Add sync property to spans

[hmdhk]

Certain operations in some languages could be executed synchronously or asynchronously and it would be useful to have this indicated on the span.

For example in the frontend applications we can create http requests that are executed synchronously and this could have negative influence on the performance of the page.

We should add sync property to spans:

{
  "sync": Boolean (optional)
}

---

UPDATE

The sync property should be searchable therefore it should be indexed!

cc @elastic/apm-agent-devs

[simitt]

@jahtalab how urgent is this? Are there also UI changes necessary to leverage this information?

[hmdhk]

@simitt , I wouldn't say it's urgent but it's an important field so I'd prefer to get it in as soon as possible.

Yes, I will create an issue for Kibana once this property is defined in the apm-server schema.

[simitt]

Should this be searchable and/or aggregatable, so we have to index it?

[hmdhk]

Yes, it should be searchable, I have updated the description!

[sorenlouv]

In the future we will need to support spans that start and/or end after the parent transaction has ended. I have heard this mentioned as "async spans". Task queues is one example.
The kind of async spans suggested in this issue is a different concept. Have we thought about whether those two terminologies collide?

[roncohen]

for future reference, I've copied the following from a discussion that happened elsewhere:

I took a look at the OpenTracing span relationships to see if that relates to this async field: They have FollowsFrom and ChildOf. The difference is that with ChildOf:

the parent Span depends on the child Span in some capacity

whereas that's not the case for FollowsFrom: https://github.com/opentracing/specification/blob/master/specification.md#references-between-spans

I think the two concepts are unrelated: To my understanding, the OT concept "ChildOf" means the parent cannot end before the child end because it depends on something the child returns/produces. While our context.async denotes if the "main thread" is blocked while the child executes. In other words, when you have "ChildOf" async can be both true and false, but with FollowsFrom "async" isn't so useful.

[watson]

@roncohen I agree that the two concepts are unrelated, but I just wanted to correct a potential misunderstanding related to "ChildOf":

To my understanding, the OT concept "ChildOf" means the parent cannot end before the child end because it depends on something the child returns/produces.

As far as I understand it, this isn't a requirement of the "ChildOf" relationship. E.g. if you look at the valid timing diagrams for "ChildOf", you'll see that "Child Span D" ends after its parent span.

The example used here says that a parent span can have a deadline for when it expects a result from the child. If the child fails to deliver the result within the deadline the parent can choose to end before the child.

So the "ChildOf" relationship is just signaling intent rather than actual timing.