Finish time of child Span (ChildOf reference)

[lookfwd]

Wondering about Child D from here. it's a bit weird, isn't it?

[eyjohn]

To me, it comes down to whether the data model sets a constraint on:

A child span can not outlive its parent

I do not see any references to this in the specification, and I would suggest that if this is not a requirement that it actually be explicitly stated that this is not guaranteed, rather than some-one inferring it from the diagram.

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As a side-point, I do see the appeal of adding such a constraint on the data-model as it does capture the nature of request/response based systems well, i.e. you can guarantee that all child requests have returned by the time the response gets sent back (timeouts + gradual degradation being the exception perhaps).

However in-practice, enforcing such a constraint in the framework is a bit tricky, especially given that say clock skew between two processes can invalidate this when looking at timestamps. Likewise it imposes a potentially difficult implementation of the tracing model. As such I would suggest this be seen more as a Best Practice rather than data model constraint.

[yurishkuro]

There is no requirement that child finishes before parent. The most common use case for this is when parent is the client span and child is the server span. The client may have a timeout that would cause the parent client span to be closed before server completes processing of the request.

In other words, childOf reference implies parent's dependency on the outcome (and one of the outcomes is "server did not finish in time"), but not the causal relationship child.end happens-before parent.end

[eyjohn]

The client may have a timeout that would cause the parent client span to be closed before server completes processing of the request.

This as interesting example and I think it would be great to explicitly include the characteristic and example in the reference.

[jmilkiewicz]

Wouldn't be the same for Child Span A and Child Span B where the former is considered a parent?