ch12.htm

corba比较入门级的介绍详细间接了corba访问发布各种细节。

<TT>Customer</TT>s, and other objects could be determined in a uniform way anywhere
in the application code.
<H3><A NAME="Heading11"></A><FONT COLOR="#000077">Query Service</FONT></H3>
<P>The Query Service supports the use of queries on objects. Queries can contain
predicates that specify objects to act on, based on attribute values. The service
also supports object indexing as well as nested queries. Query capability provides
database-like semantics to CORBA objects. Just as an application can perform queries
on tables and rows in a relational database, the Query Service allows an application
to perform queries on CORBA objects.
<H3><A NAME="Heading12"></A><FONT COLOR="#000077">Relationship Service</FONT></H3>
<P>The Relationship Service enables the representation of relationships between objects.
It provides for full constraint checking of relationship type and cardinality (one-to-many,
one-to-one, and so on) and also works in conjunction with the Life Cycle Service
to copy, move, and remove related objects. Managing relationships between objects
is, of course, possible without the Relationship Service, but this service reduces
the complexity of managing complex relationships.
<H3><A NAME="Heading13"></A><FONT COLOR="#000077">Security Service</FONT></H3>
<P>The Security Service specifies the interfaces for security features:

<UL>
	<LI><I>Identification</I> and <I>authentication</I> of users, which verify that a
	user is who he or she claims to be.<BR>
	<BR>
	
	<LI><I>Authorization</I> and <I>access control</I> determine which users are enabled
	access to which services or objects.<BR>
	<BR>
	
	<LI><I>Security auditing</I>, which provides records of users' actions.<BR>
	<BR>
	
	<LI><I>Security of communication</I>, which includes authentication of users to services
	(and vice versa), integrity protection, and confidentiality protection.<BR>
	<BR>
	
	<LI><I>Non-repudiation</I>, which provides capabilities similar to those offered
	by digital signatures; that is, the origin of data or the receipt of data can be
	proven irrefutably.<BR>
	<BR>
	
	<LI><I>Administration</I> of various security policies.
</UL>

<P>Security is a paramount issue in a number of applications; for example, in a production
bank application, virtually all aspects of the system must be made secure, from authentication
and identification of customers to security of communication between banks and ATMs.
<H3><A NAME="Heading14"></A><FONT COLOR="#000077">Time Service</FONT></H3>
<P>The Time Service enables a user to obtain the current time; it can determine event
ordering and can generate events based on timers.
<H3><A NAME="Heading15"></A><FONT COLOR="#000077">Transaction Service</FONT></H3>
<P>The Transaction Service provides the interfaces to support transaction capabilities.
It supports flat and nested transaction models as well as external TP monitors. Transaction
services can also interoperate with each other.</P>
<P>Transaction semantics are an integral part of almost every non-trivial application.
For example, in the sample <TT>Bank</TT> application, to coordinate a transfer between
accounts at different banks, a transaction should be initiated that would cause the
banks involved either to both commit the transaction or to both abort the transaction;
otherwise, inconsistent data (such as account balances) would result.
<H2><A NAME="Heading16"></A><FONT COLOR="#000077">CORBAfacilities</FONT></H2>
<P>CORBAfacilities cover both horizontal facilities (features useful to all types
of CORBA applications across various industries) and vertical facilities (functionality
that is especially useful to applications within particular vertical markets and
industries). Horizontal facilities include user interface and system management facilities
because this functionality is useful to most types of applications, regardless of
the industry in which they are used. Vertical facilities might include, for example,
general ledger and amortization functionality for use within the accounting industry,
or automated shop floor control facilities for use in the manufacturing industry.
Like CORBAservices, the OMG only specifies the interfaces for these facilities; the
implementations, where applicable, are provided by CORBA vendors. Additionally, some
CORBAfacilities only suggest interfaces to be used for particular services and types
of applications.
<H3><A NAME="Heading17"></A><FONT COLOR="#000077">Horizontal Facilities</FONT></H3>
<P>The horizontal CORBAfacilities are categorized into four types of facilities:
user interface, information management, systems management, and task management.
These categories are further broken down into other facilities (listed in the next
section). Again, horizontal facilities are advantageous to all types of applications,
regardless of industry. For example, most applications require user interfaces, methods
of information storage and retrieval, security facilities, workflow and process management,
and so on.
<H4><FONT COLOR="#000077">User Interface Common Facilities</FONT></H4>
<P>The User Interface Common Facilities cover all that relates to user interfaces,
from the tools used to develop them to the way they are presented to the user. CORBAfacilities
defines the following components of user interfaces: <I>User Interface Style</I>
is the &quot;look and feel&quot; presented to the user by the application. <I>User
Interface Enablers</I> present the user interface to the user. Enablers are grouped
into the following facilities:

