Modeling P2P Grid Information Services with Colored Petri Nets - Grid Computing: Infrastructure, Service, and Applications - page 329

Sensor

User

Monitoring service

Request

AggSource

Trigger
service

Index
service

Notification

Aggregator
framework

Notification

FIGURE 13.1 MDS4 structure.

As shown in Figure 13.1, MDS4 consists of two main services, the index
service and the trigger service. The index service keeps an index of current
state information relative to all the resources running under its container
dynamically. The trigger service waits for predei ned conditions before an
event is triggered; for example, if CPU usage is more than 80%, then a
trigger to stop accepting more jobs is executed. Both the index and trigger
services are aggregator services in that they are created using the aggre-
gator framework. It is the aggregator framework that provides the pre-
dei ned methods to register, monitor, notify, and declare the data types of
the resources. Aggregate sources (AggSource) collect and act on data from
a WSRF supported resource property (WS-Property). Three types of
aggregate sources exist. First, a query source updates its state information
on a regular basis through polling operations. Second, a subscription
source updates its data though the WS-Base notii cation mechanism.
Finally an execution source executes an external program to update the
values of resource properties. Both the index and trigger services monitor
registered resources.
Upon registering, each source specii es the data type, source type, how,
and where to get its data from, and its registration intervals. Registration
intervals are used as heartbeats to make this aggregator service self-clean-
ing; that is, automatically removes expired resources. Figure 13.2 depicts
the index services arranged in a hierarchical structure for a VO of a grid
environment. The state information of resources can be pushed up or down
to a designated index service node creating a tree structure. A single index
service node may be designated to keep track of all resources available in
a VO. Having a centralized server, users need only to query this node to
discover and monitor all of the available resources for that specii c VO.
Due to its rigid hierarchical structure, MDS4 typically works in a static
grid environment. Although MDS4 can tolerate some node failures, if any