SOA is a Set of Design Patterns for Building and Connecting Components
I’ve taken out anything client-specific.
SOA is one of those snake-oil consultancy terms that seem to mean different things depending on who you talk to. My own views on SOA have been formed from three main sources. Firstly there’s the bible on SOA, Hohpe & Woolf’s Enterprise Integration Patterns. This really is essential reading for anyone attempting to get systems to work together. Next there’s the work of Udi Dahan. He the author of the excellent NServiceBus messaging framework for .NET. I’ve attended his course, and his blog is a fantastic mine of knowledge on all things SOA. Lastly there is the fantastic series of blog posts by Bill Poole on SOA. Just check JBOWS is Bad for a taster.
So what is SOA? Software has a complexity problem. There appears to be a geometric relationship between the complexity of a monolithic system and its stability and maintainability. So a system that is twice as complex as another one will be maybe four times more expensive to maintain and a quarter as stable. There is also the human and organisational side of software. Individual teams tend to build or buy their own solutions. The organisation then needs to find ways for these disparate systems to share information and workflow. Small single purpose systems are always a better choice than large monolithic ones. If we build our systems as components, we can build and maintain them independently. SOA is a set of design patterns that guide us in building and integrating these mini-application pieces.
What is a component?
A component in SOA terms is very different from a component in Object-Oriented terms. A component in OO terms is a logical component that is not independently compiled and deployed. A component or ‘service’ in SOA is a physical component that is an independently built and deployed stand-alone application. It is usually designed as a Windows service or possibly as a web service or an executable. It may or may not have a UI, but it will have some mechanism to communicate with other services.
Each component/service should have the following characteristics:
- Encapsulated. The service should not share its internal state with the outside world. The most common way people break this rule is by having services share a single database. The service should only change state in response to input from its public interface.
- Contract. The service should have a clear contract with the outside world. Typically this will be a web-service API and/or the message types that it publishes and consumes.
- Single Purpose. A component should have one job within the system as a whole. Rendering PDFs for example.
- Context Free. A component should not be dependent on other components, it should only depend on contracts. For example, if my business process component relies on getting a flight manifest from somewhere, it should simply be able to publish a flight manifest request, it shouldn’t expect a specific flight manifest service to be present.
- Independently deployable. I should be able to deploy the component/service independently.
testable. It should be possible to test the service in isolation
without having other services in the system running.
How do components communicate?
that we’ve defined the characteristics of a component in our SOA, the
next challenge is to select the technology and patterns that they use to
communicate with each other. We can list the things that we want from
our communication technology:
- Components should only be able to communicate via a well defined contract - logically-decoupled. They should not be able to dip into each other’s internal state.
- Components should not have to be configured to communicate with specific endpoints – decoupled configuration. It should be possible to deploy a new service without having to reconfigure the services that it talks to.
- The communication technology should support ‘temporal-decoupling’. This means that all the services do not have to be running at the same time for the system to work.
- The communication should be low latency. There should be a minimal delay between a component sending a message and the consumer receiving it.
- The communication should be based on open standards.
Let’s consider how some common communication technologies meet these criteria…
|logically decoupled||decoupled configuration||temporal decoupling||low latency||open standards|
|RPC (remoting COM+)||yes||no||no||yes||no|
|SOAP Web Services||yes||no||no||yes||yes|
|Message Queue (MSMQ)||yes||no||yes||yes||no|
|Pub/Sub Messaging (AMQP)||yes||yes||yes||yes||yes|
From the table we can see that there’s only one technology that ticks all our boxes, and that is pub/sub messaging based on an open standard like AMQP. AMQP is a bit like HTTP for messaging, an open wire-level protocol for brokers and their clients. The leading AMQP implementation is RabbitMQ, and this is technology we’ve chosen as our core messaging platform.
There is a caveat however. For communicating with external clients and their systems, building endpoints with open-standards outweighs all other considerations. Now AMQP is indeed an open standard, but it’s a relatively new one that’s only supported by a handful of products. To be confident that we can interoperate with a wide variety of 3rd party systems over the internet we need a ubiquitous technology, so for all external communication we will use HTTP based web-services.
So, AMQP internally, HTTP externally.