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The concurrent connections are computed with a different granularity for
virtual servers than for virtual server classes and the global server instance.
The connection counter for an individual virtual server is incremented
atomically immediately after the request is parsed and routed to the virtual
server. It is also decremented atomically at the end of the response processing
for that request. This means that the virtual server connection statistics are
always exact at any instant.
However, the connection statistics for the virtual server class and global server
instance are not updated instantly. They are updated by the bandwidth
manager thread every recompute interval. The connection count for the virtual
server class is the sum of the connections on all virtual servers of that class; and
the global server instance connection count is the sum of connections on all
virtual server classes.
Because of the way these values are computed, the number of connections for a
virtual server is always correct (and if you've enforced a limit to the number of
connections, you can never have more than the limit), and the virtual server
class and server instance values are not quite as accurate, since they're only
computed at intervals.
SNMP Basics
SNMP is a protocol used to exchange data about network activity. With SNMP,
data travels between a managed device and a network management station (NMS).
A managed device is anything that runs SNMP: hosts, routers, your web server,
and other servers on your network. The NMS is a machine used to remotely
manage that network. Usually, the NMS software will provide a graph to display
collected data or use that data to make sure the server is operating within a
particular tolerance.
The NMS is usually a powerful workstation with one or more network
management applications installed. A network management application such as
HP OpenView graphically shows information about managed devices, such as
your web servers. For example, it might show which servers in your enterprise are
up or down, or the number and type of error messages received. When you use
SNMP with a Netscape server, this information is transferred between the NMS
and the server through the use of two types of agents, the subagent and the master
agent.
The subagent gathers information about the server and passes the information to
the server's master agent. Every Netscape server, except for the Administration
Server, has as subagent.
SNMP Basics
Chapter 10
Monitoring Servers
223
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