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A new time-advancing algorithm can be incorporated in to SOHR by
adding a TM module of the LRI and a TMS based on the new algorithm,
wh ic h ma kes SOHR a n exten sible f ra mework. T h i s h a s be en demon st rated
by the implementation of three distributed TM algorithms in SOHR [27].
18.4.7
Data Distribution Management Service
The DDMS is a grid service related to the HLA data distribution manage-
ment service group. The DDMFS dei nes a createResource operation
invoked by the FMRI to create a DDMRI for a new federation. The DDMIS
dei nes some operations corresponding to the HLA data distribution man-
agement service group. The DDMRI keeps relevant information for data
distribution management in the federation. Similar to the case of TMS, the
operations dei ned by the DDMIS and data kept in the DDMRI depend
on the DDM algorithm used in our SOHR framework. Basically a DDM
algorithm can be classii ed as a centralized algorithm or a distributed
algorithm. In a centralized DDM algorithm, a DDM coordinator calculates
the overlaps between regions (a process called matching). In a distributed
DDM algorithm, the matching is done at each federate's local site.
If a centralized DDM algorithm is utilized in SOHR, the DDMRI acts as
the DDM coordinator for its federation. It keeps information of all regions
and calculates their overlaps. It sends the updated overlapping informa-
tion to relevant federates' LRIs to update their DDM modules. The DDMIS
dei nes relevant operations for updating region information kept in the
DDMRI, such as a createRegion operation for creating a new region and
an associateRegionForUpdates operation for associating attributes of an
object instance with a region. If a distributed DDM algorithm is utilized
in SOHR, the matching is done by the DDM module of each LRI whereas
the DDMRI may simply keep some centralized information for data dis-
tribution management, such as all regions and their respective creators.
There are many DDM matching algorithms such as the region-based
approach [32], grid-based approach [33], hybrid approach [34], and sort-
based approaches [35,36]. Each algorithm has different performance, but
there is no algorithm that i ts all federation scenarios [37]. Multiple combi-
nations of a DDM module of the LRI and a DDMS can be incorporated
into SOHR based on different algorithms so that the combination with the
best performance can be used for a specii c federation scenario.
18.4.8
RTI Index Service
The RTI index service provides a system-level registry so that all other
services are able to register their EPRs here and dynamically discover
each other. A load balancing mechanism is employed in the RTI index
service to improve the overall system efi ciency. It keeps the load informa-
tion of each registered service in terms of the number of created resource
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