fix(tracing): ensure sampling decisions apply to correct trace ID

changelog/unreleased/kong/fix-tracing-sampling-rate.yml

@@ -0,0 +1,3 @@
+message: "**OpenTelemetry:** Improved accuracy of sampling decisions."
+type: bugfix
+scope: Plugin

kong/pdk/tracing.lua

@@ -575,12 +575,13 @@ local function new_tracer(name, options)
   -- @tparam number sampling_rate the sampling rate to apply for the
   -- probability sampler
   -- @treturn bool sampled value of sampled for this trace
-  function self:get_sampling_decision(parent_should_sample, sampling_rate)
+  function self:get_sampling_decision(parent_should_sample, plugin_sampling_rate)
     local ctx = ngx.ctx
 
     local sampled
     local root_span = ctx.KONG_SPANS and ctx.KONG_SPANS[1]
     local trace_id = tracing_context.get_raw_trace_id(ctx)
+    local sampling_rate = plugin_sampling_rate or kong.configuration.tracing_sampling_rate
 
     if not root_span or root_span.attributes["kong.propagation_only"] then
       -- should not sample if there is no root span or if the root span is
@@ -592,12 +593,7 @@ local function new_tracer(name, options)
       -- and Kong is configured to only do headers propagation
       sampled = parent_should_sample
 
-    elseif not sampling_rate then
-      -- no custom sampling_rate was passed:
-      -- reuse the sampling result of the root_span
-      sampled = root_span.should_sample == true
-
-    else
+    elseif sampling_rate then
       -- use probability-based sampler
       local err
       sampled, err = self.sampler(trace_id, sampling_rate)

spec/03-plugins/37-opentelemetry/04-exporter_spec.lua

@@ -243,6 +243,80 @@ for _, strategy in helpers.each_strategy() do
       end)
     end
 
+
+    describe("With config.sampling_rate unset, using global sampling rate: 0.5", function ()
+      local mock
+      local sampling_rate = 0.5
+       -- this trace_id is always sampled with 0.5 rate
+      local sampled_trace_id = "92a54b3e1a7c4f2da9e44b8a6f3e1dab"
+       -- this trace_id is never sampled with 0.5 rate
+      local non_sampled_trace_id = "4bf92f3577b34da6a3ce929d0e0e4736"
+
+      lazy_setup(function()
+        bp, _ = assert(helpers.get_db_utils(strategy, {
+          "services",
+          "routes",
+          "plugins",
+        }, { "opentelemetry" }))
+
+        setup_instrumentations("all", {}, nil, nil, nil, nil, sampling_rate)
+        mock = helpers.http_mock(HTTP_SERVER_PORT, { timeout = HTTP_MOCK_TIMEOUT })
+      end)
+
+      lazy_teardown(function()
+        helpers.stop_kong()
+        if mock then
+          mock("close", true)
+        end
+      end)
+
+      it("does not sample spans when trace_id == non_sampled_trace_id", function()
+        local cli = helpers.proxy_client(7000, PROXY_PORT)
+        local r = assert(cli:send {
+          method  = "GET",
+          path    = "/",
+          headers = {
+            traceparent = "00-" .. non_sampled_trace_id .. "-0123456789abcdef-01"
+          }
+        })
+        assert.res_status(200, r)
+
+        cli:close()
+
+        ngx.sleep(2)
+        local lines = mock()
+        assert.is_falsy(lines)
+      end)
+
+      it("samples spans when trace_id == sampled_trace_id", function ()
+        for _ = 1, 10 do
+          local body
+          helpers.wait_until(function()
+            local cli = helpers.proxy_client(7000, PROXY_PORT)
+            local r = assert(cli:send {
+              method  = "GET",
+              path    = "/",
+              headers = {
+                traceparent = "00-" .. sampled_trace_id .. "-0123456789abcdef-01"
+              }
+            })
+            assert.res_status(200, r)
+
+            cli:close()
+
+            local lines
+            lines, body = mock()
+            return lines
+          end, 10)
+
+          local decoded = assert(pb.decode("opentelemetry.proto.collector.trace.v1.ExportTraceServiceRequest", body))
+          assert.not_nil(decoded)
+          local scope_spans = decoded.resource_spans[1].scope_spans
+          assert.is_true(#scope_spans > 0, scope_spans)
+        end
+      end)
+    end)
+
     for _, case in ipairs{
       {true, true, true},
       {true, true, nil},