Traces and Spans

Parent and child spans

A trace represents one end-to-end request and is identified by a 16-byte trace ID. It consists of spans: named, timed operations (an HTTP request handled, a database query, a call to another service), each with an 8-byte span ID and the ID of its parent span. The first span is the root. Span kind describes the role: SERVER (handles an incoming request), CLIENT (makes an outgoing call), INTERNAL, PRODUCER and CONSUMER (messaging). Backends draw traces as waterfall timelines.

Following one request

Spans, their attributes and propagated context make it possible to follow a request across services.

A checkout trace as a waterfall

Illustrative timings.

trace 4bf92f3577b34da6a3ce929d0e0e4736                       0ms        500ms       1000ms
POST /checkout            [SERVER] frontend          |==========================================| 980 ms
  GET /cart               [CLIENT] frontend          |===|                                         70 ms
    GET /cart             [SERVER] cart-service      |==|                                          55 ms
  POST /payments          [CLIENT] frontend              |==================================|      780 ms
    POST /payments        [SERVER] payment-service       |=================================|       760 ms
      SELECT accounts     [CLIENT] payment-service       |=|                                       20 ms
      POST /charge        [CLIENT] payment-service          |=============================|        700 ms  <- slow

A parcel tracking history

Each scan of a parcel at a depot is a span; the full tracking history from sender to recipient is the trace.

Quick check: Which span kind describes a service making an outgoing HTTP call?

• SERVER
• CLIENT
• INTERNAL
• CONSUMER