CS603Distributed Object Systems

March 27, 2002

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

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

Distributed Object System:Distinctions

• Single language vs. multilingual– Cross-lingual?

• Platform independence– True cross-platform communication?

• “Extras”– Real-time features– Fault tolerance– Transaction support– …

The Big Three

• CORBA – DCE on steroids– Cross-lingual (primarily C++, Java)– Cross-platform– Many features

• DCOM – Microsoft’s answer– Some cross-lingual support (within Microsoft world)– Windows only– Built on DCE RPC and COM

• Java RMI– Single language, tightly integrated

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…

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

ORB Architecture

CORBA IDL

• Syntactic description of objects• Single Interface Definition Language

– Compiles to multiple binary interfaces:C, 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

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

Interfaces

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

Naming

• Object reference– Language independent “pointer”– POA: Adaptor to make server-side code accessible

to client

Indirect Binding

Message passing models

• Events– No guarantees– No persistence

• Notification– Events with filtering

Persistent Communications

• Callback model– Client provides object that

is called with result

• Polling Model– Client polls for results

• Messages Stored by ORB

Processes

• Client and Server distinct– Client processes are simple– Server potentially complex

• Agent processes– Interface to external agent system

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

Services – The Full List• CollectionGrouping 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

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

CS603CORBA

March 29, 2002

CORBA Programming

1. Select and install an Object Request Broker– More later – examples based on ORBIX, C++

2. Define the interfaces (IDL)

3. Create classes that implement interfaces

4. Write server function– Instantiates classes– Registers with ORB

5. Run Server

6. Write and Run Client

Ticket Office:IDL

// IDL – file ticket.idltypedef float Price;struct Place {

char row;unsigned long seat;

};Interface TicketOffice {

readonly attribute string namereadonly attribute unsigned long numberOfSeatsPrice getPrice (in Place chosenPlace);boolean bookSingleSeat (in Place chosenPlace, in string

creditCard);

};

Ticket Office:Compile IDL

% idl –B –S ticket.idl // Produces several files:• ticket.hh – C++ headers

#include <CORBA.h>Typedef CORBA::Float Price;Struct Place { CORBA::Char row; CORBA::ULong seat; };Class TicketOffice: public virtual CORBA::Object { public:

Virtual char* name() throw (CORBA::SystemException);…

Class TicketOfficeBOAImpl { … };

• ticketC.C // stubs for clients• ticketS.C // skeleton for server• TicketOffice_i.h, .C // Outline of implementation

TicketOffice:Implementation 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 (CORBA::SystemException);virtual CORBA::ULong numberOfSeats()

throw (CORBA::SystemException);virtual Price getPrice (const Place& chosenPlace)

throw (CORBA::SystemException);virtual CORBA::Boolean bookSingleSeat (const Place& chosenPlace,

const char* creditCard) throw (CORBA::SystemException);};

TicketOffice:Implementation

#include “ticket_i.h”TicketOffice_i::TicketOffice_i (const char *

theName, const Price theHighPrice, const Price theLowPrice) :m_highPrice(theHighPrice), m_lowPrice (theLowPrice) {

m_name = new char[strlen(theName) + 1];strcpy(m_name, theName); }

TicketOffice_i::~TicketOffice_i() {delete[] m_name; }

char* TicketOffice_i::name() throw (CORBA::SystemException) {

return CORBA::string_dup(m_name); }

CORBA::ULong TicketOffice::numberOfSeats() throw (CORBA::SystemException) {

return 9; }

Price TicketOffice::getPrice (const Place& chosenPlace) throw (CORBA::SystemException) {

if (chosePlace.row == 1) return m_lowPrice;

else return m_high_price; }

CORBA::Boolean TicketOffice::bookSingleSeat (const Place& chosenPlace,const char* creditCard)throw (CORBA::SystemException) {

unsigned long rowIndex = chosenPlace.row – ‘A’;

if (m_avail[rowIndex][chosePlace.seat]) {m_avail[rowIndex][chosePlace.seat] = 0;return 1; }

else return 0; }

TicketOffice:Server

#include “ticket_i.h”int main() {

TicketOffice_i myTicketOffice(“Loeb”, 15.00, 10.00);

CORBA::Orbix.impl_is_read(“TicketOfficeSrv”);

}• Following registers server so it is

automatically started (or just run a.out)% putit TicketOfficeSrv a.out

TicketOffice:Client

#include “Ticket.hh”int main() {

TicketOffice_var toVar;tovar = TicketOffice::_bind(“:TicketOfficeSrv”);Place p = {‘B’, 1};if (toVar->bookSingleSeat(p, “1234 5678”))

cout << “Seat B1 booked at ” << toVar->name() <<“ for $” <<toVar->getPrice(p);

elsecout << “Seat B1 taken”;

}

CS603CORBA

April 1, 2002

More on Registering a Server

• Registration – generic (not Orbix-specific)CORBA::ORB_ptr orb =

“CORBA::ORB_init(argc, argv, “Orbix”);

CORBA::BOA_ptr boa = orb->BOA_init (argc, argv, “Orbix_BOA);

boa->impl_is_ready(“TicketOfficeSrv”);

• impl_is_ready defaults to waiting forever– impl_is_ready(name, ORBA::ULong timeOut);

// return if idle for timeOut

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

Finding operations:Simple Binding

• Simple

• Optionally specify host

• Can specify additional information: Marker– Server

myTicketOffice._marker(“Loeb”);myTicketOffice._bind(“TicketOffice”);

– Clientto_var p =to::_bind(“Loeb:TicketOffice”, “blitz”);

Finding Operations:Naming Service

• Database of bindings: name X object ref

• Naming context: Hierarchical structure

• A name is a sequence– typedef sequnce<NameComponent> Name;– Struct NameComponent

• id – real name• kind – application-specific information

Finding Operations:Naming Service

• NamingContext interface– Object resolve(in Name n) – get object given name

raises ( 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 sequence<Binding> BindingList;– Interface NamingContext {

void list (in unsigned long how_many, out BindingList bl, out BindingIterator bi); };

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 CosNaming::Name(3);– name->length(3);– name[0].id = CORBA::string_dup(“tourism”);– name[0].kind = CORBA::string_dup(“”);– name[1].id = …

• LoebTicketOffices = namecontext->resolve(name);– Also facilities to iterate over or select from multiple results

More on IDL

• IDL supports inheritance, multiple inheritance– Implementations must do the same– Can redefine operations at implementation level

• Can forward reference / recursive typesinterface a;

interface b { readonly attribute a a_inst };

interface a { … };

• #include, #define, etc. supported

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);• CORBA::release(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

Casting

• Can assign object reference to object reference of parent class– Can’t use _var for target and _ptr for source

• Can Narrow: Assign object to child– child = child_class::_narrow(parent)

Dynamic Invocation(following syntax ORBIX-specific)

• Create an “on-the fly” referenceCORBA::Object_ptr target = // any objectCORBA::Request r(target, “operation”)r << CORBA::inMode << inparameter <<

CORBA::outMode << 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()

Dynamic Skeleton Interface

• Server equivalent of DII– Not required to use DII

• Intended for gateways– Server doesn’t understand calls it can process– Trusts client/server to generate legal calls– Translates to legacy server protocol

Security

• Policy Objects– Specify requirements

• Secure messaging• Authentication / signing• Trusted host list

• Security services– Know how to implement policy objects