Add loadSubset State Tracking and On-Demand Sync Mode #669

samwillis · 2025-10-12T18:19:30Z

Overview

This PR adds comprehensive loading status tracking to collections and live queries, enabling UIs to display loading indicators when more data is being fetched. It also introduces a new syncMode configuration option to control when collections load data - either eagerly on initial sync or on-demand as queries request it.

Problem

No Loading Indicators: When live queries pushed predicates down to source collections via syncMore, there was no indication that data was being loaded. This made it impossible to show proper loading states in the UI.
Always-Eager Loading: Collections always loaded all data immediately during initial sync, even when using predicate pushdown. There was no way to configure collections to only load data as it was requested by queries.

Solution

This PR implements a multi-layered loading state tracking system with configurable sync modes:

Loading State Tracking

Collection Subscriptions: Track their own loading state when requesting snapshots, with a public Subscription interface for external consumers
Collections: Track pending load promises and expose an isLoadingSubset property
Live Queries: Automatically reflect the loading state of their source collection subscriptions

Key Isolation Property: Each live query maintains its own loading state based on its own subscriptions. When a live query triggers loading via predicate pushdown, only that live query's isLoadingSubset becomes true. Other live queries that share the same source collection but don't need that specific snapshot remain unaffected.

Sync Modes

Collections can now be configured with two sync modes:

eager (default): Loads all data immediately during initial sync. loadSubset calls are bypassed.
on-demand: Only loads data as it's requested via loadSubset calls. Requires a loadSubset handler.

Changes

1. Renamed `syncMore`/`loadMore` to `loadSubset`

Files: packages/db/src/collection/sync.ts, packages/db/src/types.ts, packages/db/src/collection/subscription.ts

Renamed syncMore → loadSubset (collection method)
Renamed onLoadMore → loadSubset (sync config property)
Renamed OnLoadMoreOptions → LoadSubsetOptions (type)
Renamed syncOnLoadMoreFn → syncLoadSubsetFn (internal)
Renamed pendingLoadMorePromises → pendingLoadSubsetPromises (internal)

Rationale: "loadSubset" better reflects that this operation loads a filtered/limited subset of data, not just "more" data.

2. Added `syncMode` Configuration

Files: packages/db/src/types.ts, packages/db/src/collection/sync.ts

New syncMode property on collection config:

type SyncMode = 'eager' | 'on-demand'

interface CollectionConfig {
  syncMode?: SyncMode // defaults to 'eager'
  // ... other properties
}

Behavior:

eager (default): loadSubset is bypassed and returns undefined. All data is loaded during initial sync.
on-demand: loadSubset calls proceed normally. Data is only loaded as requested.

Validation:

If syncMode: 'on-demand' is set but no loadSubset handler is provided, throws CollectionConfigurationError with a helpful message
Validation occurs when the sync function is first called

3. Standardized `loadSubset` Return Type

Files: packages/db/src/collection/sync.ts, packages/db/src/types.ts

Changed to consistently return Promise<void> | undefined
Returns undefined when syncMode is 'eager' or no sync implementation is configured
Wraps synchronous loadSubset results in Promise.resolve()
Updated SyncConfigRes['loadSubset'] type signature

4. CollectionSubscription Status Tracking & Events

File: packages/db/src/collection/subscription.ts

Added comprehensive status tracking:

Status Property: status: 'ready' | 'loadingSubset' (readonly getter with private mutable field)
Concurrent Tracking: pendingLoadSubsetPromises: Set<Promise<void>>
Events:
- status:change - Emitted when status transitions
- status:ready - Emitted when entering ready state
- status:loadingSubset - Emitted when entering loading state
- unsubscribed - New event emitted when subscription is destroyed
Error Handling: Status returns to ready even on promise rejection
Cleanup: unsubscribe() emits unsubscribed event before clearing listeners

5. Generic `isLoadingSubset` for All Collections

Files: packages/db/src/collection/sync.ts, packages/db/src/collection/index.ts, packages/db/src/collection/events.ts

All collections now track their loading state:

Property: isLoadingSubset (boolean getter, not a method)
Tracking: pendingLoadSubsetPromises: Set<Promise<void>> in CollectionSyncManager
Events: loadingSubset:change event when state transitions
API: trackLoadPromise(promise: Promise<void>) method for internal coordination
Access: Made _sync public on Collection for internal use

6. Live Query Integration

Files: packages/db/src/query/live/collection-subscriber.ts, packages/db/src/query/live/collection-config-builder.ts

Live queries reflect loading state from their subscriptions:

CollectionSubscriber subscribes to subscription status:change events
Creates deferred promises when subscriptions enter loadingSubset state
Passes promises to result collection via liveQueryCollection.trackLoadPromise()
Result collection's isLoadingSubset reflects subscription loading states
Proper cleanup on unsubscribe (resolves pending promises)

Loading State Isolation: Each live query's subscriptions are independent. Query A triggering loadSubset doesn't affect Query B's isLoadingSubset status.

