Explaining Corba

8/8/2019 Explaining Corba

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What is an ORB?

For objects to be used across a network, a client/server relationship must be established.

The middleware that makes this happen is called the Object Request Broker (ORB). It is

the ORB's job to:

intercept a call from the client

find the correct object

pass it the parameters

invoke its method

return the results to the client

Because the ORB can make this transaction happen between applications on different

machines in heterogeneous distributed environments, this process seems transparent to

the client. ORBs provide the flexibility of communication between different operatingsystems, environments, and languages. They allow existing components to be used by

developers far and wide.

CORBA is a standard for ORBs

Of course, for this process to work well, a standard is required. A consortium of 700+

companies formed the Object Management Group (OMG) to create just such a standard.

They introduced this standard as the Common Object Request Broker Architecture(CORBA) in 1991. According to the OMG, "CORBA allows applications to

communicate with one another no matter where they are located or who has designed

them."1This standard also defined the Interface Definition Language (IDL) and theApplication Programming Interface (API) that makes client/server object interactions

work in a specific implementation of an ORB. To top of page

Why is CORBA important in a networked

environment?

CORBA offers many benefits to a networked environment.

1. Object reuse - CORBA allows wide-spread use of existing components.

2. Flexibility - CORBA allows developers to choose, for each component of their system,

the:

operating system

execution environment

programming language
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3. Processing speed - By directly invoking methods on a server, a client can pass

parameters using precompiled stubs or can generate them dynamically using CORBA's

invocation services.

4. Scalable server-to-server infrastructure - The CORBA ORB communicates with

pools of server business objects, dispatching requests to the first available object. Thisprovides a load balancing function for the incoming client requests. To top of page

How does CORBA work?

The ORB communication functions are central to how CORBA works, so let's look at

how the ORB works before we get to the details of CORBA.

First, ORB basics.

The ORB manages all communication between components. Notice the levels of

architecture in Figure 2.

Figure 2, "Distributed Object Computing with CORBA,"http://www.cs.wustl.edu/~schmidt/corba-overview.html

The OMG ORB model can be divided into two main parts:

1. Application oriented components

o Application Interfaces

o Domain Interfaces

o Common Facilities

2. System oriented components
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objects. The Implementation Repository stores the information that ORBs use to locate

and activate implementations of objects.To top of page

CORBA Overall Architecture

The overall architecture of CORBA is divided into three layers as illustrated in Figure 4.

Figure 4.

"DCOM and CORBA Side by Side, Step by Step, and Layer by Layer,"http://www.bell-labs.com/~emerald/dcom_corba/Paper.html

In the top layer (Basic Programming Architecture) a client may request an object and

invoke its methods. The server responds by creating an object instance and making it

available to the client. The middle layer (Remoting Architecture) makes it seem as if theclient and server are in the same address space. Sending data across different address

spaces requires a standard format for packing and unpacking the transmission.

Marshaling is the term used when a method call's parameters are packed at a client'sspace or when return values are packed at a server's space. Unmarshaling is the

unpacking of the standard format to a data presentation appropriate for the address space

of the receiving process. The ORB serves as the object bus. The Object Adaptor connectsthe Object to the ORB. The Libraries on the client-side and server-side are linked at

compile time. The bottom layer (Wire Protocol Architecture) is where the wire protocol

is specified for the support of a client and a server on different machines.

The basic architectures of CORBA and COM are similar. But they are different in manyways. To top of page

Why is CORBA better than COM?
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CORBA and COM both strive to accomplish communication between clients and servers,

but they do it in different ways and for different reasons.

COM Background

Microsoft created Object Linking and Embedding (OLE) technology in 1990 soWindows would have cut-and-paste capabilities. OLE2 permitted embedding of onedocument type inside another. This version was renamed the Common Object Model

(COM). The following version included a set of code libraries to provide the same

interfaces, plus it enabled communication between components on both networkedmachines and single platforms. This was called Distributed COM (DCOM). Due to

Microsoft's Internet focus in 1995 it was renamed again to ActiveX, but is now

commonly referred to as COM, this time an acronym for the Component Object Model.

COM Versus CORBA

Although CORBA and COM both allow applications to communicate in order to useobjects, they do not accomplish this task equally well. The following comparison looks atthe usability of COM and CORBA across the Object Web.

