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Chapter 11. Load Metrics

11.1. Server-Side Load Metrics

A major feature of mod_cluster is the ability to use server-side load metrics to determine how best to

balance requests.

The DynamicLoadBalanceFactorProvider bean computes the load balance factor of a node from a

defined set of load metrics.

<bean name="DynamicLoadBalanceFactorProvider" class="org.jboss.modcluster.load.i

mpl.DynamicLoadBalanceFactorProvider" mode="On Demand">

<annotation>@org.jboss.aop.microcontainer.aspects.jmx.JMX(name="jboss.web:serv

ice=LoadBalanceFactorProvider",exposedInterface=org.jboss.modcluster.load.impl.D

ynamicLoadBalanceFactorProviderMBean.class)</annotation>

</set>

</parameter>

</constructor>

</bean>

Load metrics can be configured with an associated weight and capacity:

1. T he weight (default is 1) indicates the significance of a metric with respect to the other metrics.

For example, a metric of weight 2 will have twice the impact on the overall load factor than a metric

of weight 1.

2. T he capacity of a metric serves 2 functions:

Each load metric contributes a value to the overall load factor of a node. The load factors from each

metric are aggregated according to their weights.

In general, the load factor contribution of given metric is:

(load ÷ capacity) × weight ÷ total weight

The DynamicLoadBalanceFactorProvider applies a time decay function to the loads returned by each

metric. The aggregate load, with respect to previous load values, can be expressed by the following

formula:

L = (L0 + L1⁄D + L2⁄D2 + L3⁄D3 + ... + LH⁄DH) × (1 + D + D2 + D3 + ... DH)

... or more concisely as:

H H

L = ∑ Li/Di * ∑ Di

i=0 i=0

... where D = decayFactor and H = history.

Setting history = 0 effectively disables the time decay function and only the current load for each

metric will be considered in the load balance factor computation.

The mod_cluster proxy module expects the load factor to be an integer between 0 and 100, where 0

indicates max load and 100 indicates zero load. T herefore, the final load balance factor sent to the proxy

is:

100 - (L × 100)

While you are free to write your own load metrics, the following LoadMetrics are available out of the

box:

11.2. Web Container metrics

1. ActiveSessionsLoadMetric

Requires an explicit capacity.

Uses SessionLoadMetricSource to query session managers

Analogous to method=S in mod_jk

For example:

40 Chapter 11. Load Metrics