[No QA] Add SEQUENTIAL_QUEUE.md

[adhorodyski]

@mountiny

Explanation of Change

Adds contributingGuides/SEQUENTIAL_QUEUE.md, an observational reference document for the SequentialQueue subsystem — the offline-first WRITE queue that serialises every mutating API call.

Documentation-only: the new doc plus one cspell.json word entry. No runtime code changes.

The doc mirrors the style and scope of the sibling NETWORK_STATE_DETECTION.md (problem → how it works today → sharp edges per block) and refers to code by module and function names, not line numbers, so it survives refactors. It covers:

• The coordinator (SequentialQueue) and its 7-variable state machine
• The lifecycle of one request, and restart-recovery of a persisted queue
• PersistedRequests — the durable store and its in-memory-authoritative inversion pattern
• Where a request actually hits disk: push() awaits the Onyx disk commit before flushing, the remaining fire-and-forget persist windows, and the null/MemoryOnlyProvider/retry caveats on the commit handle
• RequestThrottle — jittered exponential back-off and the give-up signal
• QueuedOnyxUpdates / queueFlushedData — the two anti-flicker buffers
• API.write / API.read / makeRequestWithSideEffects public contract and the inbound consumers that drive the queue
• The error-handling ladder (per error class: retries, Onyx data applied, modal)
• Offline behaviour, pause / data-gap sync, multi-tab leader election
• The middleware chain boundary and conflict resolution (the twice-evaluated resolver)
• Test coverage as evidence of the intended contract

Fixed Issues

$ #93422
PROPOSAL:

Tests

1. Render contributingGuides/SEQUENTIAL_QUEUE.md in a Markdown viewer (GitHub preview, VS Code, etc.) and verify headings, code blocks, and internal anchor links render correctly.
2. Confirm no broken relative links (the doc references sibling files in contributingGuides/ that already exist).

• Verify that no errors appear in the JS console

Offline tests

N/A

QA Steps

Same as tests

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
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  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
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Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[melvin-bot]

@arosiclair Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

[adhorodyski]

@codex review

[mountiny]

Reviewer Checklist

• I have verified the author checklist is complete (all boxes are checked off).
• I verified the correct issue is linked in the ### Fixed Issues section above
• I verified testing steps are clear and they cover the changes made in this PR
  • I verified the steps for local testing are in the Tests section
  • I verified the steps for Staging and/or Production testing are in the QA steps section
  • I verified the steps cover any possible failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
• I checked that screenshots or videos are included for tests on all platforms
• I included screenshots or videos for tests on all platforms
• I verified that the composer does not automatically focus or open the keyboard on mobile unless explicitly intended. This includes checking that returning the app from the background does not unexpectedly open the keyboard.
• I verified tests pass on all platforms & I tested again on:
  • Android: HybridApp
  • Android: mWeb Chrome
  • iOS: HybridApp
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• If there are any errors in the console that are unrelated to this PR, I either fixed them (preferred) or linked to where I reported them in Slack
• I verified proper code patterns were followed (see Reviewing the code)
  • I verified that any callback methods that were added or modified are named for what the method does and never what callback they handle (i.e. toggleReport and not onIconClick).
  • I verified that comments were added to code that is not self explanatory
  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified any copy / text shown in the product is localized by adding it to src/languages/* files and using the translation method
  • I verified all numbers, amounts, dates and phone numbers shown in the product are using the localization methods
  • I verified any copy / text that was added to the app is grammatically correct in English. It adheres to proper capitalization guidelines (note: only the first word of header/labels should be capitalized), and is either coming verbatim from figma or has been approved by marketing (in order to get marketing approval, ask the Bug Zero team member to add the Waiting for copy label to the issue)
  • I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
  • I verified the JSDocs style guidelines (in STYLE.md) were followed
• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I verified that this PR follows the guidelines as stated in the Review Guidelines
• I verified other components that can be impacted by these changes have been tested, and I retested again (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar have been tested & I retested again)
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• If any new file was added I verified that:
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  • The style can't be created with an existing StyleUtils function (i.e. StyleUtils.getBackgroundAndBorderStyle(theme.componentBG)
• If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
• If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar is modified, I verified that Avatar is working as expected in all cases)
• If the PR modifies a component related to any of the existing Storybook stories, I tested and verified all stories for that component are still working as expected.
• If the PR modifies a component or page that can be accessed by a direct deeplink, I verified that the code functions as expected when the deeplink is used - from a logged in and logged out account.
• If the PR modifies the UI (e.g. new buttons, new UI components, changing the padding/spacing/sizing, moving components, etc) or modifies the form input styles:
  • I verified that all the inputs inside a form are aligned with each other.
  • I added Design label and/or tagged @Expensify/design so the design team can review the changes.
• If a new page is added, I verified it's using the ScrollView component to make it scrollable when more elements are added to the page.
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• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.
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Screenshots/Videos

Android: HybridApp

Android: mWeb Chrome

iOS: HybridApp

iOS: mWeb Safari

MacOS: Chrome / Safari

[Copilot]

Pull request overview

Adds an internal reference document describing the SequentialQueue subsystem (offline-first serialized WRITE queue), intended as a sibling to existing architecture docs like NETWORK_STATE_DETECTION.md.