<UL>
	<LI><I>Rendering Management</I>, for abstracting user interface objects<BR>
	<BR>
	
	<LI><I>Compound Presentation</I>, for displaying compound documents<BR>
	<BR>
	
	<LI><I>User Support</I>, for spell checking, online help, and so on
</UL>

<P><I>Work Management System</I> maintains a user's work environment and consists
of the user's desktop, single sign-on to the system, and information used by the
user. <I>Task and Process Automation</I> enables users to write scripts to automate
their tasks and use workflows.
<H4><FONT COLOR="#000077">Information Management Common Facilities</FONT></H4>
<P>The Information Management Common Facilities consist of the following: <I>Information
Modeling</I> deals primarily with the way data is structured.</P>
<P><I>Information Storage and Retrieval</I> includes databases, information retrieval
systems, and repositories.</P>
<P><I>Information Interchange</I> enables the exchange of data between users and
between applications, consisting of the Compound Interchange Facility, the Data Interchange
Facility, and the Information Exchange Facility. <I>Data Encoding and Representation</I>
deals with how information is stored, down to the bit level, if necessary. The primary
reason for addressing this is to enable portability of data between applications,
processes, hardware and software architectures, and so on.
<H4><FONT COLOR="#000077">Systems Management Common Facilities</FONT></H4>
<P>The Systems Management Common Facilities provide interfaces for system administration.
<I>Policy<B> </B>Management</I> controls the creation, deletion, and modification
of manageable components.</P>
<P><I>Quality<B> </B>of<B> </B>Service<B> </B>Management</I> supports the selection
of service levels for availability, performance, reliability, and recovery.</P>
<P><I>Instrumentation</I> provides the capability to gather and analyze data regarding
system load, object location, system responsiveness, and so on.</P>
<P><I>Data</I><B> </B><I>Collection</I> includes capabilities such as logging and
data archival.</P>
<P><I>Security</I> provides for the management of security of system resources.</P>
<P><I>Collection</I><B> </B><I>Management</I> enables administrators to deal with
collections of objects to be managed.</P>
<P><I>Instance</I><B> </B><I>Management</I> enables objects to be associated with
other objects for management purposes.</P>
<P><I>Scheduling</I><B> </B><I>Management</I> enables tasks to be performed in a
controlled manner (for example, to occur at a certain time or as a response to a
particular event).</P>
<P><I>Customization</I> enables objects to be extended dynamically while retaining
type safety. <I>Event</I> <I>Management</I> provides for various manipulations of
events in the system.
<H4><FONT COLOR="#000077">Task Management Common Facilities</FONT></H4>
<P>Task Management Common Facilities support the processing of user tasks. Among
the Task Management Common Facilities are the following: <I>Workflow</I> provides
management and coordination of objects that are part of a work process, for example,
a purchase process. It supports production-based workflows as well as ad hoc (coordination-based)
workflows.</P>
<P><I>Agent</I> supports both static and mobile agent types. Although the definition
and discussion of the use of agents are beyond the scope of this chapter, the agent-related
facilities include the Content Service, the Communication Service, the Message Service,
the Basic Information Services, the Simple Query Services, the Multi-Response Query
Services, the Assertion Services, the Generation Services, the Capability Definition
Services, the Notification Services, the Networking Services, the Facilitation Services,
the Database Services, the Adaptation Services, the Error Correction Services, the
Automatic Re-Transmission Services, the Registration Service, Security Services,
and Management Services. (The sheer number of services suggests that the topic of
agents is far beyond the scope of this book.)</P>
<P><I>Rule</I><B> </B><I>Management</I> provides for the specification and processing
of rules, which in turn are based on events, conditions, and actions. <I>Automation</I>
provides the capability to use scripts and macros to manipulate large-grained CORBA
objects.
<H3><A NAME="Heading18"></A><FONT COLOR="#000077">Vertical Market Facilities</FONT></H3>
<P>In addition to the horizontal services and facilities offered by the OMA, there
are also a number of vertical CORBAfacilities--facilities intended for the unique
requirements of specific markets. Also, the OMG continually adds new Vertical Market
Facilities, depending on the interest in a particular specialty area. The remainder
of this section gives a brief overview of the Vertical Market Facilities specifications
available at the time of this writing.


