Async Rate Limiting Guide

All core concepts from the Rate Limiting Guide apply to async rate limiting as well.

When to Use Async Rate Limiting

You can usually just use the normal synchronous rate limiter and it will work with async functions, but for advanced use cases, such as wanting to use the return value of a rate-limited function (instead of just calling a setState side effect), or putting your error handling logic in the rate limiter, you can use the async rate limiter.

Async Rate Limiting in TanStack Pacer

TanStack Pacer provides async rate limiting through the AsyncRateLimiter class and the asyncRateLimit function.

Basic Usage Example

Here's a basic example showing how to use the async rate limiter for an API operation:

const rateLimitedApi = asyncRateLimit(
  async (id: string) => {
    const response = await fetch(`/api/data/${id}`)
    return response.json()
  },
  {
    limit: 5,
    window: 1000,
    onExecute: (limiter) => {
      console.log('API call succeeded:', limiter.store.state.successCount)
    },
    onReject: (limiter) => {
      console.log(`Rate limit exceeded. Try again in ${limiter.getMsUntilNextWindow()}ms`)
    },
    onError: (error, limiter) => {
      console.error('API call failed:', error)
    }
  }
)

// Usage
try {
  const result = await rateLimitedApi('123')
  // Handle successful result
} catch (error) {
  // Handle errors if no onError handler was provided
  console.error('API call failed:', error)
}

const rateLimitedApi = asyncRateLimit(
  async (id: string) => {
    const response = await fetch(`/api/data/${id}`)
    return response.json()
  },
  {
    limit: 5,
    window: 1000,
    onExecute: (limiter) => {
      console.log('API call succeeded:', limiter.store.state.successCount)
    },
    onReject: (limiter) => {
      console.log(`Rate limit exceeded. Try again in ${limiter.getMsUntilNextWindow()}ms`)
    },
    onError: (error, limiter) => {
      console.error('API call failed:', error)
    }
  }
)

// Usage
try {
  const result = await rateLimitedApi('123')
  // Handle successful result
} catch (error) {
  // Handle errors if no onError handler was provided
  console.error('API call failed:', error)
}

Key Differences from Synchronous Rate Limiting

1. Return Value Handling

Unlike the synchronous rate limiter which returns a boolean indicating success, the async version allows you to capture and use the return value from your rate-limited function. The maybeExecute method returns a Promise that resolves with the function's return value, allowing you to await the result and handle it appropriately.

2. Error Handling

The async rate limiter provides robust error handling capabilities:

If your rate-limited function throws an error and no onError handler is provided, the error will be thrown and propagate up to the caller
If you provide an onError handler, errors will be caught and passed to the handler instead of being thrown
The throwOnError option can be used to control error throwing behavior:
- When true (default if no onError handler), errors will be thrown
- When false (default if onError handler provided), errors will be swallowed
- Can be explicitly set to override these defaults
You can track error counts using limiter.store.state.errorCount and check execution state with limiter.store.state.isExecuting
The rate limiter maintains its state and can continue to be used after an error occurs
Rate limit rejections (when limit is exceeded) are handled separately from execution errors via the onReject handler

3. Different Callbacks

The AsyncRateLimiter supports the following callbacks:

onSuccess: Called after each successful execution, providing the result and rate limiter instance
onSettled: Called after each execution (success or failure), providing the rate limiter instance
onError: Called if the async function throws an error, providing both the error and the rate limiter instance

Both the Async and Synchronous rate limiters support the onReject callback for handling blocked executions.

