⚗️ Partial view updates (experimental)

[mormubis]

Motivation

Implement the "RUM Event Format Limitation" RFC (partial view updates) to reduce bandwidth by sending only changed fields in subsequent view events. When the partial_view_updates experimental feature flag is enabled, the SDK sends the first event per view.id as a full view event, and all subsequent updates as view_update events containing only changed fields.

Changes

Phase 1 — Foundation Types & Feature Flag:

• Added PARTIAL_VIEW_UPDATES to ExperimentalFeature enum
• Added VIEW_UPDATE to RumEventType and RawRumViewUpdateEvent interface
• Added VIEW_UPDATE to RawRumEvent union and modifiableFieldPathsByEvent

Phase 2 — Diff Engine:

• Created viewDiff.ts with computeViewDiff() and createViewDiffTracker()
• Implements all RFC update strategies: MERGE, REPLACE, APPEND, DELETE
• 21 unit tests covering all strategies and edge cases

Phase 3 — Pipeline Integration:

• Modified viewCollection.ts to conditionally emit view or view_update
• Protected view_update from beforeSend dismissal (same as view)
• Added VIEW_UPDATE to domain context type mapping and feature flag assembly hook
• Resilient degradation: diff failures fall back to full view event
• 8 new unit tests across viewCollection, assembly, and featureFlagContext

Phase 4 — E2E Testing:

• 6 Playwright E2E scenarios validating the complete feature
• Tests: event flow, document_version ordering, feature flag toggle, navigation lifecycle, required fields, view end

All changes are gated behind enableExperimentalFeatures: ['partial_view_updates']. When the flag is OFF, behavior is identical to the current SDK.

Test instructions

# Unit tests
yarn test:unit

# E2E tests (requires init first)
yarn test:e2e:init
yarn test:e2e -g "partial view"

# Enable in an app
datadogRum.init({
  enableExperimentalFeatures: ['partial_view_updates'],
  // ...other config
})

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

🤖 Generated with Claude Code

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	169.55 KiB	172.23 KiB	+2.68 KiB	+1.58%	✅
Rum Profiler	4.29 KiB	4.29 KiB	0 B	0.00%	✅
Rum Recorder	24.71 KiB	24.71 KiB	0 B	0.00%	✅
Logs	56.62 KiB	56.66 KiB	+46 B	+0.08%	✅
Flagging	944 B	944 B	0 B	0.00%	✅
Rum Slim	126.19 KiB	128.79 KiB	+2.60 KiB	+2.06%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Pending...

🧠 Memory Performance

Pending...

🔗 RealWorld

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 59.52%
• Overall Coverage: 77.03% (-0.08%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 079b043 | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[mormubis]

Why is view_update explicitly skipped here, but not view?

view_update gets an explicit guard because these events are emitted by SDK-internal timers and observers — user code never triggers them, so handlingStack is never set on their domain context. Without this guard, they'd go through getSourceUrl() only to hit computeStackTrace({ stack: undefined }) and return SKIPPED anyway. The explicit guard makes the intent clear and short-circuits pointless work.

view events don't need the same guard because they can have a meaningful handlingStack — specifically when a user explicitly calls datadogRum.startView(), which captures the call stack. In that case getSourceUrl() can resolve a real file URL and the view event legitimately receives source code context. When no handlingStack is present (the common case), it naturally returns SKIPPED via the contextByFile.get(url) miss at the end.