Elasticsearch is probably the most commonly used in-memory database. At least, if we narrow the scope to self-hosted databases. It is designed for many other scenarios, and it can be used to store (almost) any type of data. As such, it is almost perfect for storing logs which could come in many different formats. Given its flexibility, some use it for metrics as well and, as such, Elasticsearch competes with Prometheus. We'll leave metrics aside, for now, and focus only on logs.
The EFK (Elasticsearch, Fluentd, and Kibana) stack consists of three components. Data is stored in Elasticsearch, logs are collected, transformed, and pushed to the DB by Fluentd, and Kibana is used as UI through which we can explore data stored in Elasticsearch. If you are used to ELK (Logstash instead of Fluentd), the setup...