ElastAlert is an alerting project for elasticsearch maintained by YELP on GitHub : https://github.com/Yelp/elastalert
Why using two alerting modules?
While Opendistro alerting plugin is similar to Watcher and can use some complicated queries, ElastAlert is a much simpler and straight forward tool, specially with its multiple supported destinations where you can send your alerts. See documentation.
ElastAlert can be useful to build a first layer of logic where you can apply some complex monitoring set of rules using Opendistro alerting plugin afterwards. The necessity of this combination is something we will be discussing in 5.Alerting with ELK and 6.Building Detection Rules.
Cloning from repository
As root go to /etc/ an then and clone the repository and install the required packages :