E81 CSE 532S: Advanced Multi-Paradigm Software Development

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E81 CSE 532S Advanced Multi-Paradigm Software
Development
The Active Object Pattern

Chris Gill Department of Computer Science and
Engineering Washington University, St.
Louis cdgill_at_cse.wustl.edu
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Active Object Pattern

• Problem
• 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
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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 its
  ready
• Or, can post on requesting threads queue if its
  an active object as well

Active Object
Active Object
request queue
requesting thread
worker thread
result queue
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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
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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