✨ make all sampling decisions deterministic by thomas-lebeau · Pull Request #4194 · DataDog/browser-sdk

thomas-lebeau · 2026-02-17T09:48:50Z

Motivation

Decouple sampling decisions from session storage. Currently, the session store computes and persists tracking types (e.g. rum=1, logs=0) alongside the session ID. This means sampling logic lives deep in the core session store, and both RUM and Logs depend on it to make sampling decisions at session creation time.

This PR moves sampling responsibility to each product (RUM, Logs), where it's computed on demand using the deterministic isSampled() function. Since sampling is deterministic based on session ID + sample rate, there's no need to persist the decision — it can be recomputed at any time.

Additionally, it corrects a monotone correlation issue in chained deterministic sampling: when the same hash is compared against two thresholds (session → replay/tracing/profiling), the effective child rate must be adjusted to parentRate × childRate / 100 so that childRate% of tracked sessions get the child feature.

Changes

Move sampler from rum-core to core

Moved sampler.ts from packages/rum-core/src/domain/sampler/ to packages/core/src/domain/ so both RUM and Logs can use it
Updated imports in tracer.ts and profilerApi.ts

Simplify core session manager and store

SessionManager no longer carries a TrackingType generic, productKey, or computeTrackingType callback
SessionStore always generates a session ID on creation (no more "not tracked = no ID" logic)
Removed SESSION_NOT_TRACKED checks and tracking type persistence from session state
SessionContext no longer has a trackingType field
hasSessionInCache() now checks for id instead of product key
isSessionInCacheOutdated() only compares session IDs (no more tracking type comparison)

Move sampling to each product

RUM (rumSessionManager.ts): computeTrackingType() now takes (configuration, sessionId) and calls isSampled() at query time
Logs (logsSessionManager.ts): Same pattern
Both stubs updated to generate a UUID for deterministic sampling consistency
Removed RUM_SESSION_KEY, LOGS_SESSION_KEY constants and hasValidRumSession/hasValidLoggerSession validators

Apply correction factor for chained deterministic sampling

Added correctedChildSampleRate(parentRate, childRate) in core/sampler.ts
Applied at 3 call sites: replay (rumSessionManager.ts), tracing (tracer.ts), profiling (profilerApi.ts)
Wrapped isProfilingSupported in mockable() to enable testing the profiler sampling path

Test instructions

yarn test:unit --spec packages/core/src/domain/sampler.spec.ts — unit tests for correctedChildSampleRate and isSampled
yarn test:unit --spec packages/rum-core/src/domain/rumSessionManager.spec.ts — deterministic sampling with correction for replay
yarn test:unit --spec packages/rum-core/src/domain/tracing/tracer.spec.ts — correction for tracing
yarn test:unit --spec packages/rum/src/boot/profilerApi.spec.ts — correction for profiling
yarn typecheck — no type errors

Correction tests use a MID_HASH_UUID (hash ~50.7%) with sessionSampleRate=60, childRate=60. The corrected rate (36) rejects while the uncorrected rate (60) would pass — tests fail if the correction is removed.

Checklist

Tested locally
Tested on staging
Added unit tests for this change.
Added e2e/integration tests for this change.
Updated documentation and/or relevant AGENTS.md file

datadog-datadog-prod-us1 · 2026-02-17T09:54:22Z

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 75.00%
• Overall Coverage: 76.93% (-0.17%)

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: f5ae67c | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

cit-pr-commenter-54b7da · 2026-02-17T11:31:57Z

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	171.86 KiB	172.00 KiB	+144 B	+0.08%	✅
Rum Profiler	4.67 KiB	4.67 KiB	0 B	0.00%	✅
Rum Recorder	24.88 KiB	24.88 KiB	0 B	0.00%	✅
Logs	56.12 KiB	56.38 KiB	+268 B	+0.47%	✅
Flagging	944 B	944 B	0 B	0.00%	✅
Rum Slim	127.61 KiB	127.63 KiB	+20 B	+0.02%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base CPU Time (ms)	Local CPU Time (ms)	𝚫%
RUM - add global context	0.004	0.0057	+42.50%
RUM - add action	0.0134	0.0244	+82.09%
RUM - add error	0.0128	0.0214	+67.19%
RUM - add timing	0.0027	0.0031	+14.81%
RUM - start view	0.0136	0.0167	+22.79%
RUM - start/stop session replay recording	0.0007	0.0009	+28.57%
Logs - log message	0.0159	0.0212	+33.33%

🧠 Memory Performance

Action Name	Base Memory Consumption	Local Memory Consumption	𝚫
RUM - add global context	26.40 KiB	26.65 KiB	+258 B
RUM - add action	115.40 KiB	114.36 KiB	-1.03 KiB
RUM - add timing	26.81 KiB	26.53 KiB	-287 B
RUM - add error	119.60 KiB	120.24 KiB	+652 B
RUM - start/stop session replay recording	25.54 KiB	25.29 KiB	-262 B
RUM - start view	502.40 KiB	510.54 KiB	+8.14 KiB
Logs - log message	45.28 KiB	46.48 KiB	+1.20 KiB

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BenoitZugmeyer · 2026-02-18T12:55:33Z

+        if (!session || session.id === 'invalid') {
+          return
+        }


thought: Those invalid session id is somewhat bothering, but I guess it'll be removed in a follow-up step?