Comparison COM CORBA

Architecture,

Definition,

Specification

Lack of overall

architecture

Multiple retargeting andredefining in order to

promote Microsoft

products

Opposed to publicspecifications

One constant vision since

1989

Architecture and individualspecifications are vendor

neutral

Direction and definition arepublic processes, including

a broad cross-industry

consensus

Specifications are availableto everyone

Free rights to implement

software using thespecifications

Cross-Platform

Support

Implementations work on

Microsoft operatingsystems

According to Microsoftrepresentatives, where

specification and

Microsoftimplementations differ,

Implementations support

multiple platforms,including Microsoft.


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Microsoft considers its

own implementation to be

definitive.3

Cross-LanguageSupport

C++

Support for otherprogramming languages

is a problem. DCOM

requires passing C-stylememory pointers between

applications.

Designed from thebeginning to be language-

neutral to accommodate awide range of languages

including:C, C++, Ada,

Smalltalk, COBOL, Java,

Visual Basic, FORTRAN

Maturity and

Deployment

Continuously changed

specifications and code

Key documents are

available in draft form

only

Available to end-users inthe last couple years

Specifications published in

1991 Broad industry consensus

and support

Seven year use around the

world

Support for WWW

Based on native machine

code, requiring a separate

version of every objectcontrol for every browser

platform

Java mobile code easily

deals with cross-platformsupport

Security - All

distributed

communication

involves risk

Confidentiality

(using encryption)

Authentification

(confirmationmessages came from

objects entitled to

issue them)

Compiled for C++

without any security

restrictions No confidentiality

No authentification

Java applets are insulated

from direct contact with thehost system by running on a

virtual machine. This

provides a "sandbox"around the applet that

enforces security

restrictions.

Confidentiality system Authentification system

Non-repudiation system

ensures that users cannotdeny their commitments

later, accommodating

international legal

requirements for securityauditing

Scalability Not designed for large- Designed for Internet-scale


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scale networks

Uses reference counts that

require programmers tomanipulate them by hand

Uses keep alive messages(pinging) every two

minutes. After sixminutes of no response,

the connection between

client and server issevered.

applications from thebeginning.

No reference counts

necessary

No keep alive messages

necessary

To sum up this comparison, COM was never designed for Internet-scale inter-objectcommunication. It is merely an adaptation of an architecture originally designed for a

single machine with non-distributed tasks. It falls short in efficiently handling the Object

Web. Microsoft has tried to scale its architecture to the enterprise and internet to promotethe use of Microsoft products.

Conversely, CORBA was primarily designed to support interoperability across network

boundaries. The OMG has worked toward creating standards for object component

software for the good of all who use these objects. By successfully creatinginteroperability of disparate systems, CORBA is helping the evolution of the Object Web.To top of page

Conclusion

This next generation Web must be able to handle Internet, intranet, and extranet

interactions. Business-to-business and consumer-to-business transactions are pushing the

use of distributed objects. The Web is evolving into the role of a full-blown client/servermedium of distributed computing. CORBA creates a distributed objects infrastructure

which makes activating and accessing remote objects transparent. CORBA offers a

flexibility and adaptability that give it a promising future.

Footnotes

1CORBA for Beginners, http://www.omg.org/news/begin.htm2CORBA Overview, Object Management Group, 1995,http://www.infosys.tuwien.ac.at/Research/Corba/OMG/arch2.htm#446864
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3Comparing ActiveX and CORBA/IIOP, http://www.omg.org/news/activex.htmTo top ofpage

References

A Brief Tutorial on CORBA, http://www.cs.indiana.edu/hyplan/kksiazek/tuto.html

BYTE Magazine - October 1997 / Features / CORBA, Java, and the Object Web,http://www.byte.com/art/9710/sec6/art3.htm

Comparing ActiveX and CORBA/IIOP, http://www.omg.org/news/activex.htm

CORBA for Beginners, http://www.omg.org/news/begin.htm

CORBA Overview, Object Management Group, 1995,

http://www.infosys.tuwien.ac.at/Research/Corba/OMG/arch2.htm#446864

CORBA Research Overview,http://www.cs.wustl.edu/~schmidt/corba-research-overview.html

The Corporate Use of Object Technology,

http://www.cutter.com/itgroup/reports/corpuse.htm

DCOM and CORBA Side by Side, Step by Step, and Layer by Layer, http://www.bell-labs.com/~emerald/dcom_corba/Paper.html

Distributed Object Computing with CORBA,http://www.cs.wustl.edu/~schmidt/corba.html

