Think of SysML blocks like LEGO bricks—each one is a self-contained piece with specific properties and capabilities, but the real magic happens when you snap them together. Whether you’re modeling a spacecraft or a smart toaster, mastering blocks and their relationships is where system design comes to life.
Blocks: Your System’s DNA
A block isn’t just a box on a diagram—it’s the atomic unit of your system’s structure.
What Lives Inside a Block?
1. Attributes (The “What”)
These are the block’s measurable characteristics.
Example: A Drone block might have:
text
- maxAltitude: float (e.g., 5000.0 meters)
- batteryCapacity: integer (e.g., 6000 mAh)
- isGPSEnabled: boolean
Pro Tip: Use SI units in attribute names (e.g., maxSpeed_kph) to avoid ambiguity.
2. Operations (The “How”)
Actions the block can perform—like methods in object-oriented programming.
Example: For an Autopilot block:
text
- calculateRoute(waypoints: List): NavigationPath
- emergencyLand(reason: String): Status
Watch Out: Don’t overload blocks with dozens of operations. Split into smaller blocks if needed.
Real-World Block Examples
| Block | Attributes | Operations |
| ElectricMotor | rpm, voltageRating, efficiency | start(), stop(), adjustSpeed() |
| PatientMonitor | heartRate, spo2, isAlarming | alertStaff(), exportReport() |
How Blocks Relate: The Six Key Relationships
1. Associations (“Works With”)
- What it shows: Which blocks communicate or interact.
- Visual: Solid line with optional arrows (→ for direction) and multiplicities.
- Example:
| Parent | Child | Multiplicity |
|---|---|---|
| Camera | FlightController | 1 to many |
Translation: One camera feeds data to one or more flight controllers.
2. Generalization (“Is a Type Of”)
- What it shows: Inheritance hierarchies.
- Visual: Hollow triangle arrow (Child → Parent).
- Example:
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[LiDAR] → [Sensor]
Here, LiDAR inherits all attributes/operations from Sensor but adds laser-specific features.
3. Dependencies (“Needs Help From”)
- What it shows: Temporary or indirect relationships.
- Visual: Dashed arrow (Dependent → Supplier).
- Example:
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[BatteryMonitor] ⤏ [Battery]
The monitor depends on the battery but doesn’t contain it.
4. Composition (“Owns”)
- What it shows: Strong ownership—parts die with the whole.
- Visual: Filled diamond on parent (Parent ◆— Part).
- Example:
text
[Car] ◆— [Engine]
No engine? The car isn’t a car anymore.
5. Aggregation (“Has”)
- What it shows: Weaker ownership—parts can exist independently.
- Visual: Hollow diamond on parent (Parent ◇— Part).
- Example:
text
[Fleet] ◇— [Drone]
Drones can be reassigned to other fleets.
6. Interfaces (“Speaks”)
- What it shows: How blocks communicate without knowing internal details.
- Visual: Lollipop notation (⚪— for provided, ⚫— for required).
- Example:
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[SmartLock] ⚪— [LockAPI]
Where LockAPI defines methods like unlock(pin: String).
Putting It All Together: Smart Farm Example
Let’s model an automated irrigation system:
1. Blocks
SoilMoistureSensor (attributes: currentMoisture, calibrationDate)
WaterPump (operations: activate(duration: Seconds))
2. Relationships
text
[IrrigationController] ◆—1 [SoilMoistureSensor]
[IrrigationController] ◆—1 [WaterPump]
[IrrigationController] ⤏ [WeatherAPI]
Translation: The controller owns sensors and pumps but depends on an external weather service.
3. Interfaces
text
[WeatherAPI] ⚫— {
+ getForecast(location): WeatherData
}
Common Pitfalls & Pro Tips
Overcomplicating Early Models
- Start with 3-5 key blocks. You can always refine later.
Naming Matters
- Bad: Component1, PartA
- Good: GPSModule, HydraulicActuator
Traceability
- Link blocks to requirements early:
text
Requirement ID: RQ-204
Text: “System shall monitor soil moisture every 30 minutes.”
Satisfied by: [SoilMoistureSensor]
Why This Matters
Blocks and relationships aren’t just academic exercises—they force you to answer critical design questions:
- “What happens if this component fails?” (Composition vs. Aggregation)
- “Can we reuse this sensor design in another product?” (Generalization)
- “How will subsystems communicate in the field?” (Interfaces)
Master these concepts, and you’re not just drawing boxes—you’re engineering systems that work.