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and security services. They provide performance management with load
balancing and resource pooling techniques, which enable efficient use of
computing resources and thus a larger number of concurrent clients. TP
monitors map client requests through application service stateless routines
to improve system performance. They can also take some application transi-
tion logic from the client. They also provide security management where
they enable or disable access of clients to certain data and resources. TP
monitors can be viewed as middle-tier technology, and this is why they are
predecessors of today's application servers.
TP monitors have been traditionally used in legacy information systems.
They are based on the procedural model, use remote procedure calls for com-
munication between applications, and are difficult to program because of com-
plex APIs through which they provide the functionality. In spite of that, they
have been successfully used for more than 25 years. TP monitors are proprietary
products, which makes migration from one product to another very difficult.
A TP monitor can manage transactional resources on a single server or
across multiple servers, and it can also cooperate with other TP monitors in
federated arrangements. TP monitors are primarily designed to run applica-
tions that serve large numbers of clients. By interjecting themselves between
clients and servers, TP monitors can manage transactions, route them across
systems, load balance their execution, and restart them after failures. The
router subsystem of a TP monitor mediates the client request to one or more
server processes. Each server in turn executes the request and responds.
Typically, the server manages a file system, database, or other mission-critical
resources shared among several clients.
TP monitors have several drawbacks to contend with; namely, TP monitors
are much more intrusive than MOM—they demand more modification of
the applications themselves in order take advantage of the TP monitor's spe-
cific services. TP can adversely affect application performance because, from
the point of view of the transaction's requestor, the processing of a transac-
tion is synchronous. The requestor must wait until all the processing of the
transaction has completed before it can proceed with further computations.
Moreover, during the processing of a transaction, all the resources used by
it are locked until the transaction completes—no other application can use
these resources during the execution of a transaction.
5.8 Object Request Brokers
Object request brokers (ORBs) are a middleware technology that man-
ages and supports the communication between distributed objects or
components. ORBs enable seamless interoperability between distributed
objects and components without the need to worry about the details of
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