Setup database connections
There is a src_*() function to setup a connection to each
of the database backends. Each has their own unique set of parameters.
MongoDB - src_mongo()
CouchDB - src_couchdb()
Elasticsearch - src_elastic()
Redis - src_redis()
SQLite - src_sqlite()
Documentation details for each database:
MongoDB - https://docs.mongodb.com/
CouchDB - http://docs.couchdb.org/
Elasticsearch - https://www.elastic.co/guide/en/elasticsearch/reference/current
Redis - https://redis.io/documentation
SQLite/json1 - https://www.sqlite.org/json1.html