Angular Queuing Guide

Queuing stores operations in an ordered buffer and processes them individually. It is the primary Pacer strategy for work that should not be discarded when calls arrive faster than they can run.

Queues are lossless only while they have capacity. A finite maxSize, explicit clearing, expiration, or a processing error can still remove or reject work.

How queuing works#

Queuing (process one item every 2 ticks)
Timeline: [1 second per tick]
Calls:        ⬇️  ⬇️  ⬇️     ⬇️  ⬇️     ⬇️  ⬇️  ⬇️
Queue:       [ABC]   [BC]    [BCDE]    [DE]    [E]    []
Executed:     ✅     ✅       ✅        ✅      ✅     ✅
             [======================================================]
             ^ Accepted items remain queued until processed

             [Items arrive]    [Process steadily]      [Empty]

The queue can process automatically with a delay between items, or remain stopped for manual processing.

When to use queuing#

Choose queuing when:

• Every accepted operation should run individually.
• Processing order matters.
• Incoming work may temporarily exceed processing capacity.
• A maximum buffer size should reject excess work explicitly.
• FIFO, LIFO, or priority ordering is required.

Choose another utility when:

• Only the final call matters. Use debouncing.
• Intermediate calls may be discarded while work runs steadily. Use throttling.
• Calls should be rejected after a time-window quota. Use rate limiting.
• Items should run together. Use batching.
• Tasks return Promises or should run concurrently. Use async queuing.

Choose an API#

• injectQueuedValue to queue observed source values
• injectQueuedSignal to queue individual set and update operations
• injectQueuerItems for a signal of pending queue items
• injectQueuer for direct queue lifecycle and ordering control

Use the items helper when queue contents drive the UI. Use the instance API for ordering, capacity, expiration, pause, resume, flush, and manual processing.

Angular example#

import { injectQueuerItems } from '@tanstack/angular-pacer'

export class JobQueueComponent {
  readonly queued = injectQueuerItems(processJob, { wait: 500 })

  add() {
    this.queued.addItem(nextJob())
  }
}

The focused snippets later in this guide use injectQueuer and assume they run inside an Angular injection context.

Pass initialItems when work is already available at creation time. The queue applies its normal insertion and capacity rules, and automatic processing can begin immediately unless started: false is set.

Reading pending items#

queued() returns the waiting items as a readonly array. Use the addItem shortcut to enqueue items and queued.queuer to control processing:

const queued = injectQueuerItems(processItem, { started: false })
queued.addItem('first')
queued.addItem('second')
console.log(queued()) // ['first', 'second']
queued.queuer.start()

Pass a third-argument selector to expose counters or running state through queued.queuer.state(). The items signal still returns the waiting items.

Queued values#

injectQueuedValue returns the last processed source value. It queues the initial value and subsequent values observed by Angular effects. Multiple source writes before an effect runs are observed as one change.

const source = signal('')
const queued = injectQueuedValue(source, { wait: 500 })
source.set('next')
console.log(queued()) // Last processed value.

injectQueuedSignal returns an editable signal whose accepted writes are queued individually. Each updater runs against the value at processing time:

const count = injectQueuedSignal(0, { started: false })
count.set(1)
count.update((previous) => previous + 2)
count.queuer.flush()
console.log(count()) // 3

Both helpers accept a selector for their attached queuer.state(). The editable helper queues replacement values or updater functions. Updaters are evaluated against the committed value when processed, matching the delayed setter behavior of React and Solid.

Ordering items#

Automatic processing uses addItemsTo to choose where new items enter and getItemsFrom to choose where items leave.

FIFO#

FIFO processes the oldest item first. This is the default.

const queuer = injectQueuer(processItem, {
  addItemsTo: 'back',
  getItemsFrom: 'front',
  started: false,
})

queuer.addItem(1)
queuer.addItem(2)
queuer.addItem(3)
queuer.start() // Processes 1, 2, 3.

LIFO#

LIFO processes the newest item first.

const queuer = injectQueuer(processItem, {
  addItemsTo: 'back',
  getItemsFrom: 'back',
  started: false,
})

queuer.addItem(1)
queuer.addItem(2)
queuer.addItem(3)
queuer.start() // Processes 3, 2, 1.

Priority#

Provide getPriority to process higher numeric priorities first. Priority ordering takes precedence over front and back retrieval.

type Task = { name: string; priority: number }

const queuer = injectQueuer<Task>(processTask, {
  getPriority: (task) => task.priority,
  started: false,
})

queuer.addItem({ name: 'low', priority: 1 })
queuer.addItem({ name: 'high', priority: 3 })
queuer.addItem({ name: 'medium', priority: 2 })
queuer.start() // Processes high, medium, low.

