CS 603 DCOM

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CS 603DCOM

• April 5, 2002

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DCOM What is it?

• Start with COM Component Object Model
• Language-independent object interface
• Add interprocess communication

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DCOMDistributed COM

• Looks like COM to the client
• Built on DCE RPC
• Extends to support full COM functionality

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DCOM Architecture
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Locating ObjectsActivation

• CoCreateInstance(Ex)(ltCLSIDgt)
• Interface pointer to uninitialized instance
• Same as COM
• CoiGetInstanceFromFile, FromStorage
• Create new instance
• CoGetClassObject(ltCLSIDgt)
• Factory object that creates objects of ltCLSIDgt
• CoGetClassObjectFromURL
• Downloads necessary code from URL and
  instantiates
• Can take server name as parameter
• Or default to server specified in DCOM
  configuration on client machine
• HKEY_CLASSES_ROOT\APPID\ltappid-guidgt
• "RemoteServerName""ltDNS namegt
• Also store information in ActiveDirectory

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Example Auction System

• Registry on Client
• REGEDIT4
• HKEY_CLASSES_ROOT\CLSID\ltCLSID_AuctionServergt
• "AppID""ltAPPID_AuctionServergt"
• HKEY_CLASSES_ROOT \APPID\ltAPPID_AuctionServergt
  
• "RemoteServerName""auctions.r.us.com"
• Note APPID_AuctionServer can have the same
  value as CLSID_AuctionServer.
• Client Code
• IAuction Auction new AuctionService // Uses
  generated AuctionService.class file.
• Auction.PlaceBid(1324, 100000.00)
• // Auction object gets garbage collected.

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Naming Monikers

• Above examples name classes
• What if you want a specific instance?
• One option Each instance at a separate machine
• Better Monikers
• Naming object with standard IMoniker interface
• Object creates Moniker and registers it with
  libraries
• Monikers have URL as human-readable version

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Moniker Example

• HRESULT hrS_OK
• IBindCtx pBCNULL
• hrCreateBindCtx(NULL, pBC)
• if (SUCCEEDED(hr))
• DWORD dwEaten IMoniker pMonikerNULL
• // Create the moniker object.
• hrMkParseDisplayName(pBC, L"file\\\\dtwmkt\\mkt
  \\productx1\\salesQ496.xyz!Summary", dwEaten,
  pMoniker)
• if (SUCCEEDED(hr)) // Connect, create and
  initialize if necessary.
• hrpMoniker-gtBindToObject(pBC, NULL,
  IID_ISalesInfo, pSales)
• if (SUCCEEDED(hr)) // Perform the operation.
• pSales-gtAdd( 1.1, "http//www/mkt/productx/budg
  et.xyz")
• pSales-gtRelease()
• pMoniker-gtRelease()
• pBC-gtRelease()

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Invocation

• Marshaling and unmarshaling
• Built on top of DCE RPC standard
• All parameters defined with IDL
• Uses Interface ID to look up stubs / code for
  class in Registry
• DCE extension Interface Pointers
• Pointer to proxy object
• Allows callee to invoke parameter
• Call returns to source

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Additional Issues

• Garbage Collection
• Reference count on distributed objects
• Addref and Release called explicitly
• Ping / timeout to handle client failure
• Referrals
• Interface pointer refers to handling machine
• Passing interface pointer removes intermediate
  machine from call

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DCOM Security

• Access security
• Who can call an object?
• Launch security
• Who can create a new object?
• Identity
• Who is responsible for an object?
• Connection Policy
• Integrity
• Privacy
• Authentication

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DCOM Security

• Servers/clients can customize security
• CoInitializeSecurity API
• Starts with default
• Access security
• COM-provided checks
• User-specified checks using COM
  authentication/etc.
• IServerSecurity, IClientSecurity interfaces
• Kerberos for delegation
• Proxy tickets allow delegation of specific
  authorities
• Also full delegation
• IServerSecurityImpersonateClient()

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Delegation
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Threads (Apartments)

• Single Threaded
• Call blocks
• Single Thread apartment
• One thread per instance
• Multi-Threaded Apartment
• Multiple threads in an instance
• Combine both
• Allows call from single and multi-threaded

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DCOM vs. CORBA

• CORBA
• Single interface name
• Multiple inheritance
• Dynamic Invocation Interface
• C-style Exception Handling
• Explicit and Implicit reference counts
• Implemented by ORB with replaceable services

• DCOM
• Distinction between Class and Instance Identifier
• Implement multiple interfaces
• Type libraries for on-demand marshaling
• 32 Bit Error Code
• Explicit reference count only
• Implemented by many independent services