SysML Modeling Skill

When to Use This Skill

Use this skill when:

• Sysml Modeling tasks - Working on systems modeling language (sysml) for systems engineering and complex system design
• Planning or design - Need guidance on Sysml Modeling approaches
• Best practices - Want to follow established patterns and standards

Overview

Systems Modeling Language (SysML) for Model-Based Systems Engineering (MBSE) and complex system design.

MANDATORY: Documentation-First Approach

Before creating SysML models:

1. Invoke docs-management skill for systems engineering patterns
2. Verify SysML 2.0 syntax via MCP servers
3. Base all guidance on OMG SysML specification

SysML vs UML

Aspect	UML	SysML
Focus	Software systems	Systems of all types
Requirements	Not included	First-class diagrams
Structure	Classes, Components	Blocks, Parts
Parametrics	Not included	Constraint blocks
Allocation	Not included	Allocation relationships
Domain	Software engineering	Systems engineering

SysML Diagram Types

Behavior Diagrams

Diagram	Purpose	From UML
Activity	Flow of actions and data	Extended
Sequence	Object interactions over time	Same
State Machine	Lifecycle behavior	Same
Use Case	System-actor interactions	Same

Structure Diagrams

Diagram	Purpose	SysML Specific
Block Definition (BDD)	System structure hierarchy	Yes
Internal Block (IBD)	Internal component connections	Yes
Package	Model organization	Extended

Requirements Diagrams

Diagram	Purpose	SysML Specific
Requirements	Requirements and relationships	Yes
Parametric	Constraint equations	Yes

Requirements Diagram

PlantUML Syntax

@startuml
skinparam rectangle {
  BackgroundColor<<requirement>> LightBlue
  BackgroundColor<<testCase>> LightGreen
}

rectangle "<<requirement>>\nREQ-001: System Performance" as REQ001 {
  id = "REQ-001"
  text = "System shall process 1000 requests/second"
  risk = "High"
  verifyMethod = "Test"
}

rectangle "<<requirement>>\nREQ-002: Response Time" as REQ002 {
  id = "REQ-002"
  text = "System shall respond within 100ms (p95)"
  risk = "Medium"
  verifyMethod = "Test"
}

rectangle "<<requirement>>\nREQ-003: Availability" as REQ003 {
  id = "REQ-003"
  text = "System shall achieve 99.9% uptime"
  risk = "High"
  verifyMethod = "Analysis"
}

rectangle "<<testCase>>\nTC-001: Load Test" as TC001 {
  id = "TC-001"
  verifies = "REQ-001, REQ-002"
}

REQ001 <-- REQ002 : <<deriveReqt>>
REQ001 <-- REQ003 : <<deriveReqt>>
REQ002 <.. TC001 : <<verify>>
REQ001 <.. TC001 : <<verify>>

@enduml

Requirements Relationships

<<deriveReqt>>    Derived requirement (child from parent)
<<refine>>        Element refines requirement
<<satisfy>>       Design element satisfies requirement
<<verify>>        Test case verifies requirement
<<trace>>         General traceability
<<copy>>          Requirement copied (reuse)
<<containment>>   Nested requirement

Block Definition Diagram (BDD)

PlantUML Syntax

@startuml
skinparam class {
  BackgroundColor<<block>> LightYellow
  BackgroundColor<<valueType>> LightGreen
}

class "<<block>>\nVehicleSystem" as Vehicle {
  values
  --
  + maxSpeed: Speed
  + weight: Mass
  + range: Distance
  operations
  --
  + start()
  + stop()
  + accelerate(targetSpeed: Speed)
}

class "<<block>>\nPowertrainSubsystem" as Powertrain {
  values
  --
  + power: Power
  + efficiency: Real
  parts
  --
  + engine: Engine[1]
  + transmission: Transmission[1]
}

class "<<block>>\nEngine" as Engine {
  values
  --
  + displacement: Volume
  + cylinders: Integer
  + fuelType: FuelType
  operations
  --
  + ignite()
  + shutoff()
}

class "<<block>>\nTransmission" as Transmission {
  values
  --
  + gearRatios: Real[6]
  + currentGear: Integer
  operations
  --
  + shiftUp()
  + shiftDown()
}

class "<<block>>\nChassisSubsystem" as Chassis {
  parts
  --
  + wheels: Wheel[4]
  + suspension: Suspension[4]
  + brakes: BrakeSystem[1]
}

class "<<valueType>>\nSpeed" as Speed {
  unit = km/h
}

class "<<valueType>>\nMass" as Mass {
  unit = kg
}

class "<<enumeration>>\nFuelType" as FuelType {
  Gasoline
  Diesel
  Electric
  Hybrid
}