Changes:

• Added contributingGuides/SEQUENTIAL_QUEUE.md documenting SequentialQueue architecture, lifecycle, collaborators, edge cases, and test “contract” references.
• Updated cspell.json to whitelist new technical terms used by the documentation (beforeunload, reauthenticating).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cspell.json	Adds new allowed terms needed for the new guide’s vocabulary.
contributingGuides/SEQUENTIAL_QUEUE.md	New in-depth subsystem documentation for SequentialQueue behavior, boundaries, and known sharp edges/open questions.

---

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: 98675633f8

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[adhorodyski]

@MelvinBot review this PR for being true to reality. Can you confirm all the claims from the new document?

[adhorodyski]

Let me also update the PR since #91734 got merged over the weekend.

[adhorodyski]

@mountiny as you can read in the PR explanation there are some open questions for now that I couldn't address yet/guess:

An Open Questions (needs maintainer confirmation) appendix with 5 intent-ambiguous items

[mountiny]

@MelvinBot Can you do a thorough review of this PR?

[mountiny]

@codex review

[MelvinBot]

Review — [No QA] Add SEQUENTIAL_QUEUE.md

Docs-only (one new reference doc + one cspell.json word). No runtime code, so the only real risk is accuracy. I fact-checked the doc's substantive claims against the source on this branch (99ac549) — async ordering, the state machine, middleware registration order, the error ladder, exact constants, the preserved-keys allowlist, the "dead code" / "stale comment" observations, and the test-coverage map.

Verdict: accurate and genuinely useful. ~60 specific claims checked; all but one verify cleanly, including subtle ones (the 8-vs-7 state-var grouping, the four resolveIsReadyPromise sites, the three-legged all-empty guard, clear()'s untracked bare ongoing-key write, the exact 15-key allowlist count, and that persistWhenOngoing is never assigned in production). The Open Questions appendix holds up — the dead SaveResponseInOnyx guard, the stale "must be last" comment (API/index.ts:53 with FraudMonitoring added after), and the vestigial persistWhenOngoing are all real and worth a maintainer's eyes.

One correction needed

getLength() does not gate read-blocking. The doc says it twice — line 175 ("API.index reads this to decide whether a READ must wait for writes") and line 325 ("getLength() (read-only) to decide read-blocking"). In reality waitForWrites always returns waitForSequentialQueueIdle(); getLength() only feeds a log line:

function waitForWrites(command) {
    if (getPersistedRequestsLength() > 0) {
        Log.info(`[API] '${command}' is waiting on ${getPersistedRequestsLength()} write commands`);
    }
    return waitForSequentialQueueIdle();   // <-- always awaited, regardless of length
}

src/libs/API/index.ts:239-243

The actual read-blocking decision is made by isReadyPromise (it starts resolved, so a read proceeds immediately when no write is pending) — which the doc already explains well elsewhere. Suggest rewording both spots so getLength() isn't presented as the gate. The "counts the ongoing request → getLength() === 0 means truly idle" half is correct and worth keeping.

Optional nitpicks (non-blocking)

• Line 514 / line 196 etc. describe Reauthentication as throwing 'Failed to reauthenticate'. It throws new Error('Failed to reauthenticate') (Middleware/Reauthentication.ts:117) — the string is the message. Fine as shorthand; flagging only for completeness.
• Sibling links (NETWORK_STATE_DETECTION.md, philosophies/OFFLINE.md) both resolve. 👍

Nice work — this is a high-fidelity reference. Happy to approve once the getLength() wording is tweaked.

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 99ac5490e6

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[adhorodyski]

@MelvinBot thanks — both confirmed and fixed in 31d1f51:

• getLength() read-blocking: removed the claim and corrected the API.index consumer entry — it doesn't gate reads; waitForWrites always awaits waitForSequentialQueueIdle() (isReadyPromise) regardless of the count.
• Reauthentication: now reads new Error('Failed to reauthenticate') in both spots.

[adhorodyski]

I think there are now more and more open questions in the doc that need to be addressed before merging.

[adhorodyski]

Resolved all threads I've been interested in, the doc reads well for me.

[OlimpiaZurek]

LGTM, just a few more Codex findings:

[P2] SEQUENTIAL_QUEUE.md#L5 overstates the delivery guarantee. The overview says every change is delivered “once” / exactly once, but the doc itself later describes duplicate-send windows during mid-flush leadership changes and crash/retry edges. The code also relies on retry/re-drive behavior, not a client-side exactly-once lock. I’d rephrase this as “sent serially and deduplicated/reconciled where needed” rather than “delivered once.”
Source: SEQUENTIAL_QUEUE.md#L5, local evidence around promotion/retry in PersistedRequests.ts#L444.

[P2] SEQUENTIAL_QUEUE.md#L92, #L132, and #L329 describe push() arming isReadyPromise before all offline early-outs, but the code does not. In push(), if the app is already offline, it awaits persistence and returns before setIsReadyPromisePending() is called. The doc should distinguish “offline at entry” from “goes offline during the awaited persist.” Otherwise readers may think offline writes arm and release the READ gate, when they actually leave it untouched.
Source: SequentialQueue.ts#L572.

[P3] SEQUENTIAL_QUEUE.md#L105 and #L147 conflict on whether a crash can leave the same request both queued and ongoing. The restart section says that state can happen and explains head dedupe; the PersistedRequests section says the atomic multiSet prevents it. If dedupe is only defensive/legacy protection, the doc should say that. If the duplicate state is still possible, line 147 should be softened.
Source: SEQUENTIAL_QUEUE.md#L105 and SEQUENTIAL_QUEUE.md#L147.