10.1. Gruppi - Una scorciatoia sintattica
One of the most common elements of a cluster is a set of resources that need to be located together, start sequentially, and stop in the reverse order. To simplify this configuration we support the concept of groups.
Esempio 10.1. Un esempio di gruppo
<group id="shortcut">
<primitive id="Public-IP" class="ocf" type="IPaddr" provider="heartbeat">
<instance_attributes id="params-public-ip">
<nvpair id="public-ip-addr" name="ip" value="1.2.3.4"/>
</instance_attributes>
</primitive>
<primitive id="Email" class="lsb" type="exim"/>
</group>
Anche se l'esempio illustrato sopra contiene solo due risorse, non esiste limite al numero di risorse che un gruppo può contenere. L'esempio è comunque sufficiente per illustrare le proprietà fondamentali di un gruppo:
Resources are started in the order they appear in (Public-IP first, then Email)
Resources are stopped in the reverse order to which they appear in (Email first, then Public-IP)
If a resource in the group can’t run anywhere, then nothing after that is allowed to run, too.
If Public-IP can’t run anywhere, neither can Email;
but if Email can’t run anywhere, this does not affect Public-IP in any way
Il gruppo descritto è locicamente equivalente alla seguente definizione:
Esempio 10.2. Come il gruppo di risorse è visto dal cluster
<configuration>
<resources>
<primitive id="Public-IP" class="ocf" type="IPaddr" provider="heartbeat">
<instance_attributes id="params-public-ip">
<nvpair id="public-ip-addr" name="ip" value="1.2.3.4"/>
</instance_attributes>
</primitive>
<primitive id="Email" class="lsb" type="exim"/>
</resources>
<constraints>
<rsc_colocation id="xxx" rsc="Email" with-rsc="Public-IP" score="INFINITY"/>
<rsc_order id="yyy" first="Public-IP" then="Email"/>
</constraints>
</configuration>
Ovviamente a fronte di gruppi che crescono in grandezza, il risparmio di dichiarazioni in configurazione può diventare considerevole
Another (typical) example of a group is a DRBD volume, the filesystem mount, an IP address, and an application that uses them.
Tabella 10.1. Proprietà di un gruppo di risorse
Options inherited from
primitive resources:
priority, target-role, is-managed
10.1.3. Group Instance Attributes
Groups have no instance attributes, however any that are set here will be inherited by the group’s children.
Groups may only contain a collection of
Sezione 5.3, «Resource Properties» cluster resources. To refer to the child of a group resource, just use the child’s id instead of the group’s.
10.1.5. Group Constraints
Although it is possible to reference the group’s children in constraints, it is usually preferable to use the group’s name instead.
Esempio 10.3. Esempio di constraint che coinvolgono i gruppi
<constraints>
<rsc_location id="group-prefers-node1" rsc="shortcut" node="node1" score="500"/>
<rsc_colocation id="webserver-with-group" rsc="Webserver" with-rsc="shortcut"/>
<rsc_order id="start-group-then-webserver" first="Webserver" then="shortcut"/>
</constraints>
Stickiness, the measure of how much a resource wants to stay where it is, is additive in groups. Every active resource of the group will contribute its stickiness value to the group’s total. So if the default resource-stickiness is 100, and a group has seven members, five of which are active, then the group as a whole will prefer its current location with a score of 500.