Vehicle *-- Powertrain : <<block>>
Vehicle *-- Chassis : <<block>>
Powertrain *-- Engine
Powertrain *-- Transmission
Engine --> FuelType

@enduml

Block Stereotypes

<<block>>           System element (hardware, software, human)
<<constraintBlock>> Parametric constraint
<<valueType>>       Type with unit
<<flowPort>>        Flow of matter/energy/data
<<proxy>>           Proxy for external element
<<full>>            Full internal access

Internal Block Diagram (IBD)

PlantUML Syntax

@startuml
skinparam component {
  BackgroundColor<<part>> LightYellow
}

package "VehicleSystem [IBD]" {
  component "powertrain : PowertrainSubsystem" as powertrain <<part>> {
    portin "fuelIn" as p_fuel
    portout "torqueOut" as p_torque
    portout "heatOut" as p_heat
  }

  component "chassis : ChassisSubsystem" as chassis <<part>> {
    portin "torqueIn" as c_torque
    portout "motionOut" as c_motion
  }

  component "cooling : CoolingSubsystem" as cooling <<part>> {
    portin "heatIn" as cool_heat
    portout "coolantOut" as cool_out
  }

  component "fuelSystem : FuelSubsystem" as fuel <<part>> {
    portout "fuelOut" as f_out
  }

  ' Connections (item flows)
  f_out --> p_fuel : <<itemFlow>>\nfuel: Fuel
  p_torque --> c_torque : <<itemFlow>>\ntorque: Torque
  p_heat --> cool_heat : <<itemFlow>>\nheat: ThermalEnergy
}

@enduml

Parametric Diagram

Constraint Blocks

@startuml
skinparam class {
  BackgroundColor<<constraintBlock>> LightCoral
}

class "<<constraintBlock>>\nNewtonSecondLaw" as Newton {
  constraints
  --
  { F = m * a }
  parameters
  --
  F: Force
  m: Mass
  a: Acceleration
}

class "<<constraintBlock>>\nKineticEnergy" as KE {
  constraints
  --
  { E = 0.5 * m * v^2 }
  parameters
  --
  E: Energy
  m: Mass
  v: Velocity
}

class "<<constraintBlock>>\nRangeEquation" as Range {
  constraints
  --
  { R = (fuelCapacity * efficiency) / consumption }
  parameters
  --
  R: Distance
  fuelCapacity: Volume
  efficiency: Real
  consumption: VolumePerDistance
}

@enduml

Parametric Usage

@startuml
package "VehiclePerformance [Parametric]" {
  object "newton : NewtonSecondLaw" as n {
    F = thrustForce
    m = vehicleMass
    a = acceleration
  }

  object "energy : KineticEnergy" as e {
    E = kineticEnergy
    m = vehicleMass
    v = velocity
  }

  object "vehicle : Vehicle" as v {
    mass = 1500 kg
    thrust = 5000 N
  }

  n::m --> v::mass
  n::F --> v::thrust
  e::m --> v::mass
}

@enduml

Activity Diagram (Enhanced)

SysML Extensions

@startuml
title Vehicle Start Sequence [Activity]

start

:Receive Start Command;
note right: <<objectFlow>>\nStartRequest

fork
  :Validate Key Fob;
fork again
  :Check Safety Interlocks;
end fork

if (Valid?) then (yes)
  :Power On ECU;

  fork
    :Initialize Engine;
    :<<allocate>>\nEngine ECU;
  fork again
    :Initialize Transmission;
    :<<allocate>>\nTransmission ECU;
  fork again
    :Initialize Dashboard;
    :<<allocate>>\nBody Control Module;
  end fork

  :Start Engine;
  :<<objectFlow>>\nEngineStatus = Running;

  :Report Ready;
else (no)
  :Report Error;
  :<<objectFlow>>\nErrorCode;
endif

stop

@enduml

Object Flow and Control Flow

Control Flow: Sequence of actions (solid arrow)
Object Flow: Data/material flow (dashed arrow with <<objectFlow>>)
Rate: Flow rate specification { rate = 100/sec }
Probability: Branch probability { probability = 0.8 }
Streaming: Continuous flow { streaming }

