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Another two important concepts of in context of distributed tracing are span receivers and samplers. Essentially, all spans (including start/stop time, key/value annotations, timelines, ..) should be persisted (or collected) somewhere. Span receiver is a collector within a process that is the destination of spans when a trace is running (it could be a console, local file, data store, ...). Apache HTrace provides span receivers for Apache HBase, Apache Flume and Twitter Zipkin. From other side, samplers allow to control the frequency of the tracing (all the time, never, probability driven, ...). Using the sampler is the way to minimize tracing overhead (or just amount of traces) by limiting them to particular conditions.

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