Microsoft Finally Consents to Marriage of Software Models,

http://www.omg.org/news/glea.htm

OMG's Press Releases 1998, http://www.omg.org/news/pr98/9_9.htm

Overview of CORBA, http://www.cs.wustl.edu/~schmidt/corba-overview.htmlTo top ofpage

Last Update 10/9/98

Next:Client and Server PerspectivesUp:INTEGRATING CONTROL SYSTEMS TO

Previous:INTRODUCTION
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Next:THE CORBA ARCHITECTUREUp:INTEGRATING CONTROL SYSTEMS

TOPrevious:INTEGRATING CONTROL SYSTEMS TO

INTRODUCTION

A considerable number of high-level beam dynamics (BD) applications have been

developed for the operation and monitoring of the SLS accelerator facilities. Fig. 1

captures typical components required by BD applications. Their number and demand oncomputer resources motivated, in part, a desire for a distributed computing environment.

To this end, the Common Object Request Broker (CORBA), an emerging standard for

Distributed Object Computing (DOC), has been employed. Its use at the SLS has allowed

to realize the potential benefits of distributed computing, and to simultaneously exploit

features inherent to CORBA such as the interoperability between objects of different race(language) and creed (platform).

Figure 1: DOC components serving BD applications

Complex tasks, such as the modeling of the SLS accelerators, can thus be handled bydedicated computers, and developed into reusable components that can be accessed

through remote method invocations. Persevering with the notion of DOC and developing

the entire suite of BD components as CORBA objects, further elevates the level at which

applications are designed and implemented. Platforms hosting high-level softwareapplications are no longer limited to the libraries and extensions available to the host

operating system as the introduction of a CORBA middleware layer serves to extendthe

developers chosen programming language. BD application developers are, henceforth,
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able to focus on the specifics of the application at hand, such as determining user-friendly

Graphical User Interfaces (GUIs), rather than struggle with the intricate internals of

numerous Application Program Interfaces (APIs) and low-level communicationprotocols.

Next:THE CORBA ARCHITECTUREUp:INTEGRATING CONTROL SYSTEMSTOPrevious:INTEGRATING CONTROL SYSTEMS TO

Michael Boege2003-11-23

THE CORBA ARCHITECTURE

The most fundamental component of CORBA is the Object Request Broker (ORB)whose task is to facilitate communication between objects. Given an Interoperable Object

Reference (IOR), the ORB is able to locate target objects and transmit data to and from

remote method invocations. The interface to a CORBA object is specified using the

CORBA Interface Definition Language (IDL). An IDL compiler translates the IDLdefinition into an application programming language, such as C++, Java or Tcl/Tk,

generating IDL stubs and skeletons that respectively provide the framework for client-

side and server-side proxy code. Compilation of applications incorporating IDL stubsprovides a strongly-typed Static Invocation Interface (SII). Conversely, the Dynamic

Invocation Interface (DII) and the Dynamic Skeleton Interface (DSI) allows objects to be

created withoutpriorknowledge of the IDL interface. Requests and responses between

objects are delivered in a standard format defined by the Internet Inter-ORB Protocol(IIOP). Requests are marshaled in a platform independent format, by the client stub (or in

the DII), and unmarshaled on the server-side into a platform specific format by the IDL

skeleton (or in the DSI) and the object adapter, which serves as a mediator between anobject's implementation, the servant, and its ORB, thereby decoupling user code from

ORB processing. The Portable Object Adapter (POA) provides CORBA objects with a

common set of methods for accessing ORB functions, ranging from user authentication toobject activation and object persistence. It's most basic task, however, is to create object

references and to dispatch ORB requests aimed at target objects to their respective

servants. The characteristics of the POA are defined at creation time by a set of POApolicies. A server can host any number of POAs, each with its own set of policies to

govern the processing of requests. Among the more advanced features of the POA is theservant manager which assumes the role of reactivating server objects (servants) as theyare required. It also provides a mechanism to save and restore an object's state. This,

coupled with the use of the Implementation Repository (IMR), that handles the

automated start and restart of servers, realizes object persistency. Requests for server

reactivation can, alternatively, be delegated to a single default servant which providesimplementations for many objects, thereby increasing the scalability for CORBA servers.
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Figure 2: The CORBA client-server architecture

Fig. 2 shows the components of the CORBA architectural model. The ORB core isimplemented as a runtime library linked into client-server applications.

