Elasticsearch 2.3.2 with PeopleSoft was introduced in latest Peopletools release 8.55 with patch level 11 which will eventually replace Secure Enterprise Search as a search feature for Peoplesoft. Single Elasticsearch Cluster can be used with Multiple PeopleSoft Environments as long as all of the systems are on same tools release (though the patch can be different).
Elasticsearch is a search engine based on Lucene. It provides a distributed, multitenant-capable full-text search engine with an HTTP web interface and schema-free JSON documents. Elasticsearch is developed in Java and is released as open source under the terms of the Apache License. (Ref: https://www.elastic.co).
As per Oracle documentation, with the integration of Elasticsearch, PeopleSoft Search Framework builds on the existing search functionality features, and adds new features to enhance the end user experience. While the processes of creating queries, creating search definitions and categories, deploying search definitions and categories, and viewing search results, which you are familiar with, are not changed in the PeopleSoft implementation of Elasticsearch, Search Framework has enhanced the process of crawling, processing attachments, and indexing data.
Elasticsearch is typically deployed as a cluster of nodes to provide a scalable search service, which is continuously operational. PeopleSoft Search Framework integrates with Elasticsearch to allow for high availability, failover, and scalable search deployments for PeopleSoft users. For small scale PeopleSoft implementations and high availability is not a requirement, it can be deployed in single node setup as well.
Elasticsearch can be deployed using DPK as well as manually in case of issues with DPKs. Please refer to Elasticsearch Home Page (Doc ID 2205540.2) for more details on Installation and Implementation, Moving from SES to Elasticsearch And Troubleshooting and Known Issues.