Bug 1074550

A hotstandby node in the mod_cluster logic is a kind of last ressort node: when all other nodes of the cluster are down/in error the requests and the sessions will be routed to that node.
To configure a hotstandby node just configure the node with factor=0 like:
+++
        <subsystem xmlns="urn:jboss:domain:modcluster:1.2">
            <mod-cluster-config advertise-socket="modcluster" connector="ajp">
                <simple-load-provider factor="0"/>
            </mod-cluster-config>
        </subsystem>
+++
In mod_cluster-manager the node will be displayed with status OK and load=-1

Scenario with 3 node:
A factor=10
B factor=10
C facctor=0

Normal behaviour load will be balanced between A and B, if one of the 2 get in error or stopped all requests will go to the other one, if both are in error or stop all request will go to C.

I find several things being wrong in the documentation.

- naming conventions
- it's Load: 0 not Load: -1, see [2]
- swapping terms "Load" and "factor"
- somewhat misleading wording

Please, see the new version of the documentation text:

== BEGIN ==

A hot-standby node in the mod_cluster logic is the last resort node to which all requests are routed if all other nodes are down.
This is similar to the hot-standby logic in mod_proxy.
To configure a hot-standby node, replace the dynamic-load-provider in mod_cluster subsystem with a simple-load-provider with factor set to 0, e.g.:

<subsystem xmlns="urn:jboss:domain:modcluster:1.2">
    <mod-cluster-config advertise-socket="modcluster" connector="ajp">
-        <dynamic-load-provider>
-            <load-metric type="busyness"/>
-        </dynamic-load-provider>
+        <simple-load-provider factor="0"/>
    </mod-cluster-config>
</subsystem>

In mod_cluster-manager console [See Development_Guide, 9.5. Apache mod_cluster-manager Application][1], the node is displayed with OK status and Load: 0.
For instance, if there are three nodes:

    Node A, Load: 10
    Node B, Load: 10
    Node C, Load: 0

The load will be ballanced between nodes A and B, unless they are both unavailable. In such case, node C will take the load.

== END ==

[1] http://documentation-devel.engineering.redhat.com/site/documentation/en-US/JBoss_Enterprise_Application_Platform/6.3/html-single/Development_Guide/index.html#sect-Apache_mod_cluster-manager_Application

[2] https://github.com/modcluster/mod_cluster/pull/95

I'm sorry, this is wrong:

the XML configuration snippet is wrong (both providers are there at the same time), please, read my comment 5 , it says "replace", not "add". These + and - prefixes are well known diff syntax, but you might want to use your own specific formatting or convention. The only point is to show both states: before and after the change.

Furthermore, why isn't the text "refer Apache mod_cluster-manager Application section in the JBoss Enterprise Application Platform Development Guide" an actual link?

These are small bugs, so we are almost there :-)

Thanks

Topic modified/updated: 18.3.4. mod_cluster Configuration on httpd

Summary of changes/updates:
- Included information as per comment 5 after the Table 18.2. mod_cluster Derivatives

Link to topic: http://documentation-devel.engineering.redhat.com/site/documentation/en-US/JBoss_Enterprise_Application_Platform/6.3/html-single/Administration_and_Configuration_Guide/index.html#mod_cluster_Configuration_on_httpd

Build ID: JBoss_Enterprise_Application_Platform-Administration_and_Configuration_Guide-6.3-web-en-US-6.3.0-28.el6eng

It's O.K. now. THX.