MACROMEDIA COLDFUSION MX-CLUSTERCATS Manuel d'utilisateur télécharger pdf (Page 148)

138 Chapter 7 Optimizing ClusterCATS

Configuring load-balancing metrics

You have the option to customize the load-balancing metrics of web servers clustered

with ClusterCATS software. This section describes how to customize metrics to your

specific website implementation.

Overview of metrics

The JRun and ColdFusion servers record how long it takes to process each request, and

can return metrics derived from this data, upon request. These metrics include the

following:

• Average Request Time (default) the average processing time of all requests within

a one-minute moving window. The use of an average smooths the effects of brief

spikes in request volume, and in a mixture of short- and long-running requests.

• Round Trip Time the round-trip time it takes to process a request.

• Last Request Time (ColdFusion only) the time it takes to process the last request

to the server. Because it is a single, undiluted snapshot of the request time, it

immediately reflects peaks and troughs in request processing time.

To translate these metrics into a single load value for the web server, they must be

weighed against a subjective measure of server performance — a maximum acceptable

response time. The maximum indicates the upper threshold of performance at which a

server is declared “busy” for load-balancing purposes. When a server reaches this critical

threshold, ClusterCATS redirects further service requests away from the server until it

becomes more responsive to its clients.

ClusterCATS software provides a further enhancement in load-balancing options. A

ClusterCATS agent process probes a special page,

getsimpleload.jsp for JRun and

getsimpleload.cfm

for ColdFusion, every five seconds, and records the round-trip time

(RTT) for each request. From this data, it computes its own average RTT over a

one-minute moving window. You can edit this file to customize load-balancing options

for your application.

This external view of request time accounts for the processing time of the request itself,

but, more importantly, for other system overhead included in reaching the web server

and receiving an acceptable response. By factoring in external influences on web server

responsiveness, such as network load, scheduling load, and disk I/O load, the

ClusterCATS probe agent can adjust the load reported by JRun or ColdFusion, to

generate a more realistic picture of the web server's performance for its clients.

For example, if the JRun server reports a light load of requests, but the probe agent finds

significant round-trip times to and from the web server, then it reports a proportionally

higher load for server than JRun reports.

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MACROMEDIA COLDFUSION MX-CLUSTERCATS Manuel d'utilisateur Page 148