consider eliding span.sync=false because it is the typical case

tl;dr: Setting span.sync to true or false on every APM span results in unhelpful UI noise in Kibana. Let’s only set it when it is “interesting”. For Node.js the typical case is that spans are async (most traced work span event loop async operations). That leaves “span.sync==true” as the “interesting” case.

The span.sync field can be true, false, null or not set per the api-server spec.

Here is the Kibana code that adds a SyncBadge to spans in the Waterfall view and the code for the SyncBadge element. I.e. sync: true will showing “blocking” in the ui, sync: false will show “async”, sync: null (or not specified) will show nothing. E.g.:

From chat discussion (this came up when discussing elastic/apm-agent-nodejs#1878):

“Felix I remember that we’ve talked about the usefulness of having all spans marked as sync=false in Node.js. Setting sync=false makes the UI add a async flag next to the span name. But as async is the default in Node.js, it would probably make more sense to not set the sync flag in the default async case but only set sync=true when there’s a blocking operation, which makes the UI show a blocking label. This is what the RUM agent is doing.”

[trentm]

[Alan]

Also – I’d also lightly push back on the idea that Node’s default behavior is async. …

Alan, would you object to language that a “typical” Node.js traced span is async? I.e. we get away from using the term “default”.

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tl;dr: My votes are to (a) close this issue (i.e. have the Node.js APM agent report what it knows and not attempt to only report what it thinks is “interesting”) and (b) continue with https://github.com/elastic/kibana/issues/109661 (though I would say “when interesting”, rather than “when relevant”).

• AFAICT the “sync” field isn’t discussed in the APM agent specs at all. The Python async examples above show that it might be useful to at some point to have an cross-agent discussion clarifying span.sync. I don’t think we need to get into it now, however. (FWIW, my naive read of the Python APM agent code is that span.sync is set False when asyncio is used, and otherwise is almost always left blank, i.e. almost never set True.)
• I think the “async” badge in the UI on almost all spans in Node.js traces is visual clutter, so I think https://github.com/elastic/kibana/issues/109661 is a good idea.
• Note that the “blocking” badge on all the ES spans in @sqren’s screenshot in https://github.com/elastic/kibana/issues/109661 is a bug. Those ES spans are async. I hope to fix that in upcoming working.

[trentm]

My votes are to (a) close this issue …

Closing. Alan had no objections.