We have a small Elasticsearch cluster for 3 nodes: two in one datacenter and one in another for disaster recovery reasons. However, if the first two nodes fail simultaneously, the third one won't work either - it will just throw "master not discovered or elected yet".
I understand that this is intended - this is how Elasticsearch cluster should work. But is there some additional special configuration that I don't know to keep the third single node working, even if in the read-only mode?
nope, there's not. as you mentioned it's designed that way
you're probably not doing yourselves a lot of favours by running things across datacentres like that. network issues are not kind on Elasticsearch due to it's distributed nature
Elasticsearch runs in distributed mode by default. Nodes assume that there are or will be a part of the cluster, and during setup nodes try to automatically join the cluster.
If you want your Elasticsearch to be available for only node without the need to communicate with other Elasticsearch nodes. It works similar to a standalone server. To do this we can tell Elasticsearch to work in local only (disable network)
open your elasticsearch/config/elasticsearch.yml and set:
node.local: true