"ELK" is the acronym for three open source projects: Elasticsearch, Logstash, and Kibana. Elasticsearch is a search and analytics engine. Logstash is a server‑side data processing pipeline that ingests data from multiple sources simultaneously, transforms it, and then sends it to a "stash" like Elasticsearch. Kibana lets users visualize data with charts and graphs in Elasticsearch.
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This sounds like a question that can be answered by pointing to the ELK documentation. Related: https://devops.stackexchange.com/questions/2943/elk-micro-service-and-network-latency-monitoring/2952#2952