Reflection: it's not just components

The last few entries have focused on components and their DUs (located or not) and the need to recognise the 4th and 5th shortcoming of the Lego analogy for ABBs .   But these are not the only model elements that have "located equivalents" - model elements that are "somewhere"; ABB types such as

• Nodes
  To all intents and purposes, nodes represent the solution's "computing platforms" - but not just its computers; any hardware (probably plus systems software and middleware) that supports "the solution " - printers, routers, terminals, even patch panels and other passive devices.  All these things need to be located (or are locatable) somewhere.
  
  Nodes (and their ABBs) come in two flavours, which means so do Located Nodes:
  
  (1) Application Nodes, composed of usually standard collections of co-located (or locatable!) Located  DUs - application configurations of DUs that work together (in one place), such as those that make up a "Sales Server" Node ABB, which may be deployed into a regional office (which we might call it the Regional Sales Server" Located Node), call centre, or data centre. 
  
  (2) Technical Nodes, composed of standard configurations of systems software and hardware   (and which therefore form part of the system's infrastructure), probably specified by "Operations" as their recommended supported configuration that they expect Systems Designers to used as the basis for their Located Technical Nodes.  For example, Operations may specify an "Application Server" Node ABB, providing the infrastructural specification for a "Regional Application Server" Located Technical Node that provides the necessary services to underpin the "Regional Sales Server" Located application node.
  
  In both cases, the functional specification will be the same (or very similar) across all located nodes (although it may be that not all functionality is exploited in all locations) - differences will most likely be in the non-functional characteristics, such as the transaction volumes on each of the "Regional Sales" Located Nodes being different from that on the "Data Centre Sales" Located Node.

• Actors and Users
  Our system's logical actors and physical users (collectively referred to as external agents) need not be limited to accessing the system from one place, nor in using just one means of access from each place.  Therefore, the functional viewpoint's abstracted view of a "location-less external agent" (which may be based on an Actor ABB or User Implementation of that ABB) must be modelled as one or more located external agents in order to understand and create the more realistic Operational views - sometimes to such a degree that, for a given functional actor or user, one located external agent may transpire to be human (because the system's boundary is a keyboard/Video/Mouse), while another is designed as an external IT system (because the system's boundary is based on HTTP/HTML traffic) - but both based on the same (now ambiguous) specification of the functional external agent.