If an exit span was really fast, chances are that it's not relevant for analyzing latency issues. Therefore, agents SHOULD implement the strategy laid out in this section to let users choose the level of detail/cost tradeoff that makes sense for them. If an agent implements this strategy, it MUST also implement Collecting statistics about dropped spans.
Sets the minimum duration of exit spans. Exit spans with a duration lesser than this threshold are attempted to be discarded. If the exit span is equal or greater the threshold, it should be kept.
In some cases exit spans cannot be discarded. For example, spans that propagate the trace context to downstream services, such as outgoing HTTP requests, can't be discarded. However, external calls that don't propagate context, such as calls to a database, can be discarded using this threshold.
Additionally, spans that lead to an error can't be discarded.
| Type | GranularDuration |
| Default | 0ms |
| Central config | true |
If an agent implements span compression, the limit applies to the composite span.
For example, if 10 Redis calls are compressed into a single composite span whose total duration is lower than exit_span_min_duration,
it will be dropped.
If, on the other hand, the individual Redis calls are below the threshold,
but the sum of their durations is above it, the composite span will not be dropped.
The limitations are based on the premise that the parent_id of each span and transaction that's stored in Elasticsearch
should point to another valid transaction or span that's present in the Elasticsearch index.
A span that refers to a missing span via is parent_id is also known as an "orphaned span".
We only know whether to discard after the call has ended. At that point, the trace has already continued on the downstream service. Discarding the span for the external request would orphan the transaction of the downstream call.
Propagating the trace context to downstream services is also known as out-of-process context propagation.
Spans store an additional discardable flag in order to determine whether a span can be discarded.
The default value is true for exit spans and false for any other span.
According to the limitations,
there are certain situations where the discardable flag of a span is set to false:
- the span's
outcomefield is set to anything other thansuccess. So spans with outcome indicating an issue of potential interest are not discardable - On out-of-process context propagation
If the span's duration is less than exit_span_min_duration and the span is discardable (discardable=true),
the span_count.dropped count is incremented, and the span will not be reported.
We're deliberately using the same dropped counter we also use when dropping spans due to transaction_max_spans.
This ensures that a dropped fast span doesn't consume from the max spans limit.
To reduce the data loss, agents collect statistics about dropped spans. Dropped spans contribute to breakdown metrics the same way as non-discarded spans.