How is it possible through the REST api to figure out if an indexer is or was down during an export (query). The problem we would like to solve is: to ensure that a finished export contains all records. If an indexer was down during an export I assume that we are missing some data. In that case we need a possibility to figure this out somehow, because then, we need to re-start the export.
You can check the "messages" key in the job status results (what you get back when you query the http://localhost:8089/services/search/jobs/*search_id* REST endpoint for the job. This contains sets of messages grouped by priority, and within each of those, any messages generated by the search. You could also check the "searchProviders" key in the job status results and compare against a list of the desired indexers. "messages" appears not to be in the current documentation, but it does come back.