DCOM (Distributed Component Object Model) What is DCOM?

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1 DCOM (Distributed Component Object Model) Overview What is DCOM? Where does it come from? What makes it interesting? Objectives This introduction is about: DCOM Changing a local scheme (COM) into a distributed one (DCOM) Pitfalls in doing that This introduction is not about: Programming using DCOM COM/OLE/ActiveX/ Comparison to other schemes (CORBA, Java RMI, ) What is DCOM? Distributed Component Object Model Application-level protocol for -oriented remote procedure calls. Object RPC IETF Internet Draft 5/96 Microsoft RPC Version 2.0 COM with a longer wire Microsoft Press Release 9/96 1

2 The Clipboard Clipboard DDE OLE App1 Clipboard CF Text 7KH TXLFN EURZQ App2 7KH TXLFN EURZQ 7KH TXLFN EURZQ DDE (Dynamic Data Interchange) The Clipboard DDE OLE App2 App1 7KH TXLFN EURZQ EOXH IR[ MXPSHG RYHU 7KH TXLFN EURZQ EOXH IR[ MXPSHG RYHU 2

3 OLE (Object Linking and Embedding) The Clipboard class information DDE OLE App1 Clipboard Class info The quick brown fox jumped 7KH over TXLFN the EURZQ... 7KH TXLFN EURZQ presentation data presentation data The Clipboard DDE OLE visible inter-application drag-and-drop in-place editing invisible DDE gone message-based model gone COM 3

4 COM (Component Object Model) Specification describing what an is interface advertising lifetime piunknown pqueryinterface paddref prelease ObjectQI() ObjectAddRef() ObjectRelease() QueryInterface() inquire about interfaces used by particular AddRef() Release() piname pitime pqueryinterface paddref prelease pgetname psetname pqueryinterface NameQI() NameAddRef() NameRelease() GetName() SetName() TimeQI() Manage reference counters for. paddref prelease pgettime TimeAddRef() TimeRelease() COM/DCOM Architecture client client application server server implementation proxy stub channel COM Clients Proxies map method invocations into calls to COM/DCOM s COM Servers In-process local remote 4

5 Inproc (in-process) Servers Object executes in same process space as client. Server code implemented as DLL (dynamic linked library). Object has complete access to client s memory (and vice versa) client A client application 1 proxy server.dll stub server implementation client B client application 2 proxy stub stub server implementation server implementation Local Servers COM is implemented in another process. Memories are protected. client Proxy DLL... Stub DLL... server pgettime ptimeserver 5

6 Remote Servers Generally, remote servers are not different from local servers. DCOM as COM with a longer wire. ptimeserver client Proxy DLL... pgettime IPC across machine boundaries (DCE RPC) Stub DLL... server DCOM Support for Distribution Security Object References (binding) representation & communication marshaled interface references maintenance reference counting, pinging, delta pinging Optimized interfaces e.g. batched QueryInterfaces() calls Management of causally related chains of calls Non-coordinated versioning of interfaces 6

7 Issues in Supporting Distribution Example: Reference Counting Used for resource management: manage s lifetimes Local case: reference counting AddRef() Release() Reference counting is fragile: Abnormal termination of programs Distributed case: effects of communication over network Issues in Supporting Distribution Example: Reference Counting (cont) Backup mechanism: Pinging Periodically exchange keepalive messages Scalability? Large number of s: Delta Pinging Pinging of sets of s ping(set_id, additions, deletions) Large number of clients? 7

8 Specification Want to know more? N. Brown, C. Kindel, Distributed Component Object Model Protocol -- DCOM/1.0, IETF Networking Group Internet-Draft, Microsoft Corporation, May 2, 1996 Programming R. Grimes, Professional DCOM Programming, Wrox Press Ltd, A dissenting voice Comparing ActiveX and CORBA/IIOP, OMG CORBA NEWS, 4Q96, 8