional perspective by factoring out the symbolic contents of system functionalities, introducing services as symbolic hinges between enterprise and system architectures. And when defined in terms of customers, messages, contract, policy, and endpoints, services can be directly mapped to architectures capabilities.
Moreover, with services defined in terms of architecture capabilities, the divide between business and operational requirements can be drawn explicitly:
- Actual (external) entities and their symbolic counterpart: services only deal with symbolic objects (messages).
- Actual entities and their roles: services know nothing about physical agents, only about symbolic customers.
- Business logic and processes execution: contracts deal with the processing of symbolic flows, policies deal with operational concerns.
- External events and system time: service transactions are ACID, i.e from customer standpoint, they appear to be timeless.
Those distinctions are used to factor out the common backbone of enterprise and system architectures, and as such they play a pivotal role in their alignment.
Anchoring Business Requirements to Supporting Systems
Business processes are meant to met enterprise objectives given contexts and resources. But if the alignment of enterprise and system architectures is to be shielded from changes in business opportunities and platforms implementation, system functionalities will have to support a wide range of shifting business goals while securing the continuity and consistency of shared resources and communication mechanisms. In order to conciliate business changes with system continuity, business processes must be anchored to objects and activities whose identity and semantics are set at enterprise level independently of the part played by supporting systems:
- Persistent units (aka business objects): structured information uniquely associated to identified individuals in business context. Life cycle and integrity of symbolic representations must be managed independently of business processes execution.
- Functional and execution units: structured activity triggered by an event identified in business context, and whose execution is bound to a set of business objects. State of symbolic representations must be managed in isolation for the duration of process execution.
The coupling between business units (persistent or transient) identified at business level and their system counterpart can be secured through services defined with regard to business processes (customers), business objects (messages), business logic (contract), and business operations (policy).
It must be noted that while services specifications for customers, messages, contracts, and policy are identified at business level and completed at functional level, that’s not the case for endpoints since services locations are set at architecture level independently of business requirements.
Filling out Functional Requirements
Functional requirements are set in two dimensions, symbolic and operational; the former deals with the contents exchanged between business processes and supporting systems with regard to objects, activities and events, or actors; the latter deals with the actual circumstances of the exchanges: locations, interfaces, execution constraints, etc.
Given that services are by nature shared and symbolic, they can only be defined between systems. As a corollary, when functionalities are slated as services, a clear distinction should be maintained between the symbolic contents exchanged between business processes and supporting systems, and the operational circumstances of actual interactions with actors.
Depending on the preferred approach for requirements capture, symbolic contents can be specified at system boundaries (e.g use cases), or at business level (e.g users’ stories). Regardless, both approaches can be used to flesh out the symbolic descriptions of functional and persistency units.
From a business process standpoint, users (actors in UML parlance) should not be seen as agents but as the roles agents play in enterprise organization, possibly with constraints regarding the quality of service at entry points. That distinction between agents and roles is critical if functional architectures are to dissociate changes in business processes on one hand, platform implementation on the other hand.
Along that understanding actors triggering use cases (aka primary actors) can represent the performance of human agents as well as devices or systems. Yet, as far as symbolic flows are concerned, only human agents and systems are relevant (devices have no symbolic capabilities of their own). On the receiving end of use cases (aka secondary actors), only systems are to be considered for supporting services.
Hence, when requirements are expressed through use cases, and assuming they are to be realized (fully or partially) through services:
- Persistency and functional units identified by business process would be mapped to messages and contracts.
- Business processes would fit service policy.
- Use case containers (aka systems) would be registered as service customers.
Alternatively, when requirements are set from users’ stories instead of use cases, persistency and functional units have to be elicited through stories, traced back to business processes, and consolidated into features. Those features will be mapped into system functionalities possibly, but not necessarily, implemented as services.
Hence, while the mapping of business objects and logic respectively to messages and contracts will be similar with use cases and users’ stories, paths will differ for customers and policies:
- Given that use cases deal explicitly with interactions at system boundaries, they represent a primary source of requirements for services’ customers and policy. Yet, as services are not supposed to be directly affected by interactions at systems boundaries, those elements would have to be consolidated across use cases.
- Users’ stories for their part are told from a business process perspective that may take into account boundaries and actors but are not focused on them. Depending on the standpoint, it should be possible to define customers and policies requirements for services independently of the contingencies of local interactions.
In both cases, it would be necessary to factor out the non symbolic (aka non functional) part of requirements.
Non Functional Requirements: Quality of Service and System Boundaries
Non functional requirements are meant to set apart the constraints on systems’ resources and performances that could be dealt with independently of business contents. While some may target specific business applications, and others encompass a broader range, the aim is to separate business from architecture concerns and allocate the responsibilities (specification, development, and acceptance) accordingly.
Assuming an architecture of services aligned on capabilities, the first step would be to sort operational constraints:
- Customers: constraints on usability, customization, response time, availability, …
- Messages: constraints on scale, confidentiality, compliance with regulations, …
- Contracts: constraints on scale, confidentiality, …
- Policy: availability, reliability, maintenance, …
- Endpoints: costs, maintenance, security, interoperability, …
Since constraints may cut across services and capabilities, non functional requirements are not a given but the result of explicit decisions about:
- Architecture level: should the constraint be dealt with locally (interfaces), at functional level (services), or at technical level (resources).
- Services: when set at functional level, should the constraint be dealt with by business services (e.g domain or activity), or architecture ones (e.g authorization or orchestration).
The alignment of services with architecture capabilities will greatly enhance the traceability and rationality of those decisions.
A Simple Example
This example is based on the Purchase Order case published with the OMG specifications: http://www.omg.org/spec/SoaML/1.0/Beta2/PDF/
- Architecture Capabilities
- Requirements taxonomy
- Relating to functions
- Non functional Requirements
- Service Oriented Architectures
- Separation of Concerns
- Modeling Languages: Differences Matter
- Enterprise Governance & Knowledge
- Model Driven Architecture