I didn't think about it yet, but I'm taking note 👍

mormubis · 2026-02-19T11:51:16Z

-  const traceSampled = isSampled(session.id, configuration.traceSampleRate)
+  const traceSampled = isSampled(
+    session.id,
+    correctedChildSampleRate(configuration.sessionSampleRate, configuration.traceSampleRate)


IIUC, the new session manager will start regardless of the product and won't be tied to RUM. Hence, do we need child sample rates? Can we not have plain sample rates that are easier to understand?

Traces still depend on RUM, if rum don't start, we don't collect traces.
It's true that we could imagine a separate Trace product, but that's a separate topic.
currently we sample traces as: N% of collected resources will have traces. with deterministic sampling we will have N% of sampled session will have traces

Co-authored-by: Thomas Lebeau <thomas.lebeau@datadoghq.com>

- Change startRumSessionManagerWithDefaults helper to use sessionSampleRate: 100 and sessionReplaySampleRate: 100 as defaults - Remove redundant sessionSampleRate: 100 overrides from individual test cases that relied on always-tracked behavior

…rdown - Add deterministic sampling tests to rumSessionManager covering session tracking and replay sampling based on known high/low hash UUIDs - Move resetSampleDecisionCache() from sampler.spec.ts to the global forEach.spec.ts afterEach hook to ensure consistent cleanup across all test suites

…ampling - Add correctedChildSampleRate to compute the effective sample rate for child features (replay, tracing, profiling) given the parent session rate - Apply the correction factor (parentRate * childRate) / 100 at each sampling decision site so chained sampling produces the expected end-to-end probability

- Replace `{} as any` args with proper typed mocks in profilerApi test - Fix logsSessionManager stub assertion: session ID should be undefined since the stub no longer generates one

thomas-lebeau force-pushed the thomas.lebeau/v7-deterministic-sampling branch from 3878686 to f97da14 Compare February 17, 2026 11:29

thomas-lebeau changed the base branch from thomas.lebeau/v7-session-manager-from-scratch-clean to thomas.lebeau/remove-beta-encode-cookie-options February 17, 2026 11:30

thomas-lebeau marked this pull request as ready for review February 17, 2026 11:33

thomas-lebeau requested a review from a team as a code owner February 17, 2026 11:33

chatgpt-codex-connector Bot reviewed Feb 17, 2026

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Comment thread packages/rum-core/src/domain/rumSessionManager.ts Outdated

Comment thread packages/rum-core/src/domain/rumSessionManager.ts

thomas-lebeau force-pushed the thomas.lebeau/v7-deterministic-sampling branch from f97da14 to 28c6885 Compare February 17, 2026 11:43

thomas-lebeau force-pushed the thomas.lebeau/remove-beta-encode-cookie-options branch from e49d028 to 1ba1e48 Compare February 17, 2026 12:11

thomas-lebeau force-pushed the thomas.lebeau/v7-deterministic-sampling branch from 28c6885 to 45e1731 Compare February 17, 2026 12:13

Base automatically changed from thomas.lebeau/remove-beta-encode-cookie-options to v7 February 17, 2026 12:29

thomas-lebeau force-pushed the thomas.lebeau/v7-deterministic-sampling branch from 45e1731 to 5c33ff2 Compare February 17, 2026 13:15

thomas-lebeau force-pushed the v7 branch from eb57647 to ae370c4 Compare February 17, 2026 14:56

thomas-lebeau force-pushed the thomas.lebeau/v7-deterministic-sampling branch from e9f9893 to 31d3ba3 Compare February 18, 2026 07:19

thomas-lebeau changed the title ~~♻️ Move sampling out of session storage~~ ✨ make all sampling decisions deterministic Feb 18, 2026

BenoitZugmeyer reviewed Feb 18, 2026

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mormubis approved these changes Feb 19, 2026

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mormubis and others added 7 commits February 19, 2026 14:06

♻️ move sampling logic to core

54925e8

Co-authored-by: Thomas Lebeau <thomas.lebeau@datadoghq.com>

♻️ move sampling to each product

203c1f0

Co-authored-by: Thomas Lebeau <thomas.lebeau@datadoghq.com>

👌 remove sampling from logs session manager stub

dc7d420

👌 move test UUID constants to a common module

44a32ad

thomas-lebeau force-pushed the thomas.lebeau/v7-deterministic-sampling branch from cfa2fb0 to 44a32ad Compare February 19, 2026 13:07

✅ fix test setup and assertions after sampling refactor

cd9c51d

- Replace `{} as any` args with proper typed mocks in profilerApi test - Fix logsSessionManager stub assertion: session ID should be undefined since the stub no longer generates one

BenoitZugmeyer approved these changes Feb 19, 2026

View reviewed changes

Merge branch 'v7' into thomas.lebeau/v7-deterministic-sampling

f5ae67c

thomas-lebeau merged commit 32325c0 into v7 Feb 20, 2026
19 checks passed

thomas-lebeau deleted the thomas.lebeau/v7-deterministic-sampling branch February 20, 2026 08:22

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✨ make all sampling decisions deterministic#4194

✨ make all sampling decisions deterministic#4194
thomas-lebeau merged 9 commits intov7from
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