Drawing the Boundary

Online, with a lab for each exercise: thinking-in-diagrams.aryehnielsen.com

System context mapsPart I · Framing · Chapter 2 of 12 · the one chapter on system context maps

• People disagree about whose job something is.
• Something arrives that nobody expected to deal with: a bill, a request, a complaint.
• A new job, project or household starts, and nobody has listed who it deals with.
• A party outside is spoken of as if it were run from inside.
• A problem passes from one part to another, each saying it belongs elsewhere.

Priya and Sam rent a house with their ten-year-old daughter. Sam’s nephew Dev rents their spare room, furnished, and buys and cooks his own food. Sam shops and cooks for the family; Priya pays the rent and the bills.

Each month Priya’s wages arrive from her employer, and the household pays the landlord rent, the supermarket for groceries and the energy supplier for electricity. Dev pays the household rent for his room.

In January the boiler fails. Dev asks Sam when his room will be warm again; Sam phones the landlord, who sends a plumber three days later. The household owes Dev a warm room, and depends on the landlord to provide one.

A boundary is a thin, solid, rounded outline drawn around the system, with a tab carrying the system’s name. It marks what the map explains. What sits outside, the map doesn’t explain: it shows what the landlord gives the household and receives from it, not how the landlord runs a business.

Every boundary has a name, since drawing one means saying which system it bounds: here, the household. A boundary is never dashed: in this book a dashed line means a hypothesis.

An external entity is a party outside the boundary that the system exchanges something with: a person, an organization or, at the scale of a planet, the Sun. It’s drawn as its name in an oval. A party appears because something crosses between it and the system; one with nothing crossing is left off.

The system runs what is inside its boundary and only exchanges with what is outside. Dev lives in the house, but he pays for his room and runs his own meals, so he’s an external entity; the daughter, whose room and meals the household provides, is inside.

An external entity isn’t a variable: nothing on a system context map rises or falls.

An exchange is one named thing crossing the boundary between the system and an external entity: wages, rent, electricity. It’s drawn as a solid line with one arrowhead, joining an oval to the boundary, with its name beside its middle. One exchange carries one thing: rent and repairs is two exchanges. A line without a name shows that something crosses, not what.

An exchange’s direction runs from the party that gives to the party that receives. The household gives the landlord rent and receives repairs: two exchanges, pointing opposite ways.

The exchanges leaving the boundary are what the system answers for to others, such as Dev’s furnished room; those arriving are what it depends on, such as the landlord’s repairs.

A nested boundary is a boundary inside another, naming a part of the system. Nesting is read from position: a boundary drawn wholly inside another is its part. Two boundaries either nest or stay apart; overlapping ones leave it unclear which part a thing belongs to.

A part is named for what it does, not for who does it: meals, not Sam. If Priya takes over the cooking, the part stays.

An exchange’s owner is the boundary it reaches. With parts drawn, each exchange reaches the part that handles it: groceries and grocery payments reach meals, wages reach earnings, and rent, repairs, electricity and bill payments reach housing. When the boiler fails, chasing the landlord’s repairs falls to housing. An exchange drawn to the outer boundary belongs to the system as a whole.

With one or two parties, a sentence does what the map would: the household pays the landlord rent and gets repairs. When a reader needs only who the system deals with, not what crosses, as on a list of emergency phone numbers, a list of parties does it.

The map shows no amounts and no timing: not how much the rent is, nor when it’s due. Nor does it show causes, such as why the electricity bill rose; influence maps and causal loop diagrams, from chapter 3, show those. What passes between parts, such as earnings paying for meals, stays in a sentence.

What’s wrong with this map of a volunteer-run festival?

Its boundary is the organizing committee, with nested boundaries for the chair, the treasurer and the volunteer coordinator: an organization chart, showing who sits on the committee, not what the festival runs. Its five exchanges have no names and each runs one way only, so the map shows that the town council, the sponsor, bands, food stallholders and visitors deal with someone, not what they give or receive. The lab’s check reports:

5 of 5 exchanges have no name yet: from the town council to the chair; from the treasurer to bands; from food stallholders to the treasurer; from visitors to the volunteer coordinator; from the sponsor to the chair. Name the one thing each carries.

The lab checks names, not what a boundary is drawn around: the committee’s map with every exchange named would pass.

Corrected, the boundary is the festival, with parts for what it runs: the stage, the food court and the gate. The same five parties exchange named things both ways, each with the part that handles it: bands give the stage performances and receive fees; visitors give the gate entry fees and receive wristbands. The council’s event permit and the sponsor’s money reach the festival as a whole.

A new treasurer leaves the corrected map unchanged; a second stage would add a part.

Passing the checks means meeting the brief, not being right.

• Tom DeMarco, Structured Analysis and System Specification (Yourdon Press, 1978), on structured analysis’s top-level diagram: the whole system drawn as one process 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 c4model.com/diagrams/system-context, on the C4 model’s top level: 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.
• Rebecca Lindsey, “Climate and Earth’s Energy Budget” science.nasa.gov/earth/earth-observatory/climate-and-earths-energy-budget (NASA Earth Observatory, 2009), on the sunlight Earth receives and the reflected sunlight and heat it sends to space: SC 02’s situation.