Interoperability

1
Interoperability

• Eric M. Dashofy
• ICS 221
• November 12, 2002

With special thanks to David Rosenblum from whom
some of this material is blatantly stolen.
2
Overview

• Characterization of the Problem
• With a small attempt to taxonomize
• Taxonomy of Solutions
• Investigation of Specific Solutions
• CORBA, JMS, Siena, and other middleware
• XML

3
Definitions

• Interoperability
• The ability for two or more (independently-develop
  ed) (software) components to interact
  meaningfully
• Communicate meaningfully
• Exchange data or services

4
Why is Interoperability Important?

• One (perhaps the) dominant challenge in software
  engineering
• We cant live without it
• Large systems are no longer built from first
  principles (nor can they be)
• We shouldnt live without it
• Component reuse has the potential for enormous
  cost savings
• Cited by Brooks as a potential silver bullet
• We need it to maintain the living we do now
• We are burdened with un-rebuildable legacy
  systems
• cf. SAABRE, Air Traffic Control
• It is induced by the state of computing now
• Increasing connectivity of computers through the
  Internet

5
Is Interoperability the Problem?

• Interoperability is not a problem, its a
  software quality. The problem in achieving this
  quality is
• Heterogeneity!
• Components written in different programming
  languages
• Components running on different hardware
  platforms
• Components running on different operating systems
• Components using different data representations
• Components using different control models
• Components implementing different semantics or
  semantic interpretations
• Components implementing duplicate functionality
• Components implementing conflicting functionality

6
Another Characterization

• Architectural Mismatch GAO95
• Components have difficulty interoperating because
  of mismatching assumptions
• About the world they run in
• About who is in control, and when
• About data formats and how theyre manipulated
• Also assumptions about connectors
• About protocols (who says what when)
• About data models (what is said)
• Also assumptions about the global configuration
  (topology)
• and the construction process (mostly
  instantiation)

7
Syntactic vs. Semantic

• Syntactic compatibility only guarantees that data
  will pass through a connector properly
• Semantic compatibility is achieved only when
  components agree on the meaning of the data they
  exchange
• Example UNIX pipes
• Syntactic compatibility established by making all
  data ASCII
• Semantic compatibility is not addressed
• Line-oriented data?
• Field-oriented lines?
• Field separators?

8
Classic Example
American electrical plug
European electrical outlet

• Enumerate the interoperability problems here
• Are they essential or accidental?
• Are they syntactic or semantic?

9
An example of an essential power problem
American electrical plug
Flintstones Power Source
10
Achieving Interoperability
Component A
Component B
?
?
?
Give some examples of components for A and B.
Given two components and an arbitrary
connectorin between, how can we make them
interoperate?
11
Shaws Taxonomy
12
In Detail

• Change As form to Bs form
• Usually involves a complete rewrite
• Expensive but do-able
• Publish an abstraction of As form
• APIs (open or standardized)
• Projections or Views (common in databases)

13
(cont).

• Transform on the fly
• Big-endian to little-endian translations in the
  connector
• REST architectural style
• Negotiate a common form
• Requires that one or both components support
  multiple forms
• Classic example is modem protocol negotiation

14
(cont).

• Make B multilingual
• Macintosh fat binaries
• Portable code that uses ifdefs
• Import/Export Converters
• May be part of A or B, may be developed by a 3rd
  party
• Classic example word processors, Alchemy
  Mindworks Graphics Workshop

15
(cont).

• Intermediate form
• Agree on a common form, usually involves some
  sort of standardization
• IDL data definitions
• XML schema
• RTF, PostScript, PDF
• Wrap Component A
• Machine emulator
• (cf. Playstation emulators, StellaX, SAABRE)
• Piece of code that translates APIs

16
(cont).

• Maintain parallel consistent versions
• Constrain both A and B such that they have
  matching assumptions
• Whenever one changes assumptions, make the
  corresponding change in the other component
• Delicate, often impractical
• Separate essence from packaging
• Research topic
• A service without an interface
• Interfaces are provided by system integrators
• Variant exposing multiple interfaces from A
• Variant A generic interface that can be
  transformed into many interfaces automatically

17
The Solution (as offered by industry)

• Middleware
• Buzz Industry will build you a connector that
  makes interoperability magically appear
• Right?
• Hint Not Exactly ?

18
Middleware

• Popular middleware offerings
• CORBA
• COM
• RMI
• JMS
• DCE RPC (aka Xerox Courier, SunRPC, ARPC)
• Microsoft Message Queue
• MQ Series
• Siena
• KnowNow SOAP Routing
• SOAP (is this middleware?)

19
Focus CORBA

• Common Object Request Broker Architecture
• A middleware standard
• (not implementation)
• from the Object Management Group
• Like the United Nations of software organizations

20
Focus CORBA

• From the spec
• Object Request Broker, which enables objects to
  transparently make and receive requests and
  responses in a distributed environment. It is the
  foundation for building applications from
  distributed objects and for interoperability
  between applications in hetero- and homogeneous
  environments. The architecture and specifications
  of the Object Request Broker are described in
  this manual.
• Standard for middleware that enables
  interoperability among components with different
  assumptions about
• Platform
• Language (type system)

What assumptions are implicit in the OMG
definition?
21
What is CORBA?