7. Subscription Interface for External Consumers

File: packages/db/src/types.ts

New public Subscription interface:

export interface Subscription extends EventEmitter<SubscriptionEvents> {
  readonly status: SubscriptionStatus
}

export type LoadSubsetOptions = {
  where?: BasicExpression<boolean>
  orderBy?: OrderBy
  limit?: number
  subscription?: Subscription // Optional, for sync implementation use
}

Purpose: Allows sync implementations to:

Track which subscription triggered a loadSubset call
Subscribe to subscription lifecycle events (e.g., unsubscribed)
Implement advanced caching/ref-counting based on subscription lifecycle

8. Reusable Event Emitter

File: packages/db/src/event-emitter.ts (new)

Extracted event emitter logic into a reusable base class:

Type-Safe: Generic EventEmitter<TEvents> with full type safety
Methods: on, once, off, waitFor
Protected: emitInner (for subclass use), clearListeners
Error Handling: Re-throws listener errors via queueMicrotask
Usage:
- CollectionEventsManager extends it and wraps emitInner with public emit
- CollectionSubscription extends it and uses emitInner internally

9. Fixed Local-Only Collection Types

File: packages/db/src/local-only.ts

Fixed mutation function typing issues:

Wrapper functions now accept broader UtilsRecord type parameters
Properly handles contravariance in function parameter types
Removed unnecessary type casts in return statement

10. Comprehensive Test Coverage

Files: packages/db/tests/collection-subscription.test.ts, packages/db/tests/collection.test.ts, packages/db/tests/query/live-query-collection.test.ts

Added extensive test suites:

CollectionSubscription: Status tracking, event emission, concurrent promises, error handling, cleanup
Collection.isLoadingSubset: Property tracking, event emission, concurrent loads, error handling
Live Query Integration: Result collection reflects subscription states, isolation between queries
All tests using loadSubset: Updated to use syncMode: 'on-demand'

11. Enhanced `setWindow` with Loading Awareness

File: packages/db/src/query/live/collection-config-builder.ts

The setWindow utility function now returns a value that indicates whether subset loading was triggered, allowing callers to wait for data loading to complete:

Return Type: true | Promise<void>

Returns true when isLoadingSubset is false after calling the window function - no loading was triggered
Returns Promise<void> when isLoadingSubset is true - loading was triggered and the promise resolves when loading completes

Implementation Details:

After calling windowFn() and maybeRunGraphFn(), checks this.liveQueryCollection?.isLoadingSubset
If loading is active, subscribes to the loadingSubset:change event
Returns a promise that resolves when isLoadingSubset becomes false
Automatically unsubscribes from the event once loading completes

Usage Pattern:

const result = liveQuery.utils.setWindow({ offset: 10, limit: 20 })

// Type guard pattern
if (result !== true) {
  // Loading was triggered, wait for it to complete
  await result
  console.log('Data loaded and window updated')
} else {
  // No loading needed, window updated immediately
  console.log('Window updated synchronously')
}

// Or simply always await (works with both return types)
const result = liveQuery.utils.setWindow({ offset: 10, limit: 20 })
if (result !== true) {
  await result
}

Test Coverage:

Test that setWindow returns true when no loading is triggered
Integration test that validates the full async flow with fake timers:
- Verifies Promise<void> is returned when loading is triggered
- Confirms isLoadingSubset becomes true during loading
- Validates promise doesn't resolve until loading completes
- Checks isLoadingSubset returns to false after resolution
- Verifies correct data is loaded in the new window

API

Sync Mode Configuration

// Eager mode (default) - loads all data immediately
const eagerCollection = createCollection({
  getKey: (item) => item.id,
  syncMode: 'eager', // optional, this is the default
  sync: {
    sync: ({ begin, write, commit, markReady }) => {
      // Load all data here
      begin()
      allData.forEach(item => write({ type: 'insert', value: item }))
      commit()
      markReady()
    }
  }
})