Subsections

Client and Server Perspectives Exploiting the POA

The Event Service

Next:Client and Server PerspectivesUp:INTEGRATING CONTROL SYSTEMS TO

Previous:INTRODUCTION


Next:Exploiting the POAUp:THE CORBA ARCHITECTUREPrevious:THECORBA ARCHITECTURE

Client and Server Perspectives

Despite the plethora of new terms and concepts introduced, CORBA, nevertheless,

remains true to the DOC objective of providing developers with familiar object-oriented

techniques with which to program distributed applications. Indeed, from the client

perspective, once an IOR is obtained (typically from a Naming Service which mapsnames to object references) a remote method invocation essentially takes on the

appearance of a local function call: object- operation(arguments);whilst the

communication details of client-server programming are essentially hidden from the

client, a more intimate involvement with the ORB is required when developing servers.In particular appropriate POA policies need to be chosen to configure object adapters that

best fulfill the requirements of the server.

Michael Boege

2003-11-23
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Next:The Event ServiceUp:THE CORBA ARCHITECTUREPrevious:Client and

Server Perspectives

Exploiting the POA

Transient and persistent objects are two categories of objects that relate to the lifespan

policies of the POA. A transient object is short-lived with a lifetime that is bounded bythe POA in which it was created. A persistent object, on the other hand, is long-lived with

a lifetime that is unbounded. It can consequently outlive the very server process wherein

it was created. This has several advantages. A server may be shutdown whenever it is not

needed to save resources. Server updates can be implemented transparently by restartingthe server. In developing a DOC environment, the command to start a server may be

replaced with a remote shell invocation and the next server instance run remotely,

without the client being aware. Persistent objects also maintain their identify after a

server crash. Whilst the POA supports and implements persistent objects, it does nothandle the administrative aspects of server activations. This is managed by the IMR

which stores an activation record for each server process; it is consulted automaticallywhenever a (re-)launch of a server is mandated. Thus, by virtue of the capabilities of the

POA, and the activation techniques of the IMR, BD applications are never starved of the

servers they require.

Next:The Event ServiceUp:THE CORBA ARCHITECTUREPrevious:Client and

Server PerspectivesMichael Boege

2003-11-23

Next:THE SLS CORBA SERVERSUp:THE CORBA ARCHITECTUREPrevious:

Exploiting the POA

The Event Service

A reactive, event-based, form of programming is also supported by the CORBA Event

Service which provides services for the creation and management of CORBA eventchannels. These may be used by CORBA supplier/consumer clients to propagate events

asynchronously on a push or pull basis. Event channels are created and registered withthe CORBA Naming Service allowing clients to obtain object references in the usualmanner. Communication is anonymous in that the supplier does not require knowledge of

the receiving consumers. The CORBA Event Service has been usefully employed in the

monitoring of hardware devices and in the distribution of recalibrated data to clientconsumers.

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Next:A COMPLEX CORBA APPLICATIONUp:INTEGRATING CONTROL

SYSTEMS TOPrevious:The Event Service

THE SLS CORBA SERVERS

Server objects, typically of persistent type, have been developed using the CORBA 2.3

compliant product MICO [1]. The services which these objects provide are briefly

reported here (for a more details see [2] and [3]):

Accelerator Model: A dedicated host (`` Model Server'' in Fig.3) runs the servers

(``TRACY Servers'') that perform the modeling of transfer-lines, booster and storage

ring. Procedures utilize selected routines from the TRACY accelerator physics

library [4], enabling clients to employ accelerator optimization methods online.Device Controls: The CDEV C++ class controls library provides the API to the EPICS-

based accelerator device control system. The ``CDEV Server'' running on the host ``

CORBA Server'' supplies functionality for both synchronous and asynchronousinteractions with the control system. Monitored devices and recalibrated data are

propagated to clients through CORBA event channels. Recently an interface to the EZCA

C controls library (``EZCA Server'') has been added for increased performance.

Database Access: A database server provides access to Oracle instances through the

Oracle Template Library (OTL) and the Oracle Call Interface (OCI). Methods executing

SQL statements that perform database retrieval and modification operations have beenprovided.

Logging Facility: A CORBA message server has been developed using the the UNIXsyslog logging facility. Run-time messages are sent to the logger with various priority

levels, the threshold for which can be adjusted dynamically for any given servant.

