The Anatomy of a Service (Part II)

With my last post, we began the journey into the anatomy of a service by restating the definition of a service in the context of SOA. In this post, we're going to dig a bit deeper into the elements that make up a service. These elements are illustrated below.


Note that the illustration above doesn't imply any kind of layering. There are three types of service elements depicted. The yellow elements are related to the service domain. This is the logic, information model and state that drives the behaviour of the service relative to the responsibilities it holds in the overall architecture.

The green elements relate to the service boundary. The service contract describes the messages managed by the service and the endpoints through which the messages are exchanged.

The blue elements are service resources. Note that the service contains human resources. This is because service logic may in part be executed by people. This is explained in more detail here. This is a very important point. Most discussions around SOA tend to describe services only as IT constructs. A service consumer is unaware and does not care whether the service functionality is provided by people at a keyboard or software running on a server.

In addition to the human resources, a service also will very likely contain software and hardware infrastructure. The hardware infrastructure refers to the physical hardware on which the service operates - e.g. servers, SANs and communication infrastructure.

The software infrastructure refers to software that supports the operation of the service, but doesn't implement the logic defined within the service domain. So for example, this software would include operating systems, application platforms and communication software.

The distinction between software infrastructure and service domain logic is an important one. Software infrastructure can be reused between services, whereas logic related to the service domain should not be. Domain logic should be implemented and deployed only once in one service. Why? Because otherwise we have low cohesion - a single concern addressed by multiple services - and that leads to coupling between services.

So why can we reuse software infrastructure between services? This is because such software is generic. It doesn't pertain to any specific service domain. Take UI rendering logic for instance. The logic required for rendering a window doesn't relate to any specific domain addressed by any service in your business.

Other examples of software infrastructure include rules engines and workflow systems. However the rules and workflows themselves would constitute service domain logic.

An interesting example is a CRM package. A CRM package comes with quite a lot of what would seem to be domain logic. For example, a CRM may be deployed in support of the Sales function of a business without a great deal of customisation. Here, a large number of the native application features will directly contribute to the service domain.

A CRM package does not however need to deal only with customers. Most CRM packages can be highly customised to hold custom entities with complex relationships. Custom logic can be added to the CRM package to implement specific business rules around these entities. In this case the CRM package is being leveraged as an application platform, and as such is not implementing the domain logic of the service.

Okay last but not least, we need to discuss the service information model and state. Pretty much every service will have state. That state will conform to the service information model and the service domain logic executes against that information model. To that end, the service information model, state and domain logic are all indelibly linked.

Note that any state leveraged by the software infrastructure is considered part of the infrastructure itself and not part of the service state in the above illustration.

Just like service domain logic, the service state and information model should not be shared between services as it introduces coupling. Services should share data only by way of message exchange, although this is not always possible when transforming an enterprise full of legacy applications to an SOA - at least initially; but more on that in a future post.

So everything in blue in the above illustration is reusable between services. The same people can participate in many services, and the same software and hardware infrastructure is reusable between services. Everything else should not be directly reused as it introduces coupling and reduces cohesion. Of course, domain logic within one service can be reused by other services through exchanging messages with that service.

Okay well that sums up the basic elements of a service. Stay tuned for my next post in this series!

The Anatomy of a Service (Part I)

It occurred to me that to date I've been heavily focussed on defining SOA, techniques for defining service boundaries, contracts and responsibilities, and the various flavours of SOA that we see in the wild, without giving much attention to what goes on inside the service boundary. So I thought it appropriate to begin a series of blog posts on the anatomy of a service.

So let us begin by restating the definition of a service. A service (in the context of SOA) is an autonomous coarse grained unit of logic that exposes functionality by way of exchanging messages conforming to its service contract with service consumers via its endpoints.

The service contract describes the syntax (not semantics) of messages exchanged via each service endpoint, as well as the means by which messages are carried between each endpoint and service consumers. Each endpoint is located and uniquely identified by its address.