// On-demand mode - only loads data as requested
const onDemandCollection = createCollection({
  getKey: (item) => item.id,
  syncMode: 'on-demand',
  sync: {
    sync: ({ markReady }) => {
      markReady() // Don't load data initially
      
      return {
        // Required for on-demand mode
        loadSubset: async (options) => {
          const { where, limit, orderBy, subscription } = options
          // Load only the requested subset
          const data = await fetchDataSubset(where, limit, orderBy)
          // ... apply data to collection
        }
      }
    }
  }
})

Collection Subscription

const subscription = collection.subscribeChanges(callback, options)

// Status property
console.log(subscription.status) // 'ready' | 'loadingSubset'

// Event listeners
subscription.on('status:change', (event) => {
  console.log(`Status: ${event.previousStatus} → ${event.status}`)
})

subscription.on('status:loadingSubset', (event) => {
  console.log('Loading more data...')
})

subscription.on('status:ready', (event) => {
  console.log('Data loaded')
})

subscription.on('unsubscribed', (event) => {
  console.log('Subscription destroyed')
})

// Cleanup
subscription.unsubscribe()

Collection

// All collections have isLoadingSubset property
console.log(collection.isLoadingSubset) // boolean

// Listen for loading state changes
collection.on('loadingSubset:change', (event) => {
  console.log(`Loading: ${event.isLoadingSubset}`)
})

Live Query

const liveQuery = createLiveQueryCollection({
  query: (q) => q.from({ users: userCollection })
    .where(({ users }) => users.active)
})

// Result collection automatically tracks loading from subscriptions
console.log(liveQuery.isLoadingSubset) // boolean

liveQuery.on('loadingSubset:change', (event) => {
  if (event.isLoadingSubset) {
    showLoadingSpinner()
  } else {
    hideLoadingSpinner()
  }
})

Breaking Changes

Renamed Methods and Types

syncMore → loadSubset (collection method)
onLoadMore → loadSubset (sync config property)
OnLoadMoreOptions → LoadSubsetOptions (type)

Migration:

// Before
const result = await collection.syncMore({ where: expr })
sync: {
  sync: () => ({
    onLoadMore: (options) => { /* ... */ }
  })
}

// After
const result = await collection._sync.loadSubset({ where: expr })
sync: {
  sync: () => ({
    loadSubset: (options) => { /* ... */ }
  })
}

Note: loadSubset is now called via collection._sync.loadSubset() as it's an internal coordination API, not for general public use.

Migration Guide

For Collection Users

No changes required if you're just using collections - isLoadingSubset is automatically available.

For Sync Implementers

Rename onLoadMore to loadSubset:

// Before
return { onLoadMore: (options) => { /* ... */ } }

// After  
return { loadSubset: (options) => { /* ... */ } }

(Optional) Use the new subscription parameter for advanced use cases:

return {
  loadSubset: (options) => {
    // Track which subscription triggered this
    const sub = options.subscription
    
    // Can subscribe to unsubscribe event for cleanup
    sub?.on('unsubscribed', () => {
      // Clean up resources for this subscription
    })
  }
}

(Optional) Configure syncMode for on-demand loading:

createCollection({
  syncMode: 'on-demand', // Only load data as requested
  sync: {
    sync: ({ markReady }) => {
      markReady()
      return {
        loadSubset: async (options) => { /* load subset */ }
      }
    }
  }
})

…ubclass the same event emiiter implimetation

changeset-bot · 2025-10-12T18:19:35Z

🦋 Changeset detected

Latest commit: f31a67e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 12 packages

Name	Type
@tanstack/db	Patch
@tanstack/react-db	Patch
@tanstack/angular-db	Patch
@tanstack/electric-db-collection	Patch
@tanstack/query-db-collection	Patch
@tanstack/rxdb-db-collection	Patch
@tanstack/solid-db	Patch
@tanstack/svelte-db	Patch
@tanstack/trailbase-db-collection	Patch
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pkg-pr-new · 2025-10-12T18:21:02Z

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commit: 1c54b1b

github-actions · 2025-10-12T18:24:11Z

Size Change: 0 B

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github-actions · 2025-10-12T18:25:35Z

Size Change: 0 B

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…an unsunbscribed event to it

… event to the subscription, fix types

KyleAMathews

🚀

packages/db/src/collection/sync.ts

KyleAMathews · 2025-10-13T20:38:48Z

packages/db/tests/query/live-query-collection.test.ts

+
+      await liveQuery.preload()
+
+      // Calling loadSubset directly on source collection sets its own isLoadingMore


I thought the plan here was isLoadingMore is true only if a live query calls updatePredicate? Otherwise a live query would be see there isLoadingMore set to true when e.g. a joined collection needs to grab an object, etc. So any UI set to this would be flickering on and off seemingly randomly w/o any way to control it by the dev.

