Following the third part of my "Defining SOA" posts Udi Dahan left the following two questions:

How does this [layers - ARGO] play with two services talking with each other? One pubs
to the other's subs?The other requests to the first's response?
How valuable is the layered abstraction?

Considering Udi and me usually see things eye to eye. I guess that if I managed to get him confused, more clarification is warrant :) I'll do that in two posts, this one which will explain the concept of layers and the next which will explain why it is paramount  to SOA (and answer the two questions)

Usually when I review an architecture one of the first (sometimes the only) architectural artifact I am shown is a "layered diagram" of the architecture e.g. something like the following:

These sort of half layers/half block diagrams with or without the common 3 layers (which also appear in the diagram above) of "UI" "Business" and "Data" give the whole idea of layers a bad name

The key differentiator between layers and just a bunch of blocks is the limitations on the allowed communication paths between the components (layers vs. blocks). In the previous post I quoted an old (2005) definition I had for layers where I said the following

"Typically a layered is allowed to call only the layer below it and be called only by the layer above it (but there are variants e.g. a layer can call to any layer below it; vertical layers that can call multiple layers; etc. -- all is fine as long as the layers communication paths are limited by some rules)."

Alas, I was too quick on the Copy/past and everything in the brackets (bold) is wrong - it should actually say - "but there's another variant where layers are allowed to call the layer above it or below it". The other variants (like  a layer that can all any below it) just muddy the water, makes it hard to distinguish between layers and regular components and thus make layers seem unimportant. consider the following diagram:


So, in the above diagram the relations are the Component D and Component C know each other. Component D is made of two layers (A and B). Note that a more proper representation should also explain the relations allowed between the layers i.e. is it unidirectional or bidirectional (unless there's a common convention in the project)

Why is the distinction between layers and other type of components important? because Layering gives you some benefits which "just components" don't:
1. Layers allow information hiding. Since we don't know the inner working of what's beyond the layer
2. Layers allow separation  - Things beyond the immediate layer  are hidden from each other. This means that things which are beyond the layers are loosely coupled in a way that allows for  flexibility and the addition of capabilities. for instance adding a firewall between your computer and the internet.
3. Layers allows changing the abstraction level - since layers are hierarchical in nature, moving through the layer "stack" you can increase or decrease the level of abstraction you use. This allows expressing complex ideas with simple building blocks. The best known example for this is the TCP/IP stack moving from an abstraction level close to the hardware of the network interface layer to the application level protocols such as HTTP


On the downsides - layers hurt performance by adding latency. Also too many layers introduce added complexity to the overall solution (e.g. it is harder to debug).

It it interesting to note that Interfaces are in fact leaky layer abstractions (vs. for example SOA contracts which are not leaky) - since when you use an interface you still need to instantiate the object which is otherwise hidden behind the "layer" (interface). This is basically the reason we want something like dependency injection (DI) - to help make the abstraction complete and why languages like Ruby where the contract abstraction is complete - you don't need DI (I discussed Ruby and DI in another post)

Another issue which I mentioned here is the difference between logical layers and physical (or potentially  physical) layers. I usually call the first kind layers and the latter tiers. logical layers are local and can assume a lot about their neighboring layers. Tiers or physical layers can be distributed, which carries a lot of implications (something I discussed here recently in relation to MS Volta)