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Thus, services should be implemented so that each invocation is indepen-
dent and does not depend on the service maintaining client-specific conver-
sations in memory or in persistent state between the invocations.
7.3.8 Predictable Service-Level Agreements (SLAs)
Service delivery platform must provide service-level management capabili-
ties for defining, monitoring, incident logging, and metering of SLAs for
service usage. SLAs should be established early because they affect service
design, implementation, and management. There should also be provision
for fine-tuning of SLAs based on the feedback of ongoing operations.
Typically, SLAs define metrics for services such as response time, through-
put, availability, and meantime between failures. Above all, SLAs are usu-
ally tied up to a business model whereby service requesters pay more for
higher or more stringent SLAs but charge a penalty when service providers
default on their SLA commitments.
7.3.9 Design Services with Performance in Mind
Service invocation should not be modeled on local function calls since local
transparency may result in a service that is on another machine on the same
LAN or another LAN or WAN.
7.4 SOA Ingredients
Web Services are new standards for creating and delivering cooperative
applications over the Internet. Web Services allow applications to communi-
cate irrespective of the platform or the operating system.
7.4.1 Objects, Services, and Resources
Any distributed system involves sending messages to some remote entity.
Underlying the differences between many systems are the abstractions
used to model these entities; they define the architectural qualities of the
system. Three abstractions—in particular, object, resource, and service—are
commonly used to describe remote entities; their definitions, however, are
not always clearly distinguished. Yet the nature of these abstractions has
a profound effect on the distributed communication paradigms that result
from their use. One approach to identifying the similarities and differences
between them is to understand them in terms of their relationship to two
properties: state and behavior.
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