Autodesk 15606-011408-9330 - MAPGUIDE R6.3 SITE LIC-UPG R6 User Manual page 58

NSAPI and ISAPI MapAgent
Once loaded by the Web server, the ISAPI or NSAPI MapAgents remain in
memory until the Web server service is shut down. Since they persist
between requests, they are able to use a more sophisticated system of load
balancing by ranking all servers into one of the following groups:
Available—This server is online and ready to process requests. From this


group, the MapAgent will dispatch the client request to the server that is
currently processing the fewest requests.
Offline—This server has been taken offline and will not be polled again


until the MapAgent receives 50 more client requests.
Invalid/Unavailable—This machine is physically shut off or discon-


nected from the network and will not be polled again until the MapAgent
receives 100 more client requests.
Distributed Data vs. Mirrored Systems
There are two approaches to using multiple servers: distributed data and
mirrored systems. In the distributed data model, the servers contain different
resource files so that the map data is distributed across the servers. In the
mirrored model, you have multiple servers with identical setups and data
so that the additional servers act as backups if the first server is busy or
unavailable.
The distributed data model requires less maintenance than the mirrored
systems model. However, this approach is not fault tolerant—if one Autodesk
MapGuide Server goes down, Autodesk MapGuide Viewers cannot access the
data on that server. The mirrored systems model is safer than the distributed
data model because it does have fault tolerance, which provides a backup
server so that the data is still available. However, mirrored systems can
require more maintenance than distributed data systems if you need to
update the servers.
The following diagrams illustrate the two different multiple server architec-
tures.
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58 Chapter 3 Designing Your System