Integration Technologies - Guide to Cloud Computing for Business and Technology Managers - page 124

In an application program, the boundaries of a transaction are specified by
two commands:
• Begin-of-Transaction (BOT) denotes the beginning of the opera-
tion sequence - in some systems (e.g. in SQL database systems),
this command is implicitly performed after the end of the previous
transaction.
• Commit and rollback denote the end of the transaction. Commit is
the successful end and requires that all updates must be made per-
manent, while rollback is for aborting the entire sequence, that is,
undoing all effects.
The ACID properties are usually implemented by different components of a
data management solution. Maintaining isolation of transactions is achieved
by a concurrency control component implementing the concept of serial-
izibility. Atomicity and durability are guaranteed by providing recovery
strategies coping with possible failures. Finally, consistency is either explic-
itly supported by checking integrity rules or only implicitly by allowing roll-
backs of transactions.
The transaction is the mechanism that binds the client to one or more serv-
ers and is the fundamental unit of recovery, consistency, and concurrency in
a client-server system. From the perspective of application integration, trans-
actions are more than just business events. They have become an important
vehicle of guaranteeing consistency and robustness in distributed systems.
Transaction processing (TP) monitors enable building online TP by coor-
dinating and monitoring the efforts of separate applications. TP monitor
technology provides the distributed client-server environment with the
capability and capacity to efficiently and reliably develop, execute, and man-
age transaction applications. TP monitors reside between front-end appli-
cations and back-end applications and databases to manage operations on
transactional data; they manage processes and orchestrate applications by
breaking complex applications into a set of transactions. Under the control
of a TP monitor, a transaction can be managed from its point of origin—
typically on a client—across one or more servers and back to the originating
client. When a transaction ends, all parties involved agree that it either suc-
ceeded or failed. Transaction models define when a transaction starts, when
it ends, and what the appropriate units of recovery are in case of failure. A
TP monitor is needed for transactions requiring guaranteed completion of
multiple discrete functions on multiple application systems.
Transaction processing (TP) monitors are important middleware technol-
ogy in mission-critical applications. They represent the first generation of
application servers. TP monitors are based on the concept of transactions.
They monitor and coordinate transactions among different resources.
Although the name suggests that this is their only task, they have at least
two very important additional roles: providing performance management