Jaeger
Distributed tracing platform — Uber 2015, CNCF graduated 2019. Tracks a request across dozens of microservices and visualises the latency at each hop.
Built on OpenTracing/OpenTelemetry standards. Components: agent (per-host UDP collector), collector, query, UI. Storage: ElasticSearch, Cassandra, Kafka as buffer. Sampling strategies to limit volume.
Competitor: Zipkin (Twitter, older, simpler), Tempo (Grafana), Datadog APM, Honeycomb. Jaeger has lost momentum to Grafana Tempo in new deployments but dominates legacy CNCF stacks.