CS551 Object Oriented Middleware IV Dynamic Requests Chap' 6 of EDO

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CS551 Object Oriented Middleware (IV) Dynamic
Requests (Chap. 6 of EDO)

• Yugi Lee
• STB 555
• [phone]
• yugi_at_cstp.umkc.edu
• www.cstp.umkc.edu/yugi

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Outline

• Motivating Examples
• Dynamic Invocation
• The CORBA Dynamic Invocation Interface
• COM IDispatch Interfaces
• Reflection
• The CORBA Interface Repository
• The COM Type Library
• Designing Generic Applications
• Using CORBA
• Using COM

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What is a Dynamic Request?

• Sometimes clients need to be built before their
  server interfaces are defined
• They need to defer request definition until they
  are executed
• Examples
• Object browser
• Generic bridges
• Scripting language interpreter

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Motivating Example Object Browser
Use run-time type information to find out about
object types attribute names Use dynamic
invocation interfaces to obtain attribute values
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Motivating Example Generic Bridge

• Generic and request-level bridge

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Motivating Example Scripting
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Commonalities

• Discovery of type information at run-time
• Use of type information to build client objects
  that can cope with any type of server objects
• Definition of object requests at run-time
• Requires two primitives from middleware
• Dynamic invocation interfaces
• Reflection mechanisms

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Dynamic Requests Principles

• Any object request has to identify
• server object
• operation name
• actual parameters
• data structure for operation result
• In Dynamic Requests
• server object identified by object reference
• operation name identified by string
• actual parameters as list of name/value pairs
• operation result determined by an address

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Dynamic Requests in CORBA
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Dynamic Requests in CORBA

• Dynamic invocation interface (DII) supports
  dynamic creation of requests.
• Requests are objects themselves.
• Request objects have attributes for operation
  name, parameters and results.
• Request objects have operations to
• change operation parameters,
• issue the request and
• obtain the request results.

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Dynamic Request in CORBA
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Dynamic Requests in COM

• COM often used with interpreted scripting
  languages (e.g. VBScript)
• Interpreters of these languages need to make
  dynamic requests.
• Dynamic Requests in COM are defined in the
  IDispatch interface
• Any COM server that implements IDispatch can be
  requested dynamically

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Dynamic Request in COM
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Dual Interfaces

• Are accessible both via stubs and via dynamic
  invocation
• Example Interface Player
• object,dual,uuid(75DA6450-DD0E-00d1-8B59-0089C739
  15CB
• interface DIPlayer IDispatch
• id(1),propget HRESULT Name(out BSTR val)
• id(2),propget HRESULT Number(out short
  val)
• id(3) HRESULT book(in Date val)
• Interfaces have to be defined as dual!

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Transparency of Dynamic Invocation

• In both COM and CORBA
• Client programs have to be written differently Þ
  Use of dynamic invocation interfaces is not
  transparent to client programmers
• In COM
• Interfaces of server objects have to be designed
  as dual Þ Use of dynamic invocation not
  transparent in server design
• In CORBA
• Server objects are unaware of dynamic invocation
  Þ Use of DII is transparent

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Reflection Principles

• How do clients discover attributes operations
  that servers have?
• capture type information during interface
  compilation
• store type information persistently
• provide an interface for clients to obtain type
  information during run-time
• Reflection interfaces provided by
• CORBA Interface Repository
• COM Type Library

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CORBA Interface Repository

• Makes type information of interfaces available at
  run-time.
• Achieves type-safe dynamic invocations.
• Supports construction of interface browser
• Used by CORBA implementations themselves
• Persistent storage of IDL interfaces in abstract
  syntax trees (ASTs)

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Abstract Syntax Trees (ASTs)

• Interface repository persistently stores ASTs of
  IDL modules, interfaces, types, operations etc.

SoccerMgmt
module SoccerMgmt
ModuleDef
Player
Team
InterfaceDef
InterfaceDef
PlayerList
void add(in short number, in Player p)
raises(InvalidNumber)
add
TypedefDef
OperationDef
InvalidNumber
members
ExceptionDef
AttributeDef
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AST Node Types
Repository
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COM Type Library

• COMs provision of run-time type information
• Raw information generated by MIDL compiler
• Stored in tokenized form (.TLB files)
• Main interfaces

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0
..
ITypeLib
ITypeInfo
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ITypeLib

• Provides operations to browse through all
  interfaces contained in the type library
• GetTypeInfoCount (returns number of TypeInfo
  objects in the library)
• GetTypeInfo (can be used to obtain type info at a
  particular index number)
• Locate ITypeInfo objects using the GUIDs

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ITypeInfo
interface ITypeInfo IUnknown HRESULT
GetFuncDesc( UINT index, FUNCDESC ppFuncDesc)
HRESULT GetIDsOfNames( OLECHAR rgszNames,
UINT cNames, DISPID pMemId) HRESULT
GetNames(DISPID memid, BSTR rgBstrNames,
UINT cMaxNames, UINT pcNames) HRESULT
GetTypeAttr(TYPEATTR ppTypeAttr) HRESULT
GetVarDesc(UINT index, VARDESC ppVarDesc)
HRESULT Invoke(VOID pvInstance, DISPID memid,
USHORT wFlags,
DISPPARAMS pDispParams, VARIANT
pVarResult, EXCEPINFO
pExcepInfo, UINT puArgErr) ...
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Static Invocation

• Advantages
• Requests are simple to define.
• Availability of operations checked by programming
  language compiler.
• Requests can be implemented fairly efficiently.
• Disadvantages
• Generic applications cannot be build.
• Recompilation required after operation interface
  modification.

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Dynamic Invocation

• Advantages
• Components can be built without having the
  interfaces they use,
• Higher degree of concurrency through deferred
  synchronous execution.
• Components can react to changes of interfaces.
• Disadvantages
• Less efficient,
• More complicated to use and
• Not type safe!

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Key Points

• Dynamic requests are used when static requests
  are not viable
• Dynamic requests supported by both CORBA and COM
• Dynamic requests are unsafe
• Reflection mechanisms provided by COM and CORBA
  make dynamic requests safe
• IDL compilers store type information persistently
  so that reflection implementations can provide
  them