A service provider may also consume other services, and a service consumer may in fact also be a service provider. As such, the terms service provider and service consumer describe roles in a specific interaction. The rules governing communication with a service are described by its policy.

What do we mean when we say a service is autonomous? Well we mean a few things actually. Firstly, services are in control of their own state. Services are not instantiated by their consumers. A service exists as a single autonomous entity.

Secondly, we mean that services may be under different ownership domains. Those parties responsible for the service and its management may very likely be different to those responsible for other services. Services may be versioned and deployed independently of each other.

And thirdly, we mean that services should not be dependent upon the availability of other services in order to function without failure - even if that is in some limited capacity.

I'd also like to point out that there is the view that a service contract should not be devoid of semantic definition. There is the thinking that a service contract should also describe the semantics of the messages exchanged by the service, as well as the state the expected behaviour of the service.

Okay so now we've defined a service, what does a service look like? Well that's the beauty of SOA - because service implementations are encapsulated within their service boundaries, consumers don't (or shouldn't anyway) have any visibility or knowledge of the service provider's implementation.

But that doesn't really help us much as architects that need to design these things. That's the reason for this series of blog posts. We as architects need to understand the different ways to go about designing services - not just their boundaries and responsibilities, but their implementations as well.

Over the next few posts I intend to go through various different flavours of service anatomy in order to bring some clarity to the various options that exist for implementing services. So stay tuned!

SOA, EDA and MPI.NET

I had another good question from Miguel, this time referring to my post on SOA and EDA where he asked whether distributed systems built using MPI.NET conform to the SOA or EDA style of architecture.

MPI.NET is a platform for building a single distributed application where different parts of the application run on different nodes within a cluster of machines. Often, the same program runs on different nodes, but takes on different roles on each node. The role each program instance takes on each node can be determined by a unique rank assigned by the MPI environment to each MPI process.

Messages can then be passed between different processes to coordinate their activities. Often the program detects if it is rank zero and if so takes on the "root" responsibility, handing data and control messages to the other nodes, coordinating their activities and aggregating their results.

The message exchange patterns offered by MPI.NET are point-to-point, all-to-one (gathering data), one-to-all (broadcast) and all-to-all. These message exchange patterns differ from the message exchange pattern inherent to EDA, publish-subscribe.

With publish-subscribe, systems subscribe to specific topics or topic specifications. Messages published onto a given topic are routed to all subscribers that have subscribed to a matching topic specification. Alternatively, subscriptions may be defined based on message content-based rules.

With MPI.NET, there are no subscriptions as such. Messages may be broadcasted to every node, which then decides whether the message is relevant. This is a subtle difference, and perhaps one might argue it represents a highly simplified form of EDA.

With regards to SOA, it is extremely bad practice for services to share the same business rules or data representation as it couples services too tightly. The fundamental principle of MPI.NET is that the same logic can/will run on multiple nodes. The purpose is quite different from SOA.

With MPI.NET, you are distributing execution of the same logic across multiple servers for the purpose of increasing performance. With SOA each service is responsible for its own logic, encapsulated behind its service boundary.

Furthermore, the number of nodes allocated in an MPI.NET application is dynamic by design. With SOA, each service has specific rules and responsibilities. The number of services cannot be dynamically increased or decreased.

Certainly within a service the number of nodes allocated to handle messages could be dynamically adjusted based on load. However, these nodes are not directly addressable. They are not visible to the consumers of the service.

Another difference is that MPI.NET nodes do not have the concept of endpoints. One cannot deliver messages to an MPI.NET node across a variety of different bindings via different endpoints.

MPI.NET should be seen as a way to implementing a single service internally across multiple machines for the purpose of improving performance.

I would say therefore that MPI.NET is not a platform for implementing SOA. It could be considered as supporting the EDA style of architecture, but only in a very simplistic sense.