Next:A COMPLEX CORBA APPLICATIONUp:INTEGRATING CONTROL

SYSTEMS TOPrevious:The Event ServiceMichael Boege

2003-11-23

Next:The Slow Orbit FeedbackUp:INTEGRATING CONTROL SYSTEMS TO

Previous:THE SLS CORBA SERVERS

A COMPLEX CORBA APPLICATION
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One of the most complex CORBA applications at the SLS is the implementation of a

slow and the system integration of a fast global orbit feedback system. The system is

based on 72 Beam Position Monitors (BPMs) and 72 correctors in the horizontal andvertical plane distributed around the storage ring. The corrector/BPM response matrix is

``inverted'' using SVD in case of a non quadratic response matrix taking into account all

eigenvalues. The realization of the global orbit feedback has been carried out in two

steps. A Slow Orbit Feedback (SOFB) with relaxed requirements ( 3 Hz correctionrate) is in operation since August 2001 [5]. The experience gained with the various

subsystems served as a basis for the implementation of a Fast Orbit Feedback (FOFB)

(4 KHz orbit sampling rate) which is presently under commissioning [6].

Subsections

The Slow Orbit Feedback (SOFB)

The Fast Orbit Feedback (FOFB)

Michael Boege

2003-11-23

Next:The Fast Orbit FeedbackUp:A COMPLEX CORBA APPLICATIONPrevious:A COMPLEX CORBA APPLICATION

The Slow Orbit Feedback (SOFB)

Since the beginning of SLS operation global orbit corrections have been successfully

applied manually by the operators with the help of the Tcl/Tk CORBA GUI ``oco Client''.Due to the inherent modularity of the CORBA environment, thoroughly tested underlyingCORBA components like the ``TRACY Server'' and the ``CDEV Server'' could be

combined to implement the SOFB. In this case the operator is ``replaced'' by a C++

CORBA client (``Feedback Client'') which initiates an orbit correction at a given rate (see

Fig. 3).
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Figure: Schematic View of the SOFB: the ``Feedback Client'' on the `` Model Server''level replaces the operator driven GUI ``oco Client'' on the `` Console'' level. It gets BPM

data from the ``BPM Server'', asks the ``TRACY Server'' for the model predicted

corrector pattern and applies the correction through the ``CDEV Server''.

For the SOFB the digital BPM system [7] is operated in an injection triggered mode

which provides ``stroboscopic'' position readings averaged over 2 ms at a rate of 3 Hz

with a resolution 0.3 m. A ``BPM Server'' monitors, collects and sends the BPM

data to the ``Feedback Client'' with 2 Hz. A low pass filter is applied to several successiveBPM data sets. The data are then sent to the ``TRACY Server'' which predicts a corrector

pattern to restore the ``Golden Orbit'' which is defined by the orbit centered in the

quadrupoles. Additionally, local bumps at the location of the insertion devices are taken

into account in order to steer the photon beam according to the demands of theexperiments. Finally, the proposed correction is applied by toggling between the

horizontal and the vertical plane resulting in a SOFB correction rate of 0.4 Hz.

Next:The Fast Orbit FeedbackUp:A COMPLEX CORBA APPLICATIONPrevious:A COMPLEX CORBA APPLICATION
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Next:CONCLUSIONUp:A COMPLEX CORBA APPLICATIONPrevious:The Slow

Orbit Feedback

The Fast Orbit Feedback (FOFB)

In the SLS storage ring a properly chosen BPM/corrector layout leads to an `ìnverted''

response matrix where only the diagonal and their adjacent coefficients have non zero

values. Thus, corrector settings are only determined by position readings from nearbyBPMs [8]. This feature allows to run the FOFB decentralized, integrated in the 12 BPM

stations of the storage ring where each of the stations handles 6 BPMs and correctors.

The BPM data are transmitted over fiber optic links between adjacent stations. In order to

provide well defined starting conditions for the FOFB the SOFB corrects the orbit to

5 m rms. The FOFB gets initialized and started through the SOFB which downloads

the ``Golden Orbit'', 6 6 sub-matrices of the `ìnverted'' 72 72 response matrix, 1 6

sub-matrices of the `ìnverted'' 72 1 off-energy response matrix and the FOFB loop PID

parameters to the BPM stations (see Fig. 3). The SOFB continues to run in a ``watchdog''

like passive mode without touching any corrector other than the RF frequency. But itkeeps monitoring BPM and corrector values. Whenever a BPM is switched off, declared

faulty or a corrector is close to saturation the FOFB is stopped and restarted with adapted

settings. The off-energy content of the horizontal orbit is taken into account by the FOFB

but corrected by the SOFB.

Next:CONCLUSIONUp:A COMPLEX CORBA APPLICATIONPrevious:The Slow

Orbit FeedbackMichael Boege

2003-11-23
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