COTS Challenges for Embedded Systems

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E81 CSE 532S Advanced Multi-Paradigm Software
Development
Wrapper Façade Pattern
Chris Gill, Ryan Cooper, Gene Tien Department of
Computer Science and Engineering Washington
University, St. Louis cdgill_at_cse.wustl.edu
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Continued Reading Assignment

• CNPv1 Chapter 3
• Optional POSA 2 pp. 47-74

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Wrapper Facade

• Encapsulates functions and data
• Adapts existing procedural APIs
• Offers better interfaces
• Concise, maintainable
• Robust, portable
• Cohesive, object-oriented

ACE_IPC_SAP
ACE_SOCK
ACE_SPIPE
ACE_SOCK_Stream
ACE_SOCK_Dgram
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Wrapper Façade Intent Why?

• To protect the application programmer from
  worrying about platform-specific APIs
•  To simplify use of the APIs (less
  tedious/error-prone)
• To do this while paying only a small
  performance cost
•  Other related patterns have different intents
• Façade
• Decorator
• Bridge
• Adapter

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Wrapper Façade Context When?

• When extensibility can be achieved at a suitably
  low cost (vs. direct use of API)
• When extensibility is more important than
  performance improvements
• Other design patterns can give even more
  significant flexibility improvements, but the
  performance costs often increase as well
• E.g., the Extension Interface pattern

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Wrapper Façade Context Where?
ACE_INLINE ACE_thread_t ACE_OSthr_self
(void) // ACE_OS_TRACE ("ACE_OSthr_self")
if defined (ACE_HAS_THREADS) if defined
(ACE_HAS_PTHREADS) // Note, don't use "" here
since the // following call is often a macro.
ACE_OSCALL_RETURN (pthread_self (), int, -1)
elif defined (ACE_HAS_STHREADS)
ACE_OSCALL_RETURN (thr_self (), int, -1)
elif defined (ACE_HAS_WTHREADS) return
GetCurrentThreadId () elif defined
(ACE_PSOS) // there does not appear to be a way
to get // a task's name other than at creation
return 0 elif defined (VXWORKS) return
taskName (taskIdSelf ()) endif /
ACE_HAS_STHREADS / else return 1 // Might as
well make it the first thread -) endif /
ACE_HAS_THREADS /

• Most commonly used with networking, threading,
  and other low-level APIs

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Wrapper Façade Implementation How? (from POSA2)

• Identify cohesive abstractions and relationships
  among low level APIs
• E.g., Every IPC SAP has a handle and every
  socket is an IPC SAP, but not every IPC SAP is a
  socket.

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Wrapper Façade Implementation, cont.
2. Cluster cohesive groups of functions into
wrapper façade classes and methods 2.1 -
Create cohesive classes 2.2 - Coalesce multiple
individual functions into a single method (e.g.,
bind/listen) 2.3 - Automate creation and
destruction 2.4 - Select level of
indirection 2.5 - Determine where to
encapsulate platform-specifics
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Wrapper Façade Implementation, cont.

• 3. Consider allowing selective access to some
  implementation details, e.g., ACE_IPC_SAPget_han
  dle() is a controlled violation of
  encapsulation.
• 4. Develop error handling (language support for
  features such as exceptions may matter)
• 5. Define related helper classes

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Example Thread Management

•  Thread_Mutex, Thread_Condition, Thread_Manager
•  Automatic creation, initialization and
  destruction
•  Extensible templates parameterized by mutex
  types

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Common Problems and Issues

• Exceptions
• Language support
• Different ways of handling
• Resource management (e.g., memory leaks in C)
• Overhead
• Loss of functionality
• Poor cross-language support
• Performance costs relative to application needs

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For Discussion

•  What problem is addressed by the Wrapper Façade
  pattern?
• Compare and contrast socket programming code
  with and without ACE.
•  How can wrapper façade increase type safety? At
  the same time, how can it increase type
  compatibility?