I am looking for an observer event that is triggered when any re-index starts.
I want to create an observer that sends out an email whenever an index has been initiated for re-indexing.
You can probably use the indexer_state_save_before
event which is one of the standard events that get fired before a model gets saved in the DB.
This event should be triggered when a new entry is stored in the indexer_state
table which happens every time an indexer runs.
I cannot tell you any event before the indexer starts working, but this event fires just after the reindexer has finished:
clean_cache_after_reindex