feat: Thread FDv1 fallback TTL through polling and streaming data sources by joker23 · Pull Request #1806 · launchdarkly/js-core

joker23 · 2026-07-16T20:31:11Z

This commit will add in fallback TTL reading support as well as allow RN native event source read headers.

NOTE: while working this PR, I found a lot of gaps in the RN eventsource. Internally tracked in ticket 2721 in sdk project.

Note

Medium Risk
Changes FDv2 data-source orchestration, selector lifetime, and when clients fall back to FDv1—behavior-critical paths with broad test coverage but real production impact if directives are misread.

Overview
Adds centralized FDv1 fallback directive parsing (x-ld-fd-fallback, x-ld-fd-fallback-ttl in seconds → ms, plus in-band protocolFallbackTTL on goodbye) and threads FallbackDirective through FDv2SourceResult helpers instead of a bare boolean.

Polling reads directives on every response; a goodbye paired with fallback is reported as terminal_error so the orchestrator switches off FDv2. Polling initializer skips retries when a recoverable error carries a fallback directive.

Streaming defers fallback from onopen headers until the next queued result (pendingFallback / resolveFallback / putWithFallback), merges deferred TTL into ping-triggered polls without mutating the poll result object, and clears committed/pending fallback on clean reconnect. Error, malformed JSON, ping failure, and error-filter paths can surface deferred fallback with TTL.

Orchestrator (FDv2DataSource) checks FDv1 fallback after all result handling so a transfer-none changeSet with fdv1Fallback: true stops the initializer chain. FDv2DataManagerBase no longer clears the selector when a none payload arrives (e.g. HTTP 304).

CI bumps @launchdarkly/js-client-sdk-common ESM size limit 39300 → 41500.

^{Reviewed by Cursor Bugbot for commit 0f10c0e. Bugbot is set up for automated code reviews on this repo. Configure here.}

github-actions · 2026-07-16T20:32:11Z

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26360 bytes
Compressed size limit: 29000
Uncompressed size: 129188 bytes

github-actions · 2026-07-16T20:32:30Z

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 41308 bytes
Compressed size limit: 41500
Uncompressed size: 224012 bytes

github-actions · 2026-07-16T20:32:36Z

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32476 bytes
Compressed size limit: 34000
Uncompressed size: 116188 bytes

github-actions · 2026-07-16T20:32:37Z

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 180103 bytes
Compressed size limit: 200000
Uncompressed size: 833386 bytes

joker23 · 2026-07-16T21:56:04Z

@cursor review

joker23 · 2026-07-17T14:02:30Z

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joker23 · 2026-07-17T16:56:13Z

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joker23 · 2026-07-17T21:06:05Z

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joker23 · 2026-07-20T15:56:53Z

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joker23 · 2026-07-20T16:38:03Z

@cursor review

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joker23 · 2026-07-20T16:54:55Z

  terminalError,
 } from './FDv2SourceResult';
-
-function getFallback(headers: { get(name: string): string | null }): boolean {


replacing with

js-core/packages/shared/sdk-client/src/datasource/fdv2/fallbackDirective.ts

Line 27 in e430b61

export function readFallbackDirective(headers: {

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.

…rces This commit will add in fallback TTL reading support as well as allow RN native event source read headers.

This will allow us to resolve directives that should override the fallback behavior such as goodbye or error

kinyoklion · 2026-08-05T22:10:27Z

+        // Recoverable error: save it and retry if attempts remain
        lastResult = result;

        if (attempt < maxRetries) {


We should be checking the fallback before the retries.

tanderson-ld · 2026-08-06T13:51:04Z

Probably makes sense to wait for @kinyoklion's review since he left a comment.

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joker23 force-pushed the skz/SDK-2687/fdv1-fallback-fix-fallback-ttl-source-threading branch from e4fa9cb to 1400310 Compare July 17, 2026 14:01

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joker23 commented Jul 17, 2026

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Comment thread packages/sdk/react-native/src/fromExternal/react-native-sse/EventSource.ts Outdated

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joker23 force-pushed the skz/SDK-2687/fdv1-fallback-fix-fallback-ttl-source-threading branch from 46de8d4 to 6604149 Compare July 17, 2026 16:56

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Comment thread packages/sdk/react-native/src/fromExternal/react-native-sse/EventSource.ts Outdated

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Comment thread packages/sdk/react-native/src/fromExternal/react-native-sse/EventSource.ts

joker23 force-pushed the skz/SDK-2687/fdv1-fallback-fix-fallback-ttl-source-threading branch from 29aefb7 to 73eeb30 Compare July 17, 2026 22:00

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joker23 marked this pull request as ready for review July 20, 2026 16:55

joker23 requested a review from a team as a code owner July 20, 2026 16:55

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joker23 added 8 commits July 22, 2026 11:15

feat: Thread FDv1 fallback TTL through polling and streaming data sou…

6b5e7d5

…rces This commit will add in fallback TTL reading support as well as allow RN native event source read headers.

ci: bump client common size limit

80e3d41

fix: RN event source does not capture headers in a timely manner

3ada250

fix: fallback variables need to be read on stream errors

7b48590

fix: error responses are not propagated in RN eventsource

17e856c

refactor: revert the changes for react-native

3a7388a

chore: bot comment

ff0a597

chore: pr comments

9d6f7f9

joker23 force-pushed the skz/SDK-2687/fdv1-fallback-fix-fallback-ttl-source-threading branch from e95c2f2 to 005c842 Compare July 22, 2026 15:16

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chore: extending resolveFallback to take in incoming directives

adfcb4d

This will allow us to resolve directives that should override the fallback behavior such as goodbye or error

joker23 force-pushed the skz/SDK-2687/fdv1-fallback-fix-fallback-ttl-source-threading branch from 005c842 to adfcb4d Compare July 22, 2026 16:07

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joker23 requested a review from tanderson-ld July 22, 2026 16:35

chore: bot comments

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kinyoklion requested changes Aug 5, 2026

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tanderson-ld approved these changes Aug 6, 2026

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chore: addressing PR comments

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chore: addressing bot comment

0f10c0e

joker23 requested a review from kinyoklion August 6, 2026 19:21

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