Integration Technologies - Guide to Cloud Computing for Business and Technology Managers - page 108

designing the integration architecture, particularly if our systems are mis-
sion critical and are used by a large number of concurrent clients.
Middleware is connectivity software that is designed to help manage the
complexity and heterogeneity inherent in distributed systems by building
a bridge between different systems, thereby enabling communication and
transfer of data. Middleware could be defined as a layer of enabling soft-
ware services that allow application elements to interoperate across net-
work links, despite differences in underlying communications protocols,
system architectures, operating systems, databases, and other application
services. The role of middleware is to ease the task of designing, program-
ming, and managing distributed applications by providing a simple, con-
sistent, and integrated distributed programming environment. Essentially,
middleware is a distributed software layer, or platform , that lives above
the operating system and abstracts over the complexity and heterogeneity
of the underlying distributed environment with its multitude of network
technologies, machine architectures, operating systems, and programming
languages.
The middleware layers are interposed between applications and Internet
transport protocols. The middleware abstraction comprises two layers. The
bottom layer is concerned with the characteristics of protocols for communi-
cating between processes in a distributed system and how the data objects,
for example, a customer order, and data structures used in application pro-
grams can be translated into a suitable form for sending messages over a
communications network, taking into account that different computers may
rely on heterogeneous representations for simple data items. The layer above
is concerned with interprocess communication mechanisms, while the layer
above that is concerned with non-message- and message-based forms of
middleware. Message-based forms of middleware provide asynchronous
messaging and event notification mechanisms to exchange messages or react
to events over electronic networks. Non-message-based forms of middle-
ware provide synchronous communication mechanisms designed to sup-
port client-server communication.
Middleware uses two basic modes of message communication:
1. Synchronous or time dependent: The defining characteristic of a syn-
chronous form of execution is that message communication is syn-
chronized between two communicating application systems, which
must both be up and running, and that execution flow at the client's
side is interrupted to execute the call. Both sending and receiving
applications must be ready to communicate with each other at all
times. A sending application initiates a request (sends a message) to
a receiving application. The sending application then blocks its pro-
cessing until it receives a response from the receiving application.
The receiving application continues its processing after it receives
the response.