= SolrCloud Autoscaling Trigger Actions
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`TriggerAction` implementations process events generated by triggers in order to ensure the cluster's
health and good use of resources.

Currently two implementations are provided: `ComputePlanAction` and `ExecutePlanAction`.

== Compute Plan Action

The `ComputePlanAction` uses the policy and preferences to calculate the optimal set of Collection API
commands which can re-balance the cluster in response to trigger events.

The following parameters are configurable:

`collections`::
A comma-separated list of collection names. If this list is not empty then
the computed operations will only calculate collection operations that affect
listed collections and ignore any other collection operations for collections
not listed here. Note that non-collection operations are not affected by this.

Example configuration:

[source,json]
----
{
 "set-trigger" : {
  "name" : "node_added_trigger",
  "event" : "nodeAdded",
  "waitFor" : "1s",
  "enabled" : true,
  "actions" : [
   {
    "name" : "compute_plan",
    "class" : "solr.ComputePlanAction",
    "collections" : "test1,test2",
   },
   {
    "name" : "execute_plan",
    "class" : "solr.ExecutePlanAction",
   }
  ]
 }
}
----

In this example only collections `test1` and `test2` will be potentially
replicated / moved to an added node, other collections will be ignored even
if they cause policy violations.

== Execute Plan Action

The `ExecutePlanAction` executes the Collection API commands emitted by the `ComputePlanAction` against
the cluster using SolrJ. It executes the commands serially, waiting for each of them to succeed before
continuing with the next one.

Currently, it has no configurable parameters.

If any one of the commands fail, then the complete chain of actions are
executed again at the next run of the trigger. If the Overseer node fails while `ExecutePlanAction` is running,
then the new Overseer node will run the chain of actions for the same event again after waiting for any
running Collection API operations belonging to the event to complete.

Please see <<solrcloud-autoscaling-fault-tolerance.adoc#solrcloud-autoscaling-fault-tolerance,SolrCloud Autoscaling Fault Tolerance>> for more details on fault tolerance within the autoscaling framework.
