This skill builds on db-core and mutations-optimistic. Read those first.

TanStack DB — Offline Transactions

Setup

import {
  startOfflineExecutor,
  IndexedDBAdapter,
} from '@tanstack/offline-transactions'
import { todoCollection } from './collections'

const executor = startOfflineExecutor({
  collections: { todos: todoCollection },
  mutationFns: {
    createTodo: async ({ transaction, idempotencyKey }) => {
      const mutation = transaction.mutations[0]
      await api.todos.create({
        ...mutation.modified,
        idempotencyKey,
      })
    },
    updateTodo: async ({ transaction, idempotencyKey }) => {
      const mutation = transaction.mutations[0]
      await api.todos.update(mutation.key, {
        ...mutation.changes,
        idempotencyKey,
      })
    },
  },
})

// Wait for initialization (storage probe, leader election, outbox replay)
await executor.waitForInit()

Core API

createOfflineTransaction

const tx = executor.createOfflineTransaction({
  mutationFnName: 'createTodo',
})

// Mutations run inside tx.mutate() — uses ambient transaction context
tx.mutate(() => {
  todoCollection.insert({ id: crypto.randomUUID(), text: 'New todo' })
})
tx.commit()

If the executor is not the leader tab, falls back to createTransaction directly (no offline persistence).

createOfflineAction

const addTodo = executor.createOfflineAction({
  mutationFnName: 'createTodo',
  onMutate: (variables) => {
    todoCollection.insert({
      id: crypto.randomUUID(),
      text: variables.text,
    })
  },
})

// Call it
addTodo({ text: 'Buy milk' })

If the executor is not the leader tab, falls back to createOptimisticAction directly.

Architecture

Components

Component	Purpose	Default
Storage	Persist transactions to survive page reload	IndexedDB → localStorage fallback
OutboxManager	FIFO queue of pending transactions	Automatic
KeyScheduler	Serialize transactions touching same keys	Automatic
TransactionExecutor	Execute with retry + backoff	Automatic
LeaderElection	Only one tab processes the outbox	WebLocks → BroadcastChannel
OnlineDetector	Pause/resume on connectivity changes	navigator.onLine + events

Transaction lifecycle

Mutation applied optimistically to collection (instant UI update)
Transaction serialized and persisted to storage (outbox)
Leader tab picks up transaction and executes mutationFn
On success: removed from outbox, optimistic state resolved
On failure: retried with exponential backoff
On page reload: outbox replayed, optimistic state restored

Leader election

Only one tab processes the outbox to prevent duplicate execution. Non-leader tabs use regular createTransaction/createOptimisticAction (online-only, no persistence).

const executor = startOfflineExecutor({
  // ...
  onLeadershipChange: (isLeader) => {
    console.log(
      isLeader
        ? 'This tab is processing offline transactions'
        : 'Another tab is leader',
    )
  },
})

executor.isOfflineEnabled // true only if leader AND storage available

Storage degradation

The executor probes storage availability on startup:

const executor = startOfflineExecutor({
  // ...
  onStorageFailure: (diagnostic) => {
    // diagnostic.code: 'STORAGE_BLOCKED' | 'QUOTA_EXCEEDED' | 'UNKNOWN_ERROR'
    // diagnostic.mode: 'online-only'
    console.warn(diagnostic.message)
  },
})

executor.mode // 'offline' | 'online-only'
executor.storageDiagnostic // Full diagnostic info

When storage is unavailable (private browsing, quota exceeded), the executor operates in online-only mode — mutations work normally but aren't persisted across page reloads.

Configuration

interface OfflineConfig {
  collections: Record<string, Collection> // Collections for optimistic state restoration
  mutationFns: Record<string, OfflineMutationFn> // Named mutation functions
  storage?: StorageAdapter // Custom storage (default: auto-detect)
  maxConcurrency?: number // Parallel execution limit
  jitter?: boolean // Add jitter to retry delays
  beforeRetry?: (txs) => txs // Transform/filter before retry
  onUnknownMutationFn?: (name, tx) => void // Handle orphaned transactions
  onLeadershipChange?: (isLeader) => void // Leadership state callback
  onStorageFailure?: (diagnostic) => void // Storage probe failure callback
  leaderElection?: LeaderElection // Custom leader election
  onlineDetector?: OnlineDetector // Custom connectivity detection
}