• At its core
• It is RPC with objects
• Along with a fairly competent IDL (interface
  definition language)
• Plus some pre-defined services provided by the
  middleware and exposed through the RPCObjects
  mechanism (CORBAServices)
• Naming
• Trading
• Events
• Etc.

22
Example
PublicInterface of B
Component A
Object B
How do we make this call (one way only, here)?
23
Example
PublicInterface of B
Component A
Object B
First, we mustturn this interfaceinto something
thatis comprehensible in As world
Solution define the interface in a
platform-neutralinterface definition language
(IDL)
Why might this be harder than it looks?
24
Exercise Convert this Java signature to be
called from C

• public int foo(int p1, Vector v)
• public int start(Thread t)

What do we need to know about the source and
target language to do this effectively?
Can I do this for any arbitrary function?
25
Example cont.
PublicInterface of B
Component A
Object B
IDL Compiler for A-world
IDL Compiler for B-world
Are these the same?
26
Example cont.
PublicInterface of B
Component A
Object B
(or maybe)
(or maybe)
Stub Compiler for A-world
Skeleton Compiler for B-world
Bs Stub forA-world
Skeleton for B-world
27
Example cont.
PublicInterface of B
Component A
Object B
Skeleton for B-world
NB B is oftencalled the trueobject
Bs Stub forA-world
Via proprietaryprotocol, probably TCP-basedif a
network is involved, maybe through some more
efficientOS-based mechanism likenamed-pipes if
the call is allbeing made on one machine.
28
Semantic Sugar CORBAservices

• CORBAservices are basically standardized APIs for
  doing common tasks.
• The True Objects providing the services are
  usually provided by your ORB vendor.

A snippet of the IDL for the Naming service
void bind(in Name n, in Object obj)
raises(NotFound, CannotProceed,
InvalidName,AlreadyBound) void rebind(in Name n,
in Object obj) raises(NotFound, CannotProceed,
InvalidName)
29
Funny Side-note IIOP

• It turns out that the proprietary protocols
  between stubs and skeletons caused
  interoperability problems between ORBS
• Solution standardize yet another protocol for
  Inter-ORB Interoperability
• This became IIOPthe Internet Inter-Orb Protocol

30
For Discussion

• What kinds of heterogeneity/interoperability
  issues are solved by CORBA
• Which are not?
• Are the problems that are addressed syntactic or
  semantic?
• Does CORBA induce any additional assumptions
  (i.e. does it worsen interoperability)?
• What assumptions?
• How?
• Where does CORBA fit in Shaws taxonomy?

31
Can we taxonomize middlewares?
RPC with Objects - CORBA - COM - RMI -
SOAP-RPC
RPC with Services - DCE RPC - Q (U.
Colorado) - Corba w/C binding
Oneway Messages (low multicast) - JMS - MSMQ
- MQ Series - SOAP (at core) - CORBA oneway
calls
Oneway Messages (high multicast) - Siena -
KnowNow SOAP routing - Precache Secret
Project (presumably)
32
Focus XML

• XML Extensible Markup Language
• Buzz Finally, a standard for encoding data! XML
  will solve your interoperability problems!
• Right?
• Hint Not exactly ?

33
What is XML?

• From the spec
• Extensible Markup Language, abbreviated XML,
  describes a class of data objects called XML
  documents and partially describes the behavior of
  computer programs which process them. XML is an
  application profile or restricted form of SGML,
  the Standard Generalized Markup Language ISO
  8879. By construction, XML documents are
  conforming SGML documents.
• XML documents are made up of storage units called
  entities, which contain either parsed or unparsed
  data. Parsed data is made up of characters, some
  of which form character data, and some of which
  form markup. Markup encodes a description of the
  document's storage layout and logical structure.
  XML provides a mechanism to impose constraints on
  the storage layout and logical structure.

What assumptions are implicit in the W3C
discussion?
34
What is XML, really?

• A way of organizing and decorating textual data
• Blatantly hierarchical, but works well in the
  context of a running document
• Supported by meta-languages that define allowable
  constructs in the hierarchy

35
Example
Eric M. Dashofy, b. 1977 tofather Mark and
motherSusan, currently acomputer scientist at
UCIrvine, hobbies includeplaying guitar and
making Star Wars fan-films.
Erics Personal Information,unstructured.
36
With XML
Is this better or worsethan the bio on the
previous slide? Howso? What can we do with this
bio that we couldnt with the previous one? What
assumptions are being made in this
biography? What agreement(s) do two components
have to come to to make use of this bio?
Eric
M Dashofy
1977
Mark Susan
Computer
Scientist UC
Irvine nameguitar / /
37
For Discussion

• What kinds of heterogeneity/interoperability
  issues are solved by XML?
• Which are not?
• Are the problems that are addressed syntactic or
  semantic?
• Does XML induce any additional assumptions (i.e.
  does it worsen interoperability)?
• What assumptions?
• How?
• Where does XML fit in Shaws taxonomy?

38
Future Directions

• Interoperability over the Web
• SOAP
• XML for control instead of data
• Solves, introduces same issues as XML
• Web Services
• The Semantic Web
• 2nd Generation Middleware
• Which is largely geared toward interoperability
  between 1st generation middleware packages
• Enterprise Application Integration (EAI)
• A whole market driven by people with experience
  making systems interoperate