An Index Timeout alert is a Threshold Alert that notifies you by email when more than some number of /update requests have timed out over a period of some minutes. This creates an Incident in the Managed Solr Dashboard.
An indexing request times out after 180 seconds. You get an error message in the solr.log file, but the incoming documents are not lost. They are queued up in Zookeeper.
This usually means that the Solr system is overloaded and is just too busy to process index requests in a timely manner.
System Load is Too High
Index Timeout alerts can result when the system is under heavy stress. Check the System Load/CPU graphs to evaluate the load. Also, check the Service Graph to see if there is a spike.
This is often associated with a heavy episode of indexing. Solr Admin > Cloud > Tree > Overseer can show if there are too many items queued up. If the alerts are associated with indexing events, consider throttling back the flow of /update messages.
This situation can be relieved by performing a rolling restart to clear the queues. Contact SearchStax Support if you need any help there.
Query Volume is Too High
It is possible for a cluster to become too busy handling search events, such as during a holiday spike in traffic. Evaluate if the number of timeouts is decreasing and one should just wait, else open a ticket with SearchStax support.