Failover to Basic Search when Elasticsearch fails

Problem to solve

If Elasticsearch goes down then GitLab will throw a 500 error on search.

Further details

Proposal

Failover to the Basic Search when a connection cannot be made to Elasticsearch.

What does success look like, and how can we measure that?

1. Stop elasticsearch service
2. Search issues
3. Confirm that issues are returned without any error

Links / references

https://gitlab.zendesk.com/agent/tickets/105913

https://gitlab.zendesk.com/agent/tickets/105924

(Issues are for internal use only)