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Chapter 10. Server-Side Load Metrics

10.1. Web Container metrics
10.1.1. ActiveSessionsLoadMetric
10.1.2. BusyConnectorsLoadMetric
10.1.3. ReceiveTrafficLoadMetric
10.1.4. SendTrafficLoadMetric
10.1.5. RequestCountLoadMetric
10.2. System/JVM metrics
10.2.1. AverageSystemLoadMetric
10.2.2. SystemMemoryUsageLoadMetric
10.2.3. HeapMemoryUsageLoadMetric
10.3. Other metrics
10.3.1. ConnectionPoolUsageLoadMetric

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.impl.DynamicLoadBalanceFactorProvider" mode="On Demand">
      <set elementClass="org.jboss.modcluster.load.metric.LoadMetric">
        <inject bean="BusyConnectorsLoadMetric"/>
        <inject bean="HeapMemoryUsageLoadMetric"/>
  <property name="history">9</property>
  <property name="decayFactor">2</property>

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

The 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.

The 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:

L = (∑Hi=0 Li/Di) * (∑Hi=0 Di)

... 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 load balancer expects the load factor to be an integer between 0 and 100, where 0 indicates max load and 100 indicates zero load. Therefore, the final load factor sent to the load balancer

LFinal = 100 - (L * 100)

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