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Date: Mon, 04 Nov 1996 23:45:54 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Mon, 06 May 1996 23:58:23 GMTContent-length: 13364<HTML><HEAD><TITLE>Computing with Distributed Objects</TITLE></HEAD><BODY><P><H1>Computing with Distributed Objects</H1><H3>Steven Fought</H3><P><HR><H2>Talk Sources</H2><P><UL><LI> <EM>The Essential Distributed Objects Survival Guide</EM> (R. Orfali, D. Harkey, J. Edwards, Wiley)<LI> Usenet news</UL><P><HR><H2>Object Oriented Programming Review</H2><P><EM>Objects</EM> are individual sets of data coupled with related procedures used to access and alter those data.  <P>Important features:<P><DL><DT>Encapsulation<DD> Data are hidden behind methods<DT>Polymorphism<DD> Objects can be interchangeable, but different<DT>Inheritance<DD> Objects can be extended and altered, but remaininterchangeable with the older models</DL><P>Related features:<UL><LI> Early and late binding<LI> Frameworks </UL><P><HR><H2>OpenDoc</H2><P>OpenDoc is a Compound Document programming model:<P><UL><LI> Documents are divided into <EM>parts</EM><LI> Parts consist of typed data in persistent store, and an associated<EM>part editor</EM><LI> Part editors are subclassed from a 60 method superclass that is standard over all implementations (not all methods need to be overloaded)<LI> Many parts can nest within other parts</UL><P><HR><H2>OpenDoc Frameworks</H2><P>Two competing drawing frameworks:<P><DL><DT>OpenDoc Development Framework<DD> From Apple.  No rendering facilities.Instead, prebuilt editor parts are subclassed<DT>OpenDoc Framework<DD> From IBM.  Built on OpenClass, a cross-platformrendering library.  Currently available for OS/2 and Windows, Mac and AIXlater.</DL><P>Clearly, rendering is currently an OpenDoc weak-point.<P><HR><H2>OpenDoc Compound Documents</H2><P><EM>Document shells</EM> are used to interact with documents.  They:<P><UL><LI> Create a <EM>session object</EM> (which provide parts with informationand manage geometries)<LI> Retrieve documents from storage<LI> Dynamically bind and load part editors<LI> Dispatch events and manage global document data</UL><P><HR><H2>Bento: OpenDoc Persistent Storage</H2><P>Bento is used to store compound documents.<P><UL><LI> A system of structured files and references between files.<LI> No concurrent multiuser access (currently)<LI> Internal versioning system (Drafts)<LI> Persistent references (32 bit IDs) used to identify individual parts</UL><P><HR><H2>OpenDoc Data Transfers</H2><P><UL><LI> Drag and Drop<LI> Clipboard (cut/copy/paste)<LI> Copy links</UL><P>Persistent references are recursively located within parts to determine whatto copy.  <P>Copy links push new data to other documents when they are updated.  <P><HR><H2>Scripting and Extensions</H2><P>Part editor superclasses are designed so that part operations can be scripted<P>A semantic-level event system extends the OS event model<P>Standardized and registered extra interfaces to parts called Extensions are also possible.  <P><HR><H2>OLE/COM</H2><P>OLE is not based strictly on the object model:<P><UL><LI> Interfaces are not individually instantiated, and therefore have nounique associated state<LI> Inheritance is not supported.  Instead, aggregation is available<LI> Object location is not transparent.  Communications mechanisms differ depending on the environment of the object (in-process, local servers,remote servers)<LI> Remote servers are currently unimplemented (might be in NT 4 beta)</UL><P><HR><H2>OLE Interfaces</H2><P>Interfaces are called through vtables laid out in MS Visual C++ format.  <P>Programmers using compilers with different vtables must currently lay out OLE tables by hand.<P>In-process servers are implemented as DLLs loaded into the process' address space<P>Local servers are contacted using MS Lightweight RPC<P><HR><H2>More on Interfaces</H2><P>All client/server communication starts with the IUnknown interface<P>New interfaces are discovered with the <TT>QueryInterface</TT> method ofIUnknown<P>This allows multiple interfaces to be supported via aggregation<P><HR><H2>OLE features</H2><P>OLE has features similar to OpenDoc<P><UL><LI> Scripting and automation support<LI> Data transfer via drag-and-drop, clipboard and linking (through changeadvisories)<LI> Compound documents</UL><P>In addition the storage architecture supports locked regions, and commit/reverttransactions (the product currently does not).