I have a service A that creates an email and sends it to a customer. The customer will receive the email and will, eventually, click on the link in the body to trigger service B.
How can I correlate two different and completely isolated services that are part of the same business process with sleuth?
Should I leave the span "opened" or is there a way to "embed" the trace id somehow on the email?
You can use asynchronous communication (http://cloud.spring.io/spring-cloud-sleuth/spring-cloud-sleuth.html#_asynchronous_communication) for example via a trace representation of the ExecutorService called the TraceableExecutorService (http://cloud.spring.io/spring-cloud-sleuth/spring-cloud-sleuth.html#_executor_executorservice_and_scheduledexecutorservice). You emit a completable future that will process the data in a separate thread. At some point you block and then you can retrieve the data. The trace representation of the ExecutorService will take care of passing of tracing data.
UPDATE: If however, these are completely two separate processes then I'd close the span and create a completely separate span the moment someone clicks on the link. You should never leave spans explicitly open. What will bind the 2 processes will be the trace id. Zipkin doesn't yet support these long living tasks in the best possible way from the point of view of the UI but there's some work in progress going on to improve it (via so-called linked spans)