Can a Service have Tiers?


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03 February 2005

One last post on SOA from my coffee-buzzed, Chicago-traffic-addled mind.

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Can a Service have Tiers?   Certainly a Service can have layers. Any good software will have layers. In the case of a Service these layers will likely be:   1.      Interface 2.      Business 3.      Data access 4.      Data management   This only makes sense. You’ll organize your message-parsing and XML handling code into the interface layer, which will invoke the business layer to do actual work. The business layer may invoke the Data access layer to get/save data into the Data management (database) layer.   But layers are logical constructs. They are just a way of organizing code so it is maintainable, readable and reusable. Layers say nothing about how the code is deployed – that is the realm of tiers.   So the question remains, can a Service be divided into tiers?   I’ll argue yes.   You deploy layers onto different tiers in an effort to get a good trade-off between performance, scalability, fault-tolerance and security. More tiers mean worse performance, but may result in better scalability or security.   If I create a service, I may very well need to deploy it such that I can provide high levels of scalability or security. To do this, I may need to deploy some of my service’s layers onto different tiers.   This is no different – absolutely no different – than what we do with web applications. This shouldn’t be a surprise, since a web service is nothing more than a web application that spits out XML instead of HTML. It seems pretty obvious that the rules are the same.   And there are cases where a web application needs to have tiers to scale or to be secure. It follows then that the same is true for web services.   Thus, services can be deployed into multiple tiers.   Yet the SOA purists would argue that any tier boundary should really be a service boundary. And this is where things get nuts. Because a service boundary implies lack of trust, while a layer boundary implies complete trust. Tiers are merely deployments of layers, so tiers imply complete trust too.   (By trust here I am not talking about security – I’m talking about data trust. A service must treat any caller – even another service – as an untrusted entity. It must assume that any inbound data breaks rules. If a service does extend trust then it instantly becomes unmaintainable in the long run and you just gave up the primary benefit of SOA.)   So if a tier is really a service boundary, then we’re saying that we have multiple services, one calling the next. But services pretty much always have those four layers I mentioned earlier, so now each “was-tier-now-is-service” will have those layers.   Obviously this is a lot more code to write, and a lot of overhead, since the lower-level service (that would have been a tier) can’t trust the higher level one and must replicate much of its validation and possibly other business logic.   To me, at this point, it is patently obvious that the idea of entirely discarding tiers in favor of services is absurd. Rather, a far better view is to suggest that services can have tiers – private, trusting communications between layers, even across the network between the web server hosting the service interface and the application server hosting the data access code.   And of course this ties right back into my previous post for today on remoting. Because it is quite realistic to expect that you’ll use DCOM/ES/COM+ or remoting to do the communication between the web server and application server for this private communication.   While DCOM might appear very attractive (and is in many cases), it is often not ideal if there’s a firewall between the web server and application server. While it is technically possible to get DCOM to go through a firewall, I gotta say that this one issue is a major driver for people to move to remoting or web services.   And web services might be very attractive (and is in many cases), it is not ideal if you want to use distributed OO concepts in the implementation of your service.   And there we are back at remoting once again as being a perfectly viable option.   Of course there’s a whole other discussion we could have about whether there’s any value to using any OO design concepts when implementing a service – but that can be a topic for another time.