<BLOCKQUOTE>
	<P>
<HR>
<B>Note:</B>Although now a part of the OMG's Facilities Architecture, the Vertical
	Market Facilities are largely being superceded by work done by the OMG's various
	Domain Task Forces. Each of these Task Forces produces specifications for the vertical
	application domain to which it is assigned. An overview of the work completed (or
	in progress) by the Task Forces at the time of this writing appears in Appendix C,
	&quot;What Lies Ahead? The Future of CORBA.&quot; 
<HR>


</BLOCKQUOTE>

<P><I>Imagery</I> supports the access and interchange of imagery and related data.</P>
<P><I>Information Superhighways </I>consists of a set of networks, protocols, and
rules, information repositories connected through these networks, and a collection
of tools for transparent access to this information.</P>
<P><I>Manufacturing </I>represents the integration of manufacturing functions and
resources with other aspects of the business enterprise.</P>
<P><I>Distributed Simulation </I>supports distributed simulations of air traffic
control, video games and entertainment, and other needs.</P>
<P><I>Oil and Gas Industry Exploration and Production</I> provides a foundation for
defining specifications for exploration and production (E&amp;P). Requirements for
E&amp;P include dealing with large quantities of data, complex algorithms, and long-term
data storage.</P>
<P><I>Accounting </I>provides an interoperable approach to accounting interfaces
and seeks to remove the complexity from accounting service providers and end users.</P>
<P><I>Application Development </I>covers the selection, development, building, and
evolution of the applications needed to support an enterprise's information systems
strategy. <I>Mapping </I>provides a cohesive means of manipulating the flow of data
from databases through constructed analysis modules into either presentation tools
or secondary data applications.
<H2><A NAME="Heading19"></A><FONT COLOR="#000077">Enhancing the Bank Example with
CORBAservices</FONT></H2>
<P>Of course, today wouldn't be complete without a discussion and example of how
to integrate CORBAservices and CORBAfacilities with the <TT>Bank</TT> example. This
section will do just that. First, you'll examine which CORBAservices are of use to
the <TT>Bank</TT> application; then, you'll modify the application to employ those
services. There isn't room in this chapter (or this book, for that matter) to make
use of all the CORBAservices and CORBAfacilities that might be applicable, so you
will focus on just a couple.
<H3><A NAME="Heading20"></A><FONT COLOR="#000077">Choosing CORBAservices</FONT></H3>
<P>Now, briefly review the available CORBAservices for a moment and analyze the suitability
of each service to the <TT>Bank</TT> application:

<UL>
	<LI><I>The Concurrency Control Service.</I> Because it is quite feasible that objects
	in the <TT>Bank</TT> application (particularly <TT>Bank</TT>s) might be accessed
	by different objects simultaneously, the use of the Concurrency Control Service has
	merit. However, the type of concurrency used might be better serviced by the Transaction
	Service.<BR>
	<BR>
	
	<LI><I>The Event Service.</I> If the <TT>Bank</TT> application were to expand to
	include messages other than those used by the account update service added on Day
	9, &quot;Using Callbacks to Add Push Capability,&quot; the Event Service would prove
	beneficial.<BR>
	<BR>
	
	<LI><I>The Externalization Service.</I> Externalization of objects isn't a terribly
	important feature to the <TT>Bank</TT> application.<BR>
	<BR>
	
	<LI><I>The Licensing Service.</I> Bank services are generally not licensed, so you
	can safely overlook the Licensing Service for the purposes of the <TT>Bank</TT> application.<BR>
	<BR>
	
	<LI><I>The Life Cycle Service.</I> The <TT>Bank</TT> application would theoretically
	benefit from the use of the Life Cycle Service but is served well enough by the standard
	CORBA mechanisms for managing object life cycle.<BR>
	<BR>