What is an Active Object?

 

 What is an Active Object?

An active object is an object that has its own thread of control. That means it can:

  • Run independently of other objects.
  • Initiate actions, like sending messages or triggering operations.
  • Often manage its own behaviour, including timing, coordination, or communication.

This is a key concept when modeling  asynchronous or parallel systems, especially in real-time or distributed computing.

 Examples of Active Objects

Context Example Description
UML/Modeling A Sensor Monitor object Periodically polls a sensor and sends alerts.
Operating Systems A Process or Thread Runs independently and can initiate I/O or computations.
Embedded Systems A Controller task Continuously monitors and adjusts machinery.
Actor Model An Actor Processes messages independently and asynchronously.

 Passive Object vs Active Object

Feature Passive Object Active Object
Control Does not initiate control Can initiate control
Thread of Execution No Yes (has its own thread/task)
Role Data holder, utility Behaviour initiator, controller
Example Invoice, Customer Billing Service, Sensor Monitor

 Active Object Pattern (from Software Design)

There’s even a design pattern called the Active Object Pattern, which is used to decouple method execution from method invocation.

Components of this pattern:

  1. Proxy: Interface for clients.
  2. Scheduler: Manages which requests run when.
  3. Queue: Stores method requests.
  4. Method Request: Represents a command to execute.
  5. Servant: The actual object doing the work.
  6. Thread(s): Execute queued method requests.

Used in systems where you need asynchronous method calls, such as:

  • Java’s Future and Executor Service
  • C++ with std::async or active object libraries

 In UML Collaboration or Communication Diagrams

In UML:

  • Active objects are often depicted with a thicker border or labeled explicitly.
  • They play a key role in scenarios where events, processes, or control flow must be visualized.

Diagram Snippet Example:

[SensorMonitor] --> [AlarmSystem]
(SensorMonitor initiates this)

Here, SensorMonitor is the active object initiating control.

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