I downloaded the https://github.com/richardwilly98/elasticsearch-river-mongodb/downloads
and tried the example: https://github.com/richardwilly98/elasticsearch-river-mongodb/blob/master/README.md.
It failed with on the following command:
curl -XGET "http://localhost:9200/testmongo/_search?q=firstName:John"
I have set the mongodb replica set - so the oplog is present.
Thanks for your help,