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Thinking in Diagrams

System context map

Systems & feedbackField guide · Systems & feedback · 1 of 6

What is inside the system you're responsible for?

In place of variables and effects, a system context map shows a named boundary, the parties outside it in ovals, and named exchanges crossing it.

Marks

Section titled “Marks”
  • System context map: the house Boundary the house contains no external entities. External entities outside every boundary: landlord. Exchanges: rent, from the house to landlord (owner: the house). the house rent landlord
    boundary
  • System context map External entities: landlord. landlord
    external entity
  • System context map: the house Boundary the house contains no external entities. External entities outside every boundary: landlord. Exchanges: rent, from the house to landlord (owner: the house); repairs, from landlord to the house (owner: the house). the house rent repairs landlord
    exchange

How each mark is drawn and read: chapter 2, What the notation expresses.

A line with an arrowhead

Section titled “A line with an arrowhead”

A line with an arrowhead is an exchange: one named thing passing between the boundary and an external entity, from the party that gives to the party that receives. It carries the thing, not a change: nothing on a system context map rises or falls.

A household with a lodger

Section titled “A household with a lodger”
System context map: the household Boundary the household contains no external entities. External entities outside every boundary: Priya's employer; the landlord; the supermarket; the energy supplier; Dev. Exchanges: wages, from Priya's employer to the household (owner: the household); rent, from the household to the landlord (owner: the household); repairs, from the landlord to the household (owner: the household); groceries, from the supermarket to the household (owner: the household); grocery payments, from the household to the supermarket (owner: the household); electricity, from the energy supplier to the household (owner: the household); bill payments, from the household to the energy supplier (owner: the household); a furnished room, from the household to Dev (owner: the household); room rent, from Dev to the household (owner: the household). the household wages rent repairs groceries grocery payments electricity bill payments a furnished room room rent Priya's employer the landlord the supermarket the energy supplier Dev

Dev rents a room but runs his own meals, so he is an external entity beside the landlord, Priya’s employer, the supermarket and the energy supplier; each exchanges one named thing each way with the household, except the employer, whose wages only arrive (chapter 2’s household).

When to draw one

Section titled “When to draw one”

Draw one when people disagree about whose job something is, when something arrives that nobody expected to deal with, or when a problem passes from one part to another, each saying it belongs elsewhere (chapter 2’s signals).

When not to draw one

Section titled “When not to draw one”

With one or two parties, a sentence does, and when only who the system deals with is in question, a list of parties. Amounts, timing and causes aren’t drawn: an influence map or a causal loop diagram shows causes (chapter 2, what stays informal).

Reasoning and polish

Section titled “Reasoning and polish”
  • Reasoning3 of 5
  • Polish4 of 5boundary sketch → architecture plate
  1. One boundary and the main external entities
  2. Named exchanges, their direction, and the system's responsibilities
  3. Nested boundaries, trust boundaries and owners
Section titled “Related types”
  • There are parties on both: on a system context map, those outside a system’s boundary, with what passes each way across it; on a stakeholder map, those inside an organization and outside it, placed by power and interest.

In this book

Section titled “In this book”

Sources

Section titled “Sources”
  • Tom DeMarco, Structured Analysis and System Specification (Yourdon Press, 1978), on the diagram at the top of structured analysis’s set: the whole system drawn as one shape among the parties outside it.
  • Edward Yourdon, Modern Structured Analysis (Yourdon Press, 1989), on the environmental model, the part of structured analysis that draws a system’s surroundings.
  • Simon Brown, The C4 model: System context diagram, on the C4 model’s zoomed-out view: a software system drawn as one box among the people and other systems it deals with.
  • Donella H. Meadows, Thinking in Systems: A Primer (Chelsea Green, 2008), chapter 4, on boundaries as choices made afresh for each problem.
  • John D. Sterman, Business Dynamics: Systems Thinking and Modeling for a Complex World (Irwin/McGraw-Hill, 2000), chapter 3, on the model boundary chart, which lists what a model treats as inside, as outside, and leaves out.