Tracing is simple, in theory. As a request flows from one component to another in a system, through ingress and egress points, tracers add logic where possible to perpetuate a unique trace ID that's generated when the first request is made. As a request arrives at a component along its journey, a new span ID is assigned for that component and added to the trace. A trace represents the whole journey of a request, and a span is each individual hop along the way, each request. Spans may contain tags, or metadata, that can be used to later contextualize the request. Spans typically contain common tags like start timestamps and stop timestamp, though it's easy to associate semantically relevant tags like an a business entity ID with a span.

Spring Cloud Sleuth (org.springframework.cloud:spring-cloud-starter-sleuth), once added to the CLASSPATH, automatically instruments common communication channels:

requests over messaging technologies like Apache Kafka or RabbitMQ (or any other Spring Cloud Stream binder

HTTP headers received at Spring MVC controllers

requests that pass through a Netflix Zuul microproxy

requests made with the RestTemplate, etc.