commit 3d5f6d857462fbc45adf528768fa7b6ff75934fa
author Vadim Shtayura <vadimsh@chromium.org> Fri Aug 02 22:13:26 2024
committer LUCI CQ <infra-scoped@luci-project-accounts.iam.gserviceaccount.com> Fri Aug 02 22:13:26 2024
tree a6eb40106474acd417463f1f91696aea69e7aa3b
parent a4ea684ac4ad64e7137327186b3e623bca8880ba

[server] Sample requests even if the parent trace is unsampled.

This affects only non-serverless code path (basically just GKE).

It seems Google Cloud Load Balancer adds X-Cloud-Trace-Context header
with "not sampled" flag to every request that doesn't have a trace
context yet. I'm not sure if this is a new behavior or it was always
like that.

Before this CL, we unconditionally followed the existing tracing
context sampling status (using our own sampler only if there were
no tracing context at all). As a result, most requests passing
GCLB were unsampled. Only requests that had a tracing header (with
sampling on) set *before* hitting GCLB were sampled, since GCLB
passed this header through. This header was set for requests done
by other LUCI services running on GAE.

Requests done by Cloud itself (Cloud Tasks, Cloud PubSub) never
have tracing context populated, so they were never sampled.

Now unsampled parent context is treated the same way as no context
at all: we'll use our local sampler to decide when to trace such
requests (resulting in 0.1qps of requests sampled with default
settings).

R=nqmtuan@google.com

Change-Id: Idb9314585e15e1eaa8f6d14c5f99e52a7ce416ff
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/5760710
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Tuan Nguyen <nqmtuan@google.com>