CS603 Distributed Object Systems

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CS603Distributed Object Systems

• March 27, 2002

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Distributed Object Systems

• What are they?
• CORBA
• DCOM
• Java/RMI
• But what does it mean?
• RPC for the object crowd
• With all the tradeoffs/choices/distinctions of
  RPC systems

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Distributed Objects as RPC

• Interface Description Language
• Defines external view of objects
• Compilers / Precompilers
• Language extensions
• Run Time System
• Directory Services
• Data Conversion
• Security / Authentication

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Distributed Object SystemDistinctions

• Single language vs. multilingual
• Cross-lingual?
• Platform independence
• True cross-platform communication?
• Extras
• Real-time features
• Fault tolerance
• Transaction support

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The Big Three

• CORBA DCE on steroids
• Cross-lingual (primarily C, Java)
• Cross-platform
• Many features
• DCOM Microsofts answer
• Some cross-lingual support (within Microsoft
  world)
• Windows only
• Built on DCE RPC and COM
• Java RMI
• Single language, tightly integrated

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CORBA Background

• Object Management Group
• Corporate-sponsored standards body
• Members develop and vote on standards
• Current specs include UML, CORBA
• Started April 1989
• Working groups for extensible specifications
• RealTime CORBA
• Fault-Tolerant CORBA
• Embedded CORBA
• Many more

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CORBA Basics(Thanks to Doug Schmidt, Andrew
Tannenbaum, and OMG for figures)

• Object Request Broker
• Object Services
• Naming, Trading (property-based location),
  security, etc.
• Common Facilities
• General interfaces, e.g., OpenDoc
• Domain interfaces Standards
• Application interfaces IDL specifications for a
  particular application

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ORB Architecture
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CORBA IDL

• Syntactic description of objects
• Single Interface Definition Language
• Compiles to multiple binary interfacesC, C,
  Java, Smalltalk, Ada, COBOL, ?
• Assign Repository Identifier
• Register interface in interface repository
• Generate Proxy
• Client-side stub
• Marshals invocation request
• Unmarshals result
• Also Dynamic Invocation Interface
• Invoke object when interface not known until
  runtime

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Key ORB facilities

• Manipulate object references
• Marshal/Unmarshal
• Comparison
• Service Discovery
• By name
• By property
• Interface repository and Implementation
  repository
• ORB/Proxy interface can be vendor specific

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Interfaces
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Invocation Models

• Default Synchronous Semantics
• Block for response
• Exception on failure
• At-most-once semantics
• One-Way Request
• No response needed/possible
• Non-blocking
• Best effort semantics
• Deferred Synchronous
• Caller can continue and later wait for result

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Naming

• Object reference
• Language independent pointer
• POA Adaptor to make server-side code accessible
  to client

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Indirect Binding
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Message passing models

• Events
• No guarantees
• No persistence
• Notification
• Events with filtering

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Persistent Communications

• Callback model
• Client provides object that is called with result
• Polling Model
• Client polls for results
• Messages Stored by ORB

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Processes

• Client and Server distinct
• Client processes are simple
• Server potentially complex
• Agent processes
• Interface to external agent system

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Common Services

• Collection service
• List/queue/etc. objects
• Iterator, get methods
• Class library for CORBA
• Query service
• Construct collections searchable through
  declarative query language
• Concurrency control service
• Locking mechanisms
• Transaction service

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Services The Full List

• Collection Grouping objects into lists, queue,
  sets, etc.
• Query Querying collections of objects in a
  declarative manner
• Concurrency Allow concurrent access to shared
  objects
• Transaction Flat and nested transactions on
  method calls over multiple objects
• Event Asynchronous communication through events
• Notification Event-based asynchronous
  communication
• Externalization Marshaling and unmarshaling of
  objects
• Life cycle Creation, deletion, copying, and
  moving of objects
• Licensing Attaching a license to an object
• Naming Systemwide naming of objects
• Property Associating (attribute, value) pairs
  with objects
• Trading Publish and find the services an object
  has to offer
• Persistence Persistently storing objects
• Relationship Expressing relationships between
  objects
• Security Secure channels, authorization, and
  auditing
• Time Current time within specified error margins

