CS551 Object Oriented Middleware (IV) Dynamic Requests (Chap. 6 of EDO) Outline. Why Dynamic Request?

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1 CS551 Object Oriented Middleware (IV) Dynamic Requests (Chap. 6 of EDO) Yugi Lee STB #555 (816) yugi@cstp.umkc.edu 1 Outline 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 2 Why 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 3

2 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 4 Motivating Example: Generic Bridge Generic and request-level bridge Client DSI DII Obj. Imp. ORB Core ORB Core 5 Motivating Example: Scripting 6

3 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 7 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 8 Dynamic Requests in CORBA Client Object Implementation Dynamic Invocation Client Stubs ORB Interface Implementation Skeletons Object Adapter ORB Core 9

4 Dynamic Requests in CORBA Dynamic invocation interface (DII) supports dynamic creation of requests (objects). 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. 10 Dynamic Request in CORBA :Client r=create_request(, Op, ) :Server add_arg() invoke() r:request r Op() delete() 11 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 12

5 Dynamic Request in COM :Client QueryInterface(IID_IDispatch) :Server :ITypeInfo GetIDsOfNames( Op ) Invoke() GetIDsOfNames( Op ) Invoke() Op() 13 Dual Interfaces Are accessible both via stubs and via dynamic invocation Example: Interface Player: [object,dual,uuid(75da6450-dd0e-00d1-8b c73915cb] 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! 14 Transparency of Dynamic Invocation In both COM and CORBA: Client programs have to be written differently Use of dynamic invocation interfaces is not transparent to client programmers In COM: Interfaces of server objects have to be designed as dual Use of dynamic invocation not transparent in server design In CORBA: Server objects are unaware of dynamic invocation Use of DII is transparent 15

6 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 16 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) 17 Abstract Syntax Trees (ASTs) Interface repository persistently stores ASTs of IDL modules, interfaces, types, operations etc. 18 SoccerMgmt module SoccerMgmt { interface Player; ModuleDef interface Team { typedef sequence<player> Player Team PlayerList; exception Invalid {}; InterfaceDef InterfaceDef attribute ATMList ATMs; void add(in short number, PlayerList add in Player p); raises(invalidnumber) TypedefDef OperationDef }; InvalidNumber members }; ExceptionDef AttributeDef

7 AST Node Types IRObject Contained Container OperationDef InterfaceDef ModuleDef Repository ExceptionDef ConstantDef TypedefDef AttributeDef 19 COM Type Library COM s provision of run-time type information Raw information generated by MIDL compiler Stored in tokenized form (.TLB files) Main interfaces: ITypeLib 1 0..* ITypeInfo 20 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 21

8 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... }; 22 wflags, DISPPARAMS *pdispparams, VARIANT *pvarresult, EXCEPINFO *pexcepinfo, UINT *puargerr); 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. 23 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! 24

9 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 25