Allocation

Allocation Relationships

@startuml
skinparam rectangle {
  BackgroundColor<<requirement>> LightBlue
  BackgroundColor<<block>> LightYellow
  BackgroundColor<<activity>> LightGreen
}

rectangle "<<requirement>>\nREQ-001: Process Orders" as R1

rectangle "<<activity>>\nProcessOrder" as A1

rectangle "<<block>>\nOrderProcessor" as B1

R1 <.. A1 : <<satisfy>>
A1 <.. B1 : <<allocate>>

note bottom of B1
  Function ProcessOrder
  is allocated to block
  OrderProcessor
end note

@enduml

Allocation Matrix

| Function/Behavior | Allocated To (Block) |
|-------------------|----------------------|
| ProcessOrder      | OrderProcessor       |
| ValidatePayment   | PaymentGateway       |
| ShipOrder         | FulfillmentSystem    |
| NotifyCustomer    | NotificationService  |

C# Model Representation

// SysML Block as C# class
public abstract class Block
{
    public string Name { get; init; }
    public IReadOnlyDictionary<string, object> Values { get; init; }
    public IReadOnlyList<Block> Parts { get; init; }
    public IReadOnlyList<Port> Ports { get; init; }
}

public sealed class VehicleSystem : Block
{
    // Value properties
    public Speed MaxSpeed { get; init; }
    public Mass Weight { get; init; }
    public Distance Range { get; init; }

    // Parts
    public PowertrainSubsystem Powertrain { get; init; }
    public ChassisSubsystem Chassis { get; init; }
    public CoolingSubsystem Cooling { get; init; }

    // Operations
    public void Start() { /* ... */ }
    public void Stop() { /* ... */ }
    public void Accelerate(Speed targetSpeed) { /* ... */ }
}

// Value Types with Units
public readonly record struct Speed(double Value, SpeedUnit Unit = SpeedUnit.KmPerHour)
{
    public static Speed FromKmPerHour(double value) => new(value, SpeedUnit.KmPerHour);
    public static Speed FromMilesPerHour(double value) =>
        new(value * 1.60934, SpeedUnit.KmPerHour);
}

public readonly record struct Mass(double Value, MassUnit Unit = MassUnit.Kilogram);
public readonly record struct Distance(double Value, DistanceUnit Unit = DistanceUnit.Kilometer);

// Constraint Block
public sealed class NewtonSecondLaw : IConstraint
{
    public Force CalculateForce(Mass mass, Acceleration acceleration)
        => new(mass.Value * acceleration.Value);

    public Acceleration CalculateAcceleration(Force force, Mass mass)
        => new(force.Value / mass.Value);
}

// Requirement
public sealed record Requirement(
    string Id,
    string Text,
    RiskLevel Risk,
    VerificationMethod VerifyMethod,
    IReadOnlyList<string> DerivedFrom,
    IReadOnlyList<string> SatisfiedBy,
    IReadOnlyList<string> VerifiedBy);

public enum RiskLevel { Low, Medium, High }
public enum VerificationMethod { Analysis, Inspection, Demonstration, Test }

MBSE Workflow

When creating SysML models:

1. Define Requirements: Capture stakeholder needs in requirements diagrams
2. Model Structure: Create BDD for system decomposition
3. Define Interfaces: Use IBD for part connections and flows
4. Specify Behavior: Activity, sequence, and state diagrams
5. Add Constraints: Parametric diagrams for physics/math
6. Allocate Functions: Map behaviors to structural elements
7. Trace & Verify: Link requirements through to verification

Best Practices

Model Organization

Model
├── 1_Requirements/
│   ├── StakeholderNeeds.req
│   ├── SystemRequirements.req
│   └── DerivedRequirements.req
├── 2_Structure/
│   ├── SystemContext.bdd
│   ├── SystemArchitecture.bdd
│   └── Subsystems/
│       ├── PowertrainStructure.bdd
│       └── PowertrainInternal.ibd
├── 3_Behavior/
│   ├── UseCases.uc
│   ├── SystemSequences.seq
│   └── StateMachines/
│       └── VehicleStates.stm
├── 4_Parametrics/
│   └── PerformanceConstraints.par
└── 5_Allocation/
    └── FunctionAllocation.alloc

Naming Conventions

Element	Convention	Example
Block	PascalCase noun	VehicleSystem
Part	camelCase noun	powertrain
Port	camelCase + In/Out	fuelIn, torqueOut
Requirement	REQ-###	REQ-001
Constraint	PascalCase equation	NewtonSecondLaw

References

For detailed guidance:

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Last Updated: 2025-12-26