Loki
Grafana Labs' open-source log storage/query engine — the direct open-source alternative to Elasticsearch when Elasticsearch's job is storing and querying logs (not general full-text search or analytics). Unlike Elasticsearch, Loki indexes only labels (service, container, level, etc.), not full log text — dramatically lighter on storage and CPU, at the cost of weaker full-text search across log bodies. Paired with Promtail (or the newer unified Grafana Alloy agent) as the log shipper — the role Filebeat/Logstash play for ELK, or that Fluent_Bit plays generically — and with Grafana for visualization, forming the "PLG stack" (Promtail + Loki + Grafana): the direct open-source alternative to the ELK/EFK stack (Elasticsearch + Logstash/Filebeat + Kibana).