Custom storage adapter

interface StorageAdapter {
  get: (key: string) => Promise<string | null>
  set: (key: string, value: string) => Promise<void>
  delete: (key: string) => Promise<void>
  keys: () => Promise<Array<string>>
  clear: () => Promise<void>
}

Error Handling

NonRetriableError

import { NonRetriableError } from '@tanstack/offline-transactions'

const executor = startOfflineExecutor({
  mutationFns: {
    createTodo: async ({ transaction, idempotencyKey }) => {
      const res = await fetch('/api/todos', { method: 'POST', body: ... })
      if (res.status === 409) {
        throw new NonRetriableError('Duplicate detected')
      }
      if (!res.ok) throw new Error('Server error')
    },
  },
})

Throwing NonRetriableError stops retry and removes the transaction from the outbox. Use for permanent failures (validation errors, conflicts, 4xx responses).

Idempotency keys

Every offline transaction includes an idempotencyKey. Pass it to your API to prevent duplicate execution on retry:

mutationFns: {
  createTodo: async ({ transaction, idempotencyKey }) => {
    await fetch('/api/todos', {
      method: 'POST',
      headers: { 'Idempotency-Key': idempotencyKey },
      body: JSON.stringify(transaction.mutations[0].modified),
    })
  },
}

React Native

import {
  startOfflineExecutor,
} from '@tanstack/offline-transactions/react-native'

// Uses ReactNativeOnlineDetector automatically
// Uses AsyncStorage-compatible storage
const executor = startOfflineExecutor({ ... })

Outbox Management

// Inspect pending transactions
const pending = await executor.peekOutbox()

// Get counts
executor.getPendingCount() // Queued transactions
executor.getRunningCount() // Currently executing

// Clear all pending transactions
await executor.clearOutbox()

// Cleanup
executor.dispose()

Common Mistakes

CRITICAL Not passing idempotencyKey to the API

Wrong:

mutationFns: {
  createTodo: async ({ transaction }) => {
    await api.todos.create(transaction.mutations[0].modified)
  },
}

Correct:

mutationFns: {
  createTodo: async ({ transaction, idempotencyKey }) => {
    await api.todos.create({
      ...transaction.mutations[0].modified,
      idempotencyKey,
    })
  },
}

Offline transactions retry on failure. Without idempotency keys, retries can create duplicate records on the server.

HIGH Not waiting for initialization

Wrong:

const executor = startOfflineExecutor({ ... })
const tx = executor.createOfflineTransaction({ mutationFnName: 'createTodo' })

Correct:

const executor = startOfflineExecutor({ ... })
await executor.waitForInit()
const tx = executor.createOfflineTransaction({ mutationFnName: 'createTodo' })

startOfflineExecutor initializes asynchronously (probes storage, requests leadership, replays outbox). Creating transactions before initialization completes may miss the leader election result and use the wrong code path.

HIGH Missing collection in collections map

Wrong:

const executor = startOfflineExecutor({
  collections: {},
  mutationFns: { createTodo: ... },
})

Correct:

const executor = startOfflineExecutor({
  collections: { todos: todoCollection },
  mutationFns: { createTodo: ... },
})

The collections map is used to restore optimistic state from the outbox on page reload. Without it, previously pending mutations won't show their optimistic state while being replayed.

MEDIUM Not handling NonRetriableError for permanent failures

Wrong:

mutationFns: {
  createTodo: async ({ transaction }) => {
    const res = await fetch('/api/todos', { ... })
    if (!res.ok) throw new Error('Failed')
  },
}

Correct:

mutationFns: {
  createTodo: async ({ transaction }) => {
    const res = await fetch('/api/todos', { ... })
    if (res.status >= 400 && res.status < 500) {
      throw new NonRetriableError(`Client error: ${res.status}`)
    }
    if (!res.ok) throw new Error('Server error')
  },
}

Without distinguishing retriable from permanent errors, 4xx responses (validation, auth, not found) will retry forever until max retries, wasting resources and filling logs.

See also: db-core/mutations-optimistic/SKILL.md — for the underlying mutation primitives.

See also: db-core/collection-setup/SKILL.md — for setting up collections used with offline transactions.