Distributed Systems

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Distributed Systems

• Topic 6
• Resolving Heterogeneity
• Dr. Michael R. Lyu
• Computer Science Engineering Department
• The Chinese University of Hong Kong

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Outline

• Heterogeneous Programming Languages
• Common Object Model
• Common Interface Definition Language
• Programming Language Bindings
• Heterogeneous Middleware
• Interoperability
• Interworking
• Heterogeneous Data Representation
• Standardized Data Representation
• Application-Level Transport Protocol
• Virtual Machines

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1.1 Heterogeneous Languages Motivation

• Components of distributed systems are written in
  different programming languages
• Programming languages may or may not have their
  own object model
• Object models largely vary
• Differences need to be overcome in order to
  facilitate integration and portability

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1.1 Heterogeneous Programming Languages

• 1 Polylingual applications.
• 2 Standardization of bindings.
• 3 Available bindings.
• 4 What bindings need to address.
• 5 An example IDL/C.

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1.1 Polylingual Applications

• Polylingual applications have components in
  different programming languages.
• Language bindings address language
  interoperability.
• Problem with n different languages, n(n-1)
  different language bindings needed.
• Solution One language (such as IDL) as a
  mediator. Requires only n bindings.

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1.1 Why do we use an IDL?
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1.2 Interface Definition Language

• Language for expressing all concepts of the
  middlewares object model
• Should be
• programming-language independent
• not computationally complete
• Bindings to different programming languages are
  needed
• As an example OMG object model and OMG/IDL

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1.2 Polylingual Applications with IDL
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1.3 CORBA Programming Language Bindings
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1.4 What Bindings Need to Address

• Atomic data types and type constructors
• Constants
• Interfaces and multiple inheritance
• Object references
• Attribute accesses
• Operation execution requests
• Exceptions
• Invocation of ORB operations

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1.5 An Example IDL/C

• 1 Modules
• 2 Atomic Types
• 3 Records
• 4 Interfaces
• 5 Attributes
• 6 Operations
• 7 Inheritance
• 8 Exceptions
• 9 Operation Execution Requests

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1.5.1 Modules
module BankApplication ...

• IDL
• C

namespace BankApplication ...
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1.5.2 Atomic Types

• IDL C
• short CORBAShort
• long CORBALong
• unsigned short CORBAUShort
• unsigned long CORBAULong
• float CORBAFloat
• double CORBADouble
• char CORBAChar
• boolean CORBABoolean
• octet CORBAOctet
• enum enum

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1.5.3 Records

• IDL
• C

struct info long height short
weight
struct info CORBALong height
CORBAShort weight
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1.5.4 Interfaces
interface person ...

• IDL
• C

class person ...
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1.5.5 Attributes

• IDL
• C

interface person attribute readonly string
name attribute address lives_at
class person public string name()
void lives_at(address ) address
lives_at()
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1.5.6 Operations

• IDL
• C

interface person void move(in address
new)
class person public void move(const
address new)
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1.5.7 Inheritance

• IDL
• C

interface employee person ... person
boss()
class employee public virtual person
public person boss()
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1.5.8 Exceptions

• IDL
• C

interface employee person exception
too_young... void retire() raises
(too_young)
class employee public virtual person
public class too_youngpublic UserException
too_young (const char id) too_young
() void retire() throw (too_young)
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1.5.9 Operation Execution Requests

• C

employee emp ... try emp-gtretire()
catch (const too_young IsTooYoung) //
Handle the Specific Exception catch (const
SystemException SysEx) switch
(SysEx-gtminor() ) case BAD_PARAM ...
break case NO_MEMORY ... break
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2 Need for Heterogeneous Middleware

• Middleware implementations differ
• Available programming language bindings
• Available services facilities
• Supported hardware platforms
• Supported operating system platforms
• Separation of security domains.
• Large scale distributed systems.

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2.1 Heterogeneous Middleware
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2.2 Middleware Integration
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2.2 Bridging

• in-line

• request-level

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2.3 Interoperability vs. Interworking

• Interoperability denotes the ability of different
  implementations of the same middleware standard
  to work together. Requires
• Definition of interaction protocols
• Interworking defines how different middleware
  standards are integrated. Requires
• Mapping of object models
• Definition of interaction protocols

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2.3 Interoperability vs. Interworking

• Interoperability between different
  implementations of the same standard (CORBA)
• Interworking between different standards
• CORBA DCE
• CORBA COM/DCOM/OLE
• First CORBA interoperability
• Then COM/CORBA interworking

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2.4 Interoperability Protocols
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2.4.1 General Inter-ORB Protocol (GIOP)

• Define eight messages
• Request
• Reply
• Fragment
• Locate Request
• Locate Reply
• Cancel request
• Close Connection
• Message Error

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2.4.2 Common Data Representation

• Defined as part of GIOP.
• Presentation layer implementation to support
  heterogeneity.
• Mapping of IDL data types to transport byte
  stream.
• Encodings of
• primitive types
• constructed types
• interoperable object references

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2.4.3 Internet Inter-ORB Protocol (IIOP)

• Maps GIOP to TCP/IP.
• Provides operations to open and close TCP/IP
  connections.
• Is required from ORBs for CORBA compliance.
• Supported by Netscape Communicator and others.

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2.5 Motivation for Interworking

• COM and OLE/Automation are widely used for
  desktop integration of PC applications
• Compound documents
• Visual Basic / Visual C User Interfaces.
• OMG has no support for compound documents and
  user interfaces yet.
• COM and OLE do not support distribution.
• CORBA was designed to support distribution.

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2.5.1 COM-CORBA Interworking

• Goal provide transparent bi-directional mapping
  between COM/OLE and CORBA.
• Adopted specification was submitted by consortium
  of 11 ORB vendors. Most of them have COM/OLE
  Interworking implemented in their ORB products.
• Microsoft decided not to be involved in this
  effort but rather pursue its own distributed
  object environment (DCOM).
• CORBA2.2 specification defines interworking
  between CORBA and DCOM.

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2.5.2 Interworking Architecture
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2.5.3 Architecture Instantiations
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2.5.5 CORBA COM

• Enables COM Clients to access CORBA objects
• Fairly straight-forward mapping
• IDL atomic types map closely to COM primitives
• Constructed types also map closely
• CORBA object references map to COM interface
  pointers
• Inherited CORBA interfaces may be represented as
  multiple COM interfaces
• CORBA attributes mapped to set and get
  operations in COM.

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3 Heterogeneous Data Representation

• Examples of Heterogeneity
• Big-endian versus little-endian data
  representation
• Different encoding schemes for character set
• Different ways complex data types are implemented
  by run-time environment of programming languages
• Resolution Standardized Data Representation
• Resolution Application-Level Transport Protocol
• Resolution Virtual Machines

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4 Summary

• Polylingual applications.
• IDL programming language bindings.
• Heterogeneous Middleware
• Heterogeneous Data Representation.