Design
Tier is a Natural Boundary
When talking about multi-tiered architectures, we need to remember that the tier boundary is significant. The tier boundary is where distribution happens and if you remember the "fallacies of distributed computing", you know not to take that lightly. A tier is a physical boundary (versus, for example, an Edge in a SOA which is a logical boundary within the service). The implications of that are numerous. For instance, you need to consider:- Trust--who do you let in?
- Security--what do you send out?
- Performance--you need to serialize to pass the boundary, and remote data is expensive to fetch.
- Availability--what happens if you crash?
- Manageability--can anyone see what's your state? Help you recover?
- Temporal coupling--can you afford to make synchronous calls?
- and many similar questions.
[Originally posted Aug. 2006]
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