1. The Active Object Pattern
E81 CSE 532S: Advanced Multi-Paradigm Software Development
The Active Object Pattern
Chris Gill
Department of Computer Science and Engineering
Washington University, St. Louis
Title slide

2. Active Object Pattern Problem Solution: “activate” the object
Concurrent access risks races or deadlock, blocking, etc.
Solution: “activate” the object
I.e., launch thread “inside” it
Decouples object method execution from invocation
Helps prevent deadlock
Helps increase concurrency
Implementation concerns
Ensuring thread safety
Increasing concurrency
Code and concurrency interact
Active Object
queue
requesting
thread
worker
thread

3. Details: Retrieving Results
Completion of work is asynchronous
Blocking the request thread becomes a variant of the Monitor Object pattern
Can combine Active Object and ACT patterns
Encapsulate general completion rendezvous points as “Futures”
Can poll for result, or just block until it’s ready
Or, can post on requesting thread’s queue if it’s an active object as well
Active Object
Active Object
request
queue
requesting
thread
worker
thread
result
queue

4. Details: Managing Queue Access
Need to synchronize access to queue methods
Particularly mutator methods
Want classic supplier-consumer behavior
Supplier sleeps when queue is full
Consumer sleeps when queue is empty
Need to manage carefully
Use condition variables
Increase concurrency opportunity
Maintain thread safety
Use high/low water marks
Avoid “silly window syndrome”
Active Object
queue
requesting
thread
worker
thread

5. Design and Implementation Concerns
Key trade-off for lock-based data structures
Ensuring thread safety (no meaningful race conditions)
Increasing concurrency opportunities (deadlock == none)
Scope of serialization matters
How long is a lock held, and over how much data?
Minimize protected regions, use different locks if possible
I.e., “right mutex is locked … held for … minimum … time”
Data structure and concurrency semantics interact
Queue interface motivates use of condition variables
Note that notify_all() may be needed for exception safety!
Move expensive copying/allocation outside locked regions
Associating a lock per item also reduces lock granularity
Decoupling items is then essential (hash vs. tree vs. array example)
Using a readers/writer lock also may be helpful in some cases