This bit of the code all dates to when I was working over the weekend. Will update...

I do wander if we need to expose that it is grabbing data from the server though, joins lazy appearing without the dev being able to put a placeholder/spinner does feel messy.

Maybe we need to separate flags?

Also not that this version from the weekend made the isLoadingMore prop a standard on all collections. For base collections it's true when their own loadSubset is pending, for live query collections it's when a subscription trigger a loadSubset that returns a promise.

Maybe we need an isLoadingMore and a isLoadingSubset, the latter whenever any subset triggered by the collection is loading, and isLoadingMore for when it's triggered by an offset/limit change (which do not isn't complete yet, and not in this PR)

To fully implement this as only triggered when the offset/limit is changed we will have to rebase this PR on #663 or wait till thats merged. I suggest we leave the implementation as is now (setting isLoading on any subscription triggering a loadSubset) and fix in a future PR.

db/packages/db/src/query/live/collection-subscriber.ts

Lines 77 to 84 in 9ad1169

const statusUnsubscribe = subscription.on(`status:change`, (event) => {

// TODO: For now we are setting this loading state whenever the subscription

// status changes to 'loadingMore'. But we have discussed it only happening

// when the the live query has it's offset/limit changed, and that triggers the

// subscription to request a snapshot. This will require more work to implement,

// and builds on https://github.com/TanStack/db/pull/663 which this PR

// does not yet depend on.

if (event.status === `loadingMore`) {

The question is if collection. isLoadingMore should be collection.isLoadingSubset and that isLoadingMore is a live query only thing?

kevin-dp

Nice work, left a few minor comments.

packages/db/src/collection/sync.ts

kevin-dp · 2025-10-14T12:03:17Z

packages/db/src/collection/index.ts

+   * Tracks a load promise for isLoadingMore state.
+   * @internal This is for internal coordination (e.g., live-query glue code), not for general use.
   */
-  public syncMore(options: OnLoadMoreOptions): void | Promise<void> {


I would rather keep this method such that we can keep the _sync property private

I disagree, syncMore is an internal method that shouldering be presented on the front he public api. The managers having the underscore marks them as "internal implementation but exposed for debugging and internal communication". I would prefer to not present syncMore on the prompts when people are interacting with a collection.

kevin-dp · 2025-10-14T12:06:30Z

packages/db/src/collection/subscription.ts

    })

+    // Track the promise if it's actually a promise (async work)
+    if (syncResult instanceof Promise) {


Should we also check that the promise is not yet resolved? Because if it is already resolved then there is no need to do the below as that may lead to some flickering in the UI if one would e.g. show a spinner when more is loading.

Unfortunately there is no api to do this, thats why loadSubset can return true to indicate that it has the data synchronously.

packages/db/src/collection/subscription.ts

packages/db/src/types.ts

* feat: implement useLiveInfiniteQuery hook for React * use the new utils.setWindow to page through the results improve types add test that checks that we detect new pages on more rows syncing changeset tweaks * isFetchingNextPage set by promise from setWindow --------- Co-authored-by: Sam Willis <sam.willis@gmail.com>

Updated changeset to correctly describe: - isLoadingSubset property (not isLoadingMore) - loadingSubset:change events - syncMode configuration options - Comprehensive loading state tracking - Enhanced setWindow utility 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

…features (#679) Fix changeset for PR #669 to accurately describe features Updated changeset to correctly describe: - isLoadingSubset property (not isLoadingMore) - loadingSubset:change events - syncMode configuration options - Comprehensive loading state tracking - Enhanced setWindow utility 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude <noreply@anthropic.com>

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude <noreply@anthropic.com>

github-actions · 2025-10-15T18:53:52Z

🎉 This PR has been released!

