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abstract class
Channel
;
Message
to compose and decompose messages; and
ServerSocket
and
ClientSocket
to send and receive messages by members.
3.
G-EEG-Extend
provides extended messaging services through the interfaces
I-Vertical
,
I-Member
and
I-Channel
to enable, configure, query and dis-
able the three types of extensions. The component contains the following
classes:
VerticalManager
,
MemberManager
and
ChannelManager
to man-
age these types of extensions, and abstract classes
VerticalExtension
,
ChannelExtension
and
MemberExtension
to define abstract operations that
each extension must implement. Three channel-oriented extensions are cur-
rently implemented:
Auditing
,
Transformation
and
Validation
.
4.
G-EEG-Schemas
contains XML definitions for messages and parameter files,
and
G-EEG-Database
defines classes for object-relational mapping.
G-EEG
is implemented in Java using open-source technologies. MySQL [16] is
used as a database engine. Hibernate [9] represents the object-relational map-
ping. Messages are written using XML [17] and are composed and decomposed
programmatically using XMLBeans [3]. The validation extension relies on XML
Schema [24] and the transformation extension applies XSLT [25] templates.
7
Related Work
The aim of the work presented here is to enable rigorous development of messag-
ing services, with particular view on Electronic Government as the application
domain. Such services are also provided by existing MOM solutions, both com-
mercial and open-source, enabling software applications to produce and consume
messages using MOM-supplied APIs, and to transfer them through messages
queues. Concrete implementations of MOM include JMS - Java Message Ser-
vice [19] which provides messaging services for Java applications with standard
API widely adopted by the industry. Other MOM products include Microsoft's
MSMQ [5], IBM WebSphere [11] or WebMethods Enterprise [26], all offering pre-
defined functionality for authentication, encryption and routing of messages,
among others. The limitations of existing MOM solutions include reliability -
lack of formal models to enable rigorous development, extensibility - fixed func-
tionality offered, and genericity - inability to address the problems peculiar to
Electronic Government. These limitations were described in Section 1.
This work presents the formal model to serve as the foundation for messaging
services. At the specification level, we followed a state-based paradigm to specify
a set of basic operations to allow the definitions of syntax and semantics of
messaging services. At this level, state-based languages such as VDM [13], B [1]
or RSL [21] could be used; we opted for the last one. Our aim is to determine
how to build and evolve communication structures through messaging, focusing
on the expression of observables outcomes but ignoring implementation details.
One important detail ignored at this stage is the distributed nature of the state,
thus the inability for members to directly modify variables of other members.
At the implementation level, in contrast, remote state-changes can be only
carried out through sending messages and processing such messages locally. At
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