Automatic and manual processing#

Queues start automatically by default. The first accepted item processes immediately, then wait controls the delay before later items.

const queuer = injectQueuer(processItem, {
  wait: 1000,
})

Set started: false to collect items before processing:

const queuer = injectQueuer(processItem, { started: false })

queuer.addItem(1)
queuer.addItem(2)
queuer.start()
queuer.stop()

stop() cancels the scheduled tick and retains queued items. start() resumes automatic processing.

For manual control:

• execute() removes and processes the next item immediately.
• getNextItem() removes and returns the next item without processing it.
• peekNextItem() returns the next item without removing it.
• peekAllItems() returns a copy of the current queue.

Capacity and rejection#

Set maxSize to bound the number of waiting items. An item added to a full queue is rejected, addItem() returns false, and onReject runs.

const queuer = injectQueuer(processItem, {
  maxSize: 2,
  started: false,
  onReject: (item, queuer) => {
    console.log('Rejected:', item)
    console.log('Total rejections:', queuer.store.state.rejectionCount)
  },
})

queuer.addItem(1) // true
queuer.addItem(2) // true
queuer.addItem(3) // false

The active synchronous execution is not part of size; size counts items still waiting in the queue.

Expiring stale items#

Use expirationDuration to remove items that have waited too long:

const queuer = injectQueuer(processItem, {
  expirationDuration: 5000,
  onExpire: (item) => {
    console.log('Expired:', item)
  },
})

Use getIsExpired for custom logic:

const queuer = injectQueuer(processItem, {
  getIsExpired: (item, addedAt) => Date.now() - addedAt > item.maxAge,
})

Expiration is checked while the automatic processing loop runs. A stopped queue evaluates stale items when processing resumes.

Flushing, clearing, and resetting#

Flush#

flush() processes waiting items immediately without the configured delay. Pass a count to process only part of the queue.

queuer.flush() // Process all waiting items.
queuer.flush(2) // Process at most two waiting items.

flushAsBatch() removes all waiting items and passes them to a separate batch function:

queuer.flushAsBatch((items) => {
  saveItems(items)
})

Clear#

clear() removes all waiting items without processing them. It does not change whether the queue is running.

queuer.clear()

Reset#

reset() restores state to the default running, empty queue. It does not clear an already scheduled timeout. Call stop() before reset() when scheduled work must be canceled.

queuer.stop()
queuer.reset()

Configuring and observing the queue#

Use an options function to read reactive configuration:

import { signal } from '@angular/core'

const options = signal({ wait: 500, maxSize: 10 })
const queuer = injectQueuer(processItem, options)

options.set({ wait: 250, maxSize: 20 })

The wait option may be a function that receives the queuer instance:

const queuer = injectQueuer(processItem, {
  wait: (queuer) => (queuer.store.state.size > 20 ? 50 : 250),
})

Use callbacks for queue events:

• onItemsChange: An item was added or removed.
• onExecute: An item was processed.
• onReject: An item was rejected.
• onExpire: An item expired.

Angular lifecycle#

The adapter stops automatic processing when its owner is destroyed. Providing onUnmount replaces that default cleanup, so a custom callback must perform every required lifecycle action. When custom cleanup flushes work, remember that user callbacks can run while the component is being destroyed.

Reactive state#

Pass a selector to expose the state used by your view through state(). Without a selector, state() returns {}:

const queuer = injectQueuer(processItem, { wait: 250 }, (state) => ({
  size: state.size,
  isRunning: state.isRunning,
}))

console.log(queuer.state().size, queuer.state().isRunning)

Option callbacks receive the core utility instance. Read the selected state() signal in templates.

initialState can restore selected queue state that your app has persisted. If it includes items, they take precedence over initialItems; initialState.isRunning likewise takes precedence over started. Restore only durable fields. Pending timers are not restored.

Commonly useful state includes:

• items and size: Items still waiting.
• isRunning: Whether automatic processing is enabled.
• isIdle: Whether a running queue is empty.
• isFull: Whether maxSize has been reached.
• executionCount: Items whose wrapped function returned successfully.
• rejectionCount and expirationCount: Items removed without processing.
• status: 'idle', 'running', or 'stopped'.

See the Angular API reference for adapter signatures and the public core reference for complete option and state types.