<P><HR><H2>Monikers</H2><P>Monikers are aliases for other objects, including:<P><UL><LI> Files<LI> Items (within files)<LI> Anti monikers (which negate previous monikers)<LI> Pointers (to volatile storage)<LI> Composite (groups of monikers)</UL><P>In OLE, nested data are handled with monikers<P><HR><H2>OLE Compound Documents</H2><P><DL><DT>Container<DD> An application with some main purpose (e.g. Word) thatcan contain servers<DT>Server<DD> A smaller program that is either loaded into the address spaceor through LRPC that allows editing of some sub-document</DL><P>Pixelmaps of sub-documents are cached to allow document display without full server activation.<P><HR><H2>OCXs</H2><P>OCXs or ``OLE Custom Controls'' are Microsoft's replacement for Visual Basic controls, and are more generally applicable to OLE.  <P>Their purpose is to provide a standard architecture for most servers.<P>Each consists of an in-process server aggregated with an automation server(which allows scripting).  <P>Biggest weakness: OCXs cannot be containers.<P><HR><H2>CORBA overview: The Common Object Request Broker Architecture</H2><P>Distributed Objects:  Distributed computing with object-oriented principles<P>CORBA objects can exist anywhere on a network<P>Their location is transparent to other objects<P>The Object Request Broker (ORB) marshals communication between objects<P>Object interfaces are defined in a PL-independent Interface Definition Language(IDL)<P><HR><H2>CORBA History</H2><P>CORBA spec is controlled by the Object Management Group (OMG), a consortiumof over 500 companies<P>CORBA 1.0 was published in 1990 and specified the compliant behaviors ofindividual ORBs<P>CORBA 2.0 was published in 1996 and specified interoperability between ORBs,as well as updating other aspects of the spec<P>Five sub-specifications on different standardized services ORBs must implement have been issued.  Three exist in RFC form.<P><HR><H2>Interface Definition Language</H2><P>The IDL completely specifies communication between a client and server<P>The client/server relationship is arbitrary, and is generally linked to the initiating object<P>The IDL defines:<P><DL><DT>Modules:<DD> sets of class descriptions grouped into one namespace<DT>Interfaces:<DD> sets of method invocations.  Like a class definition withno implementation<DT>Operations:<DD> the CORBA term for methods<DT>Data types:<DD> include simple types short, long, unsigned short, unsigned long, float, double, char, boolean, and octet, and constructed types enum, string,struct, array, union, sequence and ``any.''  Aggregate types can be constructed with typedefs</DL><P><HR><H2>IDL example</H2><P><PRE>module MyAnimals{  interface Dog: Pet, Animal  {    attribute integer age;    exception NotInterested (string explanation);<P>    void Bark(in short how_long)      raises (NotInterested);<P>    void Sit(in string where)      raises (NotInterested);  }}</PRE><P>Attributes provide automatic <EM>get/set</EM> operations.<P><HR><H2>Object Request Brokers</H2><P>ORBs are also called ``object buses''  <P>All requests to other objects go through the ORB<P>An ORB is not a single process or a set of processes, it is a set of servicesand protocols<P>ORBs can be compared with plumbing<P><HR><H2>ORBs: Client side</H2><P><DL><DT>Client IDL stubs<DD> which pack up information and transport it to theserver, and are created by the IDL compiler<DT>Dynamic Invocation Interface (DII)<DD> which allows run-time binding of object interfaces<DT>Interface Repository APIs<DD> are stored in a run-time database, allowingthe DII to discover object methods<DT>ORB Interface<DD> which provides a few commonly needed services, such asconverting object reference objects to strings</DL><P><HR><H2>ORBs: Server side</H2><P><DL><DT>Server IDL stubs<DD>  created by the IDL compiler<DT>Dynamic Skeleton Interface (DSI)<DD> is the server-equivalent of the DII,allowing it to receive non IDL-marshaled requests<DT>Object Adapter<DD> provides a runtime environment for CORBA objects, including assigning IDs (object references), instantiation, and message passing<DT>Implementation Repository<DD> provides a place to store administrativedata, including the location of executable servers<DT>ORB Interface<DD> provides a few commonly needed services (same as client side)</DL><P><HR><H2>Steps