Thank you for your contribution!

samwillis added 2 commits October 12, 2025 18:48

wip

f329fb1

refactor so both CollectionEventsManager and CollectionSubscription s…

dc34f5b

…ubclass the same event emiiter implimetation

changeset

b7f45c8

samwillis added 3 commits October 13, 2025 18:12

rename loadMore to loadSubset

d6c6e7e

feed the subscription object through to the loadSubset call, and add …

7b584b1

…an unsunbscribed event to it

feed subscription through to the loadSubset callback, add unsubscribe…

5636c46

… event to the subscription, fix types

samwillis force-pushed the samwillis/load-more-tracking branch from 4d186d9 to 68dc938 Compare October 13, 2025 19:37

fix types

b1aeceb

samwillis force-pushed the samwillis/load-more-tracking branch from 68dc938 to b1aeceb Compare October 13, 2025 19:52

add sync mode to base colleciton

53fa027

samwillis changed the title ~~Add "Load More" Status Tracking to Collections and Live Queries~~ Add loadSubset State Tracking and On-Demand Sync Mode Oct 13, 2025

KyleAMathews reviewed Oct 13, 2025

View reviewed changes

samwillis added 2 commits October 14, 2025 09:50

loadSubset fn return promise or true

f78c8e1

add comment on setting is loading

9ad1169

samwillis mentioned this pull request Oct 14, 2025

Handle pushed down predicates in Electric collection #618

Open

kevin-dp self-requested a review October 14, 2025 11:50

kevin-dp approved these changes Oct 14, 2025

View reviewed changes

samwillis mentioned this pull request Oct 14, 2025

Handle pushed down predicates in query collection #646

Closed

address review

0a0bbb0

samwillis mentioned this pull request Oct 15, 2025

feat: implement useLiveInfiniteQuery hook for React #666

Merged

samwillis added 3 commits October 15, 2025 11:38

Merge branch 'main' into samwillis/load-more-tracking

8b1c68d

remove public trackLoadPromise

f8cd6fe

setWindow returns a promise when it triggers loading subset

1c54b1b

samwillis marked this pull request as ready for review October 15, 2025 11:56

samwillis mentioned this pull request Oct 15, 2025

Predicate Comparison and Merging Utilities for Predicate Push-Down #668

Open

samwillis merged commit b0687ab into main Oct 15, 2025
5 checks passed

samwillis deleted the samwillis/load-more-tracking branch October 15, 2025 17:49

github-actions bot mentioned this pull request Oct 15, 2025

ci: Version Packages #678

Merged

KyleAMathews mentioned this pull request Oct 15, 2025

Fix changeset for PR #669 - Update to accurately describe loadSubset features #679

Merged

KyleAMathews added a commit that referenced this pull request Oct 15, 2025

Change PR #669 changeset from minor to patch

1b8b562

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

KyleAMathews mentioned this pull request Oct 15, 2025

Change PR #669 changeset from minor to patch #680

Merged

KyleAMathews added a commit that referenced this pull request Oct 15, 2025

Change PR #669 changeset from minor to patch (#680)

c0f06e2

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude <noreply@anthropic.com>

KyleAMathews mentioned this pull request Oct 15, 2025

Handle pushed down predicates in query collection #681

Open


		await liveQuery.preload()

		// Calling loadSubset directly on source collection sets its own isLoadingMore

	const statusUnsubscribe = subscription.on(`status:change`, (event) => {
	// TODO: For now we are setting this loading state whenever the subscription
	// status changes to 'loadingMore'. But we have discussed it only happening
	// when the the live query has it's offset/limit changed, and that triggers the
	// subscription to request a snapshot. This will require more work to implement,
	// and builds on https://github.com/TanStack/db/pull/663 which this PR
	// does not yet depend on.
	if (event.status === `loadingMore`) {

Add loadSubset State Tracking and On-Demand Sync Mode #669

Add loadSubset State Tracking and On-Demand Sync Mode #669

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Overview

Problem

Solution

Loading State Tracking

Sync Modes

Changes

1. Renamed syncMore/loadMore to loadSubset

2. Added syncMode Configuration

3. Standardized loadSubset Return Type

4. CollectionSubscription Status Tracking & Events

5. Generic isLoadingSubset for All Collections

6. Live Query Integration

7. Subscription Interface for External Consumers

8. Reusable Event Emitter

9. Fixed Local-Only Collection Types

10. Comprehensive Test Coverage

11. Enhanced setWindow with Loading Awareness

API

Sync Mode Configuration

Collection Subscription

Collection

Live Query

Breaking Changes

Renamed Methods and Types

Migration Guide

For Collection Users

For Sync Implementers

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samwillis commented Oct 12, 2025 •

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1. Renamed `syncMore`/`loadMore` to `loadSubset`

2. Added `syncMode` Configuration

3. Standardized `loadSubset` Return Type

5. Generic `isLoadingSubset` for All Collections

11. Enhanced `setWindow` with Loading Awareness

changeset-bot bot commented Oct 12, 2025 •

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samwillis Oct 13, 2025 •

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