STPA Step 2: Model the Control Structure

Objective

Create a hierarchical model showing:

• Controllers (humans, software, hardware that make decisions)
• Control Actions (commands sent to controlled processes)
• Feedback (information returned to controllers)
• Controlled Processes (entities being controlled)

Announce at Start

"I'm using the STPA Step 2 skill to model the control structure. We'll identify controllers, control actions, and feedback paths."

Key Concepts

What is a Control Structure?

A control structure shows the hierarchical relationships between:

• Higher-level controllers that set goals/constraints
• Lower-level controllers that implement those constraints
• Controlled processes that execute actions
• Feedback loops that provide information back up

Why Control Structures?

• Traditional dataflow diagrams don't show control hierarchy
• STPA focuses on feedback paths often neglected in design
• Abstracts complex systems to 10-15 manageable boxes
• Narrows search from millions of lines of code to specific decisions

Phase 1: Identify Controllers

Questions to Ask

Q1: Who or what makes control decisions in this system?

• Human operators/users
• Software components (services, modules, agents)
• Hardware controllers (PLCs, microcontrollers)
• External systems that provide input

Q2: What is the hierarchy of control?

• Which controllers give commands to others?
• Who has override authority?
• What is the chain of command?

Controller Examples

Software System:

• API Gateway, Authentication Service, Database Controller, User

Physical System:

• Plant Manager, Control Room Operator, PLC, Valve Actuator

AI System:

• Human Operator, Orchestrator, ML Model, Action Executor

Phase 2: Identify Control Actions

Questions to Ask

Q1: What commands does [Controller] send to [Controlled Process]?

• Start/Stop commands
• Configuration changes
• Data modifications
• Enable/Disable actions

Q2: When are these control actions sent?

• On user request
• On schedule
• In response to feedback
• Based on system state

Control Action Examples

Phase 3: Identify Feedback Paths

Questions to Ask

Q1: What information does [Controlled Process] send back to [Controller]?

• Status information (running, stopped, error)
• Measurement data (temperature, pressure, counts)
• Acknowledgments (success, failure)
• Alerts and warnings

Q2: How does the controller use this feedback?

• To verify actions completed
• To adjust future control actions
• To detect anomalies
• To report to higher-level controllers

Common Missing Feedback

• Confirmation that action was received
• Confirmation that action was executed correctly
• Confirmation that desired effect was achieved
• Time taken to complete action

Graphviz/DOT Template

digraph ControlStructure {
    rankdir=TB;
    node [shape=box];

    // Controllers (top to bottom = higher to lower authority)
    User [label="User"];
    AuthService [label="Auth Service"];
    SessionManager [label="Session Manager"];
    Database [label="Database"];

    // Control Actions (solid arrows pointing down)
    User -> AuthService [label="Login Request"];
    AuthService -> SessionManager [label="Create Session"];
    SessionManager -> Database [label="Store Session"];

    // Feedback (dashed arrows pointing up)
    AuthService -> User [label="Auth Result", style=dashed];
    SessionManager -> AuthService [label="Session ID", style=dashed];
    Database -> SessionManager [label="Write Confirmation", style=dashed];
}

Diagram Conventions

Layout

• rankdir=TB - Top to bottom flow
• Higher authority at top, controlled processes at bottom

Node Style

• node [shape=box] - All nodes are rectangular boxes
• Clear, descriptive labels

Edge Style

• Solid arrows: Control actions (pointing down/to controlled)
• Dashed arrows: Feedback (pointing up/to controller)
• Labels: Describe what is being communicated

Validation Questions

Before proceeding to Step 3:

Q1: Is every controller-controlled relationship shown? Q2: Does every control action have corresponding feedback? Q3: Are there any hidden controllers we haven't identified? Q4: Are there any feedback paths that are missing or delayed?

Output Template

Record in .sgai/PROJECT_MANAGEMENT.md:

### Step 2: Control Structure

#### Controllers Identified
1. [Controller 1] - [Role/Description]
2. [Controller 2] - [Role/Description]

#### Control Structure Diagram

```dot
digraph ControlStructure {
    rankdir=TB;
    node [shape=box];

    // [Insert diagram here]
}

Control Actions Table

Notes

• [Any gaps or concerns about feedback paths]
• [Areas where control is unclear]

## When to Proceed to Step 3

Move to Step 3 when:
- [ ] All controllers are identified and placed in hierarchy
- [ ] All control actions are documented
- [ ] All feedback paths are identified (or gaps noted)
- [ ] Human partner confirms the structure looks accurate

Load: `skills({"name":"stpa/step3-unsafe-control-actions"})`