in creating a CORBA service</H2><P><OL><LI> Create your IDL definitions<LI> Precompile to create interface skeletons<LI> Add server implementation code<LI> Compile </OL><P>The compile will add APIs to the Interface Repository, create client IDLstubs and executable server objects, and register the executable with theImplementation Repository<P>When a client opens a connection to the server, the Object Adapter will check the Implementation Repository for a compatible executable, and instantiate it<P><HR><H2>Dynamic Invocations</H2><P>To allow interface discovery, a client can dynamically invoke object methodswith the DII<P>The steps are:<P><OL><LI> Obtain the method description<LI> Create argument list<LI> Create the request<LI> Invoke the method using RPC, Send/Receive, or Datagram, depending onthe method description</OL><P>Library routines are provided to aid in this process<P><HR><H2>More on Object Adapters</H2><P>Object Adapters<UL><LI> Registers server classes with the IR<LI> Instantiates new objects<LI> Manages object references<LI> Broadcasts the presence of object servers<LI> Handles incoming client calls</UL><P>The OA is effectively an inherited interface, although it can be implementedin procedural languages<P>Standardized <EM>Basic Object Adapters (BOAs)</EM> are provided for the commoncases:  <EM>shared server, unshared server, server-per-method, and persistentserver</EM><P><HR><H2>Common Object Service Specifications</H2><P>A number of standardized services have been established, and more are being worked on.  These are grouped together in Common Object Service Specifications(COSSs)<P><DL><DT>COSS1:<DD> Life Cycle, Naming, Persistence, Event Notification (1993)<DT>COSS2:<DD> Transactions, Concurrency Relationships, Externalization (1994)<DT>COSS3:<DD> Security, Time (1995)<DT>COSS4:<DD> Query, Licensing, Properties (1995)<DT>COSS5:<DD> Trader, Collections Change Management (1996)</DL><P>I will discuss some of these in the following slides<P><HR><H2>Naming, Events, Life Cycle and Trader</H2><P><DL><DT>Naming:<DD> A standardized hierarchical name service designed to encapsulate other services such as DCE CDS, ISO X.500, and Sun NIS+<DT>Trader:<DD> Provides a way of finding the object most suited to atask based on some set of criteria<DT>Life Cycle:<DD> Allows the creation, copying, deleting, and moving ofobjects.  Also allows relationships between objects to be defined in termsof graphs<DT>Events:<DD> Allows semantic level events to be defined and distributed</DL><P><HR><H2>Transactions and Concurrency</H2><P>The Transaction Object allows nested Commit/Abort style transaction to bedefined. The object interface is inherited by objects that support transactions<P>The Concurrency service includes locking facilities to allow mutual exclusionand synchronization<P><HR><H2>Persistence and Object Databases</H2><P>The Persistence object allows seamless access to one or two level stores<P>The implementation is complex, but three basic interfaces are provided:<P><UL><LI> The <B><EM>Direct Access (DA)</EM></B> protocol provides direct access to persistent data using a Data Definition Language<LI> The <B><EM>ODMG-93</EM></B> protocol provides access to object from C++ usingODMG's DDL<LI> The <B><EM>Dynamic Data Object</EM></B> protocol is used to define a structurethat contains all the data for an object</UL><P>The latter can be used for two-level stores such as relational databasesand filesystems<P><HR><H2>Notes on CORBA</H2><P>ORBs from different vendors are only compatible if they implement half-bridgesto a reference protocol that runs on top of TCP/IP (CORBA 2.0)<P>ORB vendors have great flexibility in determining transport, packaging, andserver calling mechanisms<P>The flexibility is intended to encourage high performance systems<P>Performance varies widely by implementation and relative object location<P><HR><H2>Interesting Issues</H2><P><UL><LI> Parallelism<UL><LI> Breaking applications into smaller objects that cooperate increasespotential parallelism<LI> Objects designed to work across networks will be forced to exploitconcurrency to avoid excessive blocking</UL><LI> Programming Languages<UL><LI> The smaller size of objects encourages inter-procedural analysis<LI> All new languages can use existing object components</UL></UL></BODY></HTML>
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