B2B Integration : A Practical Guide to Collaborative E-commerce

Middleware Technologies 243
maintainability. The drawbacks include slower performance due, to the
communication overhead incurred from RPC calls and brokering
performed by COM, such as marshalling.
COM+ was born from the marriage of traditional COM and MTS.
COM+ supports transactions natively — components are inherently
designated a transaction. In COM+ there is no longer a need to deploy
a component and then specify its transactional requirements. Instead,
the transactional semantics are part of a component as defined by the
programmer, thereby reducing administrative error.
COM+ also provides transaction processing monitoring capabilities
by allowing multiple COM objects to be stringed together into a single
transaction. If an error occurs in any component, COM+ will roll back
all of the work and restore the system to its original state.
DCOM is what puts the 'distributed' in COM. DCOM is an extension
of Microsoft's Component Object Model (COM) across the network. It
allows a transparent implementation of COM between two or more
computers. DCOM is now built into the Windows 2000 operating
system which has reduced the effort to deploy a distributed COM
system.
Fundamentally, DCOM specifies a network protocol for making this
transparency work. When a client and component reside on the same
computer they communicate via local procedure calls. When a client
and a component reside on different computers, DCOM inserts itself
into the mix by using remote procedure calls to access the components
across the network (see Figure 8.11).
DCOM
Protocol
LPC
DCE
RPC
Figure 8.11. — Client-server communication via DCOM protocol
CCM
Remote
Component