confusion regarding kong.tracing:active_span() API

[khaled4vokalz]

I have a custom plugin that has access and header_filter phase defined. Whenever I try to to get the active span using the kong.tracing:active_span() API, it always gets the root span instead of the actual plugin phases' span. Is this behavior expected.

Here's my Jaeger UI view of the trace, all the custom attributes I'm trying to add to the active_span all of those are getting added to the root span (kong). I am a bit unsure how this is supposed to work.. Is it even possible to add some custom attributes to an existing span? like e.g. (kong.access.plugin.proxy-cache). I just don't want to create some new spans to add some attributes, there'll be too many duplicates...

[khaled4vokalz]

From the code it seems like the tracing span is set to be the active span, but it doesn't look like from the plugin itself that span is returned, somehow it get's released to the parent span. 🤔

[khaled4vokalz]

okay, found it.... this is fixed in 3.9.0 f2adaed .... But personally it looks like could've been backported. I am currently on 3.3.1, that's a huge jump for me to 3.9.0.

@samugi , does it sound okay to backport it? I can create a PR if you want...

[samugi]

@khaled4vokalz hi, thanks for opening this discussion. The commit you linked is actually something in between a fix and a change in behavior: the active_span() function was not affected as it was already working fine, however the spans were not set as active upon creation (which is allowed), and this resulted in the behavior you describe, where the only active span returned was the root span.

Because of the small change in behavior, changes like this are usually not backported to avoid producing unexpected impact on users who might be relying on the existing behavior. I would recommend upgrading to the latest version of Kong in this case.
Let me know if you have any doubts. Thank you!

[khaled4vokalz]

@samugi ,
Thanks for the explanation. Even though always returning the root span doesn't look like a correct behavior to me. I know people may have already relied upon that, but any attributes added to the active_span from a plugins access/header_filter etc. phases would result in being added to the root span always which doesn't look like an expected behavior.

But I guess what you're saying is right that, users may have already started depending on that.

[klaude]

Greetings,

Count me among those that relied on kong.tracing.active_span() to always return the root span. What's the best way to get the root span now? The kong.tracing reference documentation isn't clear on this. Thanks!

[samugi]

hey @klaude , I would like to provide a better way to do this via the PDK, but for now please try:

local root_span = ngx.ctx.KONG_SPANS and ngx.ctx.KONG_SPANS[1]

please note: this may stop working in the future and I advice going with a PDK-provided function as soon as it's available. Thank you!

[klaude]

Hi, @samugi. Thanks for the fast response! That works perfectly. We'll do this for now and look for a PDK update.