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 how FDv2 sources signal and time FDv1 fallback (including goodbye vs terminal_error), which affects orchestrator source switching; scope is large but heavily tested.

Overview
Adds a shared FallbackDirective model and header/event parsers so FDv2 polling and streaming can carry fdv1Fallback plus fdv1FallbackTtlMs (from x-ld-fd-fallback / x-ld-fd-fallback-ttl, or in-band protocolFallbackTTL on goodbye).

Result builders (changeSet, interrupted, terminalError, goodbye) now take a directive object instead of a bare boolean; call sites and tests are updated accordingly. Polling reads the directive on every response and, when fallback is set alongside a goodbye event, emits terminal_error so the orchestrator switches off FDv2 instead of treating it as a benign goodbye. Streaming defers open-header directives until the next queued result, merges them into errors/ping paths without mutating ping handler objects, and clears pending/committed fallback state on reconnect and close. Polling initializer keeps fallback + TTL when retries exhaust.

CI package size limit for @launchdarkly/js-client-sdk-common is raised to 41500 bytes.

^{Reviewed by Cursor Bugbot for commit 20081c8. 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: 41238 bytes
Compressed size limit: 41500
Uncompressed size: 223057 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: 32439 bytes
Compressed size limit: 34000
Uncompressed size: 115992 bytes

github-actions · 2026-07-16T20:32:37Z

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 180100 bytes
Compressed size limit: 200000
Uncompressed size: 833166 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 · 2026-07-20T16:38:03Z

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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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tanderson-ld · 2026-07-21T14:01:17Z

+          fdv1FallbackTtlMs = pendingFallbackTtlMs;
+          resultQueue.put(changeSet(action.payload, true, undefined, undefined, pendingFallbackTtlMs));
+          pendingFallback = false;
+          pendingFallbackTtlMs = undefined;


Should this section use resolveFallback?

tanderson-ld · 2026-07-21T14:02:08Z

      case 'payload':
        logConnectionResult(true);
-        resultQueue.put(changeSet(action.payload, fdv1Fallback));
+        if (pendingFallback) {


Is this if check required? Is there a way to write it such that it just uses resolveFallback always?

tanderson-ld · 2026-07-21T14:03:49Z

+          fdv1Fallback = true;
+          resultQueue.put(terminalError(errorInfoFromUnknown(action.reason), true, ttlMs));
+          pendingFallback = false;
+          pendingFallbackTtlMs = undefined;


Same comment here. I thought the purpose of the resolveFallback was to remove duplication of this routine of consuming the pending fallback and clearing values.

tanderson-ld · 2026-07-21T14:06:31Z

+        const fallback = resolveFallback();
+        if (!result.fdv1Fallback && fallback.fdv1Fallback) {
+          result.fdv1Fallback = true;
+          result.fdv1FallbackTtlMs = fallback.fdv1FallbackTtlMs;


Is editing result appropriate? Seems like it shouldn't be mutated from a good practices standpoint.

tanderson-ld · 2026-07-21T14:11:44Z

Something feels off about how many spots have if checks on pending fallback and logic for moving things around and masking values on top of others. It just seems very scattered and likely a subtle tweak will mess up the logic. Is there a way to make it such that all results that go into the queue conform to some type / interface and fallback can be applied to that thing unconditonally, sometimes a no-op but sometimes a real fallback apply occurs. The pending mechanism could be something that can only apply once and then turns itself into a no-op or something.

…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 commented Jul 17, 2026

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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 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

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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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