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format. Sensor-II is deployed on the front-end node of clusters, which is
responsible for pulling resource information from Sensor-I-like data
sources through UDP messages and processing incoming data request
through TCP massages. Dynamic information of Web services are avail-
able through the service container, so the APIs can be treated as sensors
and wrapped with an adaptor. In addition, Sensor-II is also responsible
for processing control messages and adjusting behaviors of Sensor-I. All
the messages are under the protocol that will be discussed in the next
section.
4.5.1.1.2
A message protocol is designed for measurement data transmission and
sensor control. Inspired by the classical i le transfer protocol (FTP) [38]
and Supermon project [39], the protocol used in CGSV is a client-server
protocol, which is based on symbolic expressions (or s-expressions). The
underlying transfer protocol varies from UDP to TCP. S-expressions origi-
nates from LISP as a recursively dei ned, simple format for data
representation. This format of data has the following features:
Transfer and Control Protocol
Extensibility: The protocol can be extended to new data types and
new type of commands, which allows the system to be capable of
evolving.
Self-descriptive: Each measurement data is associated with its
metric name and timestamp, so the packet can be independently
interpreted without any other knowledge, which increases the
system's interoperability.
Compactness: Though the packets are self-descriptive, the format
is very compact compared to XML. This feature saves network
transmission bandwidth and memory cost for protocol interpre-
tation, which decrease the intrusiveness to host systems.
Architecture independence: This is achieved by plain textual
representation, which facilitates system portability.
To improve the protocol parsing efi ciency, command packets are encoded
in s-expressions. This unii ed data transmission and control method
simplii es the implementation of monitoring components and makes
protocol interpretation and protocol execution logically separated.
This effort also makes CGSV components loose-coupled and easy to
collaborate with other monitor systems. Table 4.1 lists the basic packet
types.
4.5.1.1.3
Service Layer
Registry: There is a logical domain monitor center in each domain
of the ChinaGrid, where registry, archive module, and monitor
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