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Interoperability

• Multiple ORB vendors
• Do you have to choose one?
• General Inter-ORB Protocol
• Framework without tranport
• Internet Inter-ORB Protocol on TCP
• Message Types
• From client Request, LocateRequest,
  CancelRequest
• From server Reply, LocateReply
• Both CloseConnection, MessageError, Fragment

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CS603CORBA

• March 29, 2002

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CORBA Programming

• Select and install an Object Request Broker
• More later examples based on ORBIX, C
• Define the interfaces (IDL)
• Create classes that implement interfaces
• Write server function
• Instantiates classes
• Registers with ORB
• Run Server
• Write and Run Client

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Ticket OfficeIDL

• // IDL file ticket.idl
• typedef float Price
• struct Place
• char row
• unsigned long seat
• Interface TicketOffice
• readonly attribute string name
• readonly attribute unsigned long numberOfSeats
• Price getPrice (in Place chosenPlace)
• boolean bookSingleSeat (in Place chosenPlace, in
  string creditCard)

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Ticket OfficeCompile IDL

• idl B S ticket.idl // Produces several
  files
• ticket.hh C headers
• include ltCORBA.hgt
• Typedef CORBAFloat Price
• Struct Place CORBAChar row CORBAULong
  seat
• Class TicketOffice public virtual CORBAObject
  public
• Virtual char name() throw (CORBASystemException
  )
• Class TicketOfficeBOAImpl
• ticketC.C // stubs for clients
• ticketS.C // skeleton for server
• TicketOffice_i.h, .C // Outline of implementation

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TicketOfficeImplementation Declaration

• class TicketOffice_i public virtual
  TicketOfficeBOAImpl
• char m_name
• Price m-highPrice Price m-lowPrice
• unsigned char m_avail 1 1 1 1 1 1 1 1
  1
• public
• TicketOffice_i (const char theName, const Price
  theHighPrice, const Price theLowPrice)
• virtual TicketOffice_i()
• virtual char name() throw (CORBASystemException
  )
• virtual CORBAULong numberOfSeats()throw
  (CORBASystemException)
• virtual Price getPrice (const Place
  chosenPlace)throw (CORBASystemException)
• virtual CORBABoolean bookSingleSeat (const
  Place chosenPlace, const char creditCard) throw
  (CORBASystemException)

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TicketOfficeImplementation

• include ticket_i.h
• TicketOffice_iTicketOffice_i (const char
  theName, const Price theHighPrice, const Price
  theLowPrice) m_highPrice(theHighPrice),
  m_lowPrice (theLowPrice)
• m_name new charstrlen(theName) 1
• strcpy(m_name, theName)
• TicketOffice_iTicketOffice_i()
• delete m_name
• char TicketOffice_iname() throw
  (CORBASystemException)
• return CORBAstring_dup(m_name)
• CORBAULong TicketOfficenumberOfSeats() throw
  (CORBASystemException)
• return 9

• Price TicketOfficegetPrice (const Place
  chosenPlace) throw (CORBASystemException)
• if (chosePlace.row 1) return m_lowPrice
• else return m_high_price
• CORBABoolean TicketOfficebookSingleSeat
  (const Place chosenPlace,const char
  creditCard)throw (CORBASystemException)
• unsigned long rowIndex chosenPlace.row A
• if (m_availrowIndexchosePlace.seat)
• m_availrowIndexchosePlace.seat 0
• return 1
• else return 0

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TicketOfficeServer

• include ticket_i.h
• int main()
• TicketOffice_i myTicketOffice(Loeb, 15.00,
  10.00)
• CORBAOrbix.impl_is_read(TicketOfficeSrv)
• Following registers server so it is automatically
  started (or just run a.out)
• putit TicketOfficeSrv a.out

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TicketOfficeClient

• include Ticket.hh
• int main()
• TicketOffice_var toVar
• tovar TicketOffice_bind(TicketOfficeSrv)
• Place p B, 1
• if (toVar-gtbookSingleSeat(p, 1234 5678))
• cout ltlt Seat B1 booked at ltlt toVar-gtname()
  ltlt for ltlttoVar-gtgetPrice(p)
• else
• cout ltlt Seat B1 taken

