feat: Improve sampling algorithm

Author: medikoo

node/packages/aws-lambda-sdk/instrument/index.js

@@ -11,6 +11,7 @@ const sendTelemetry = require('./lib/send-telemetry');
 const flushSpans = require('./lib/auto-send-spans').flush;
 const filterCapturedEvent = require('./lib/filter-captured-event');
 const invocationContextAccessor = require('./lib/invocation-context-accessor');
+const isApiEvent = require('./lib/is-api-event');
 const pkgJson = require('../package');
 
 const serverlessSdk = require('./lib/sdk');
@@ -98,15 +99,34 @@ const reportResponse = async (response, context, endTime) => {
   await sendTelemetry('request-response', payloadBuffer);
 };
 
+let requestCounter = 0;
+let lastCounterResetTime = 0;
+
 const reportTrace = ({ isErrorOutcome }) => {
-  // Sample out 80% of traces in production mode if no warning or error event was reported
   const isSampledOut =
     (() => {
+      // This function determines if a trace should be sampled or not
+      //
+      // Do not sample when invocation ends with error
       if (isErrorOutcome) return false;
+      // Do not sample when in debug mode
       if (serverlessSdk._isDebugMode) return false;
+      // Do not sample when in dev mode
       if (serverlessSdk._isDevMode) return false;
+      // Do not sample when any error or warning event is captured
       if (capturedEvents.some(({ name }) => alertEventNames.has(name))) return false;
-      return Math.random() > 0.2;
+      const currentTime = Date.now();
+      if (currentTime - lastCounterResetTime > 1000) {
+        lastCounterResetTime = currentTime;
+        requestCounter = 0;
+      }
+      ++requestCounter;
+      // In case of API backed function sample out 3rd and following invocations,
+      // that happen in a frame of a second
+      if (isApiEvent()) return requestCounter > 2;
+      // In other cases do not sample 1st invocation, and sample out 90% of following invocations
+      if (requestCounter === 1) return false;
+      return Math.random() > 0.1;
     })() || undefined;
   const payload = (serverlessSdk._lastTrace = {
     isSampledOut,