SOA: Is it preferable to implement a service instead of just writing service-ready code, when no external access is needed?

I'm working on the initial architecture f开发者_如何学Goor a solution for which an SOA approach has been recommended by a previous consultant. From reading the Erl book(s) and applying to previous work with services (and good design patterns in general), I can see the benefits of such an approach. However, this particular group does not currently have any traditional needs for implementing web services -- there are no external consumers, and no integration with other applications.

What I'm wondering is, are there any advantages to going with web services strictly to stick to SOA, that we couldn't get from just implementing objects that are "service ready"?

To explain, an example. Let's say you implement the entity "Person" as a service. You have to implement: 1. Business object/logic 2. Translator to service data structure 3. Translator from service data structure 4. WSDL 5. Service data structure (XML/JSON/etc) 6. Assertions

Now, on the other hand, if you don't go with a service, you only have to implement #1, and make sure the other code accesses it through a loose reference (using dependency injection, or a wrapper, etc). Then, if it later becomes apparent that a service is needed, you can just have the reference instead point to #2/#3 logic above in a wrapper object (so all caller objects do not need updating), and implement the same amount of objects without a penalty to the amount of development you have to do -- no extra objects or code have to be created as opposed to doing it all up front.

So, if the amount of work that has to be done is the same whether the service is implemented initially or as-needed, and there is no current need for external access through a service, is there any reason to initially implement it as a service just to stick to SOA?

Generally speaking you'd be better to wait.

You could design and implement a web service which was simply a technical facade that exposes the underlying functionality - the question is would you just do a straight one for one 'reflection' of that underlying functionality? If yes - did you design that underlying thing in such a way that it's fit for external callers? Does the API make sense, does it expose members that should be private, etc.

Another factor to consider is do you really know what the callers of the service want or need? The risk you run with building a service is that (as you're basically only guessing) you might need to re-write it when the first customers / callers come along. This can could result in all sorts of work including test cases, backwards compatibility if it drives change down to the lower levels, and so on.

having said that the advantage of putting something out there is that it might help spark use of the service - get people thinking - a more agile principled approach.

If your application is an isolated Client type application (a UI that connects to a service just to get data out of the Database) implementing a SOA like architecture is usually overkill. Nevertheless there could be security, maintainability, serviceability aspects where using web services is a must. e.g. some clients needs access to the data outside the firewall or you prefer to separate your business logic/data access from the UI and put it on 1 server so that you don’t need to re-deploy the app every time some bus. rules changes.

Entreprise applications require many components interacting with each other and many developers working on it. In this type of scénario using SOA type architecture is the way to go. The main reason to adopt SOA is to reduce the dependencies. Enterprise Applications usually depends on a lot of external components (logic or data) and you don’t want to integrate these components by sharing assemblies.
Imagine that you share a component that implements some specific calculation, would you deploy this component to all the dependent applications? What will happen if you want to change some calculation logic? Would you ask all teams to upgrade their references and recompile and redeploy their app?
I recently posted on my blog a story where the former Architect had also choosed not to use web services and thought that sharing assemblies was fine. The result was chaos. Read more here.

As I mentioned, it depends on your requirements. If it’s a monolithically application and you’re sure you’ll never integrate this app and that you’ll never reuse the bus. Logic/data access a 2 tier application (UI/DB) is good enough.
Nevertheless this is an Architectural decision and as most of the architectural decisions it’s costly to change. Of course you can still factor in a web service model later on but it’s not as easy as you could think. Refactoring an existing app to add a service layer is usually a difficult task to accomplish even when using a good design based on interfaces. Example of things that could go wrong: data structure that are not serializable, circular references in properties, constructor overloading, dependencies on some internal behaviors…