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

BEGIN_COMMIT_OVERRIDE
chore: Thread FDv1 fallback TTL through polling and streaming data sources
END_COMMIT_OVERRIDE

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

Overview
Extends FDv2 polling and streaming so FDv1 fallback directives carry optional TTL (x-ld-fd-fallback / x-ld-fd-fallback-ttl, plus in-band protocolFallbackTTL on goodbye). Result builders now take a shared FallbackDirective instead of a bare boolean, and header TTL is interpreted in seconds and converted to milliseconds.

Orchestration and data manager: FDv2DataSource checks fdv1Fallback on any initializer result (including transfer-none / HTTP 304-style payloads) and stops the initializer chain when the server directs FDv1. FDv2DataManagerBase no longer overwrites the stored selector when a none payload arrives without state, so a prior full payload’s basis survives 304/none confirmations.

Polling: Goodbye paired with a fallback directive is surfaced as terminal_error so the orchestrator moves off FDv1; recoverable errors with fallback skip initializer retries. Streaming: Open headers defer fallback until the next queued result; reconnect without fallback clears pending/committed fallback; ping-triggered polls merge deferred TTL without mutating the handler’s result object.

CI package size limit for @launchdarkly/js-client-sdk-common is raised from 39300 to 44000 bytes.

^{Reviewed by Cursor Bugbot for commit 18c0cfd. 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: 41502 bytes
Compressed size limit: 44000
Uncompressed size: 224541 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: 32444 bytes
Compressed size limit: 34000
Uncompressed size: 116232 bytes

github-actions · 2026-07-16T20:32:37Z

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179887 bytes
Compressed size limit: 200000
Uncompressed size: 833430 bytes

joker23 · 2026-07-16T21:56:04Z

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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 · 2026-07-20T16:54:55Z

  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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View in Devin Review to see 2 additional findings.

tanderson-ld · 2026-08-06T13:51:04Z

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

kinyoklion · 2026-08-12T20:22:41Z

+    // A 'none' payload (e.g. an HTTP 304, or a streaming 'none' intent)
+    // carries no state -- it confirms existing data is still current and
+    // must not clear a selector already obtained from a prior payload.
+    if (payload.type !== 'none') {


I think we really need to verify that an intent of none would never change the selector. Because if it did, then I assume we could get drift in the value used for the basis and then trigger a full transfer on a reconnect.

An aside is that a 304 in a browser is a replay, as it is in RN, but maybe it isn't for node client.

In that the HTTP library in both cases re-presents the cached values.

…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

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

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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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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

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Comment thread packages/shared/sdk-client/src/datasource/fdv2/PollingInitializer.ts

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joker23 added 12 commits August 13, 2026 10:34

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

01d0932

…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

966bf2e

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

15a1a11

fix: fallback variables need to be read on stream errors

0f811c2

fix: error responses are not propagated in RN eventsource

f992d63

refactor: revert the changes for react-native

413d599

chore: bot comment

35372da

chore: pr comments

2485fc3

chore: extending resolveFallback to take in incoming directives

b7bd0b9

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

chore: bot comments

e25be5a

chore: addressing PR comments

95d33f4

chore: addressing bot comment

92a909d

joker23 force-pushed the skz/SDK-2687/fdv1-fallback-fix-fallback-ttl-source-threading branch from 0f10c0e to 92a909d Compare August 13, 2026 14:34

chore: bumping size limit again

18c0cfd

joker23 merged commit 65c543f into main Aug 13, 2026
56 checks passed

joker23 deleted the skz/SDK-2687/fdv1-fallback-fix-fallback-ttl-source-threading branch August 13, 2026 14:59

github-actions Bot mentioned this pull request Aug 13, 2026

chore: release main #1855

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