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CS603CORBA

• April 1, 2002

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More on Registering a Server

• Registration generic (not Orbix-specific)
• CORBAORB_ptr orb CORBAORB_init(argc, argv,
  Orbix)
• CORBABOA_ptr boa orb-gtBOA_init (argc, argv,
  Orbix_BOA)
• boa-gtimpl_is_ready(TicketOfficeSrv)
• impl_is_ready defaults to waiting forever
• impl_is_ready(name, ORBAULong timeOut)//
  return if idle for timeOut

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Wrapping Existing Code

• TIE approach
• Creates object that interfaces CORBA to
  identically declared existing C classes
• Execute DEF_TIE macros to create/bind
• Multiple inheritance approach
• CORBA interface implementation inherits
• BOAImpl
• Legacy class

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Finding operationsSimple Binding

• Simple
• Optionally specify host
• Can specify additional information Marker
• ServermyTicketOffice._marker(Loeb)myTicketOff
  ice._bind(TicketOffice)
• Clientto_var p to_bind(LoebTicketOffice,
  blitz)

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Finding OperationsNaming Service

• Database of bindings name X object ref
• Naming context Hierarchical structure
• A name is a sequence
• typedef sequnceltNameComponentgt Name
• Struct NameComponent
• id real name
• kind application-specific information

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Finding OperationsNaming Service

• NamingContext interface
• Object resolve(in Name n) get object given
  nameraises ( NotFound, CannotProceed,
  InvalidName )
• Void bind(in Name n, in Object o) -- bind
  object to name
• Void bind_context(in Name n, in NamingContext nc)
  put name in context
• CosNaming interface
• Struct Binding Name binding_name nobject,
  ncontext binding_type
• Typedef sequenceltBindinggt BindingList
• Interface NamingContext void list (in unsigned
  long how_many, out BindingList bl, out
  BindingIterator bi)

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Naming Use

• Create hierarchy of naming contexts
• E.g., tourism theatre, hotel,
• Bind specific name to context
• tourism.theatre.loeb
• Resolution Create name and resolve
• name new CosNamingName(3)
• name-gtlength(3)
• name0.id CORBAstring_dup(tourism)
• name0.kind CORBAstring_dup()
• name1.id
• LoebTicketOffices namecontext-gtresolve(name)
• Also facilities to iterate over or select from
  multiple results

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More on IDL

• IDL supports inheritance, multiple inheritance
• Implementations must do the same
• Can redefine operations at implementation level
• Can forward reference / recursive types
• interface a
• interface b readonly attribute a a_inst
• interface a
• include, define, etc. supported

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Object References

• Can pass objects
• Given class foo, get class foo_ptr (C pointer)
• Must manually maintain reference counts
• foo_ptr p2 p1 p2 foo_duplicate(p1)
• CORBArelease(p2)
• Class foo_var Automatically maintains reference
  counts
• Use _ptr for parameters
• Passing objects Server understands them
• Parameters defined in IDL
• Server must have valid implementation (including
  parameters) to invoke them

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Casting

• Can assign object reference to object reference
  of parent class
• Cant use _var for target and _ptr for source
• Can Narrow Assign object to child
• child child_class_narrow(parent)

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Dynamic Invocation(following syntax
ORBIX-specific)

• Create an on-the fly reference
• CORBAObject_ptr target // any object
• CORBARequest r(target, operation)
• r ltlt CORBAinMode ltlt inparameter ltlt
  CORBAoutMode ltlt outparameter
• r.invoke()
• Can use repository to identify name/parameters to
  construct request
• Only way to make deferred synchronous calls
• r.send_deferred(), r.poll_response(),
  r.get_response()
• Also r.send_oneway()

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Dynamic Skeleton Interface

• Server equivalent of DII
• Not required to use DII
• Intended for gateways
• Server doesnt understand calls it can process
• Trusts client/server to generate legal calls
• Translates to legacy server protocol

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Security

• Policy Objects
• Specify requirements
• Secure messaging
• Authentication / signing
• Trusted host list
• Security services
• Know how to implement policy objects