Example:

const asyncLimiter = new AsyncRateLimiter(async (id) => {
  await saveToAPI(id)
}, {
  limit: 5,
  window: 1000,
  onExecute: (rateLimiter) => {
    // Called after each successful execution
    console.log('Async function executed', rateLimiter.store.state.successCount)
  },
  onReject: (rateLimiter) => {
    // Called when an execution is rejected
    console.log(`Rate limit exceeded. Try again in ${rateLimiter.getMsUntilNextWindow()}ms`)
  },
  onError: (error) => {
    // Called if the async function throws an error
    console.error('Async function failed:', error)
  }
})

const asyncLimiter = new AsyncRateLimiter(async (id) => {
  await saveToAPI(id)
}, {
  limit: 5,
  window: 1000,
  onExecute: (rateLimiter) => {
    // Called after each successful execution
    console.log('Async function executed', rateLimiter.store.state.successCount)
  },
  onReject: (rateLimiter) => {
    // Called when an execution is rejected
    console.log(`Rate limit exceeded. Try again in ${rateLimiter.getMsUntilNextWindow()}ms`)
  },
  onError: (error) => {
    // Called if the async function throws an error
    console.error('Async function failed:', error)
  }
})

4. Sequential Execution

Since the rate limiter's maybeExecute method returns a Promise, you can choose to await each execution before starting the next one. This gives you control over the execution order and ensures each call processes the most up-to-date data. This is particularly useful when dealing with operations that depend on the results of previous calls or when maintaining data consistency is critical.

For example, if you're updating a user's profile and then immediately fetching their updated data, you can await the update operation before starting the fetch.

Dynamic Options and Enabling/Disabling

Just like the synchronous rate limiter, the async rate limiter supports dynamic options for limit, window, and enabled, which can be functions that receive the rate limiter instance. This allows for sophisticated, runtime-adaptive rate limiting behavior.

Framework Adapters

Each framework adapter provides hooks that build on top of the core async rate limiting functionality to integrate with the framework's state management system. Hooks like createAsyncRateLimiter, useAsyncRateLimitedCallback, or similar are available for each framework.

State Management

The AsyncRateLimiter class uses TanStack Store for reactive state management, providing real-time access to execution state, error tracking, and rejection statistics.

Accessing State

When using the AsyncRateLimiter class directly, access state via the store.state property:

const asyncLimiter = new AsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// Access current state
console.log(asyncLimiter.store.state.isExecuting)

const asyncLimiter = new AsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// Access current state
console.log(asyncLimiter.store.state.isExecuting)

Framework Adapters

When using framework adapters like React or Solid, the state is exposed directly as a reactive property:

// React example
const asyncLimiter = useAsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// Access state directly (reactive)
console.log(asyncLimiter.state.successCount) // Reactive value
console.log(asyncLimiter.state.isExecuting) // Reactive value

// React example
const asyncLimiter = useAsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// Access state directly (reactive)
console.log(asyncLimiter.state.successCount) // Reactive value
console.log(asyncLimiter.state.isExecuting) // Reactive value

Initial State

You can provide initial state values when creating an async rate limiter:

const asyncLimiter = new AsyncRateLimiter(asyncFn, {
  limit: 5,
  window: 1000,
  initialState: {
    successCount: 3, // Start with 3 successful executions
    errorCount: 1, // Start with 1 error
    rejectionCount: 2, // Start with 2 rejections
    lastResult: 'initial-result', // Start with initial result
    executionTimes: [Date.now() - 500], // Start with one execution timestamp
  }
})

const asyncLimiter = new AsyncRateLimiter(asyncFn, {
  limit: 5,
  window: 1000,
  initialState: {
    successCount: 3, // Start with 3 successful executions
    errorCount: 1, // Start with 1 error
    rejectionCount: 2, // Start with 2 rejections
    lastResult: 'initial-result', // Start with initial result
    executionTimes: [Date.now() - 500], // Start with one execution timestamp
  }
})

Subscribing to State Changes

The store is reactive and supports subscriptions:

const asyncLimiter = new AsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// Subscribe to state changes
const unsubscribe = asyncLimiter.store.subscribe((state) => {
  console.log('Success count:', state.successCount)
  console.log('Error count:', state.errorCount)
  console.log('Rejection count:', state.rejectionCount)
  console.log('Currently executing:', state.isExecuting)
})

// Unsubscribe when done
unsubscribe()

const asyncLimiter = new AsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// Subscribe to state changes
const unsubscribe = asyncLimiter.store.subscribe((state) => {
  console.log('Success count:', state.successCount)
  console.log('Error count:', state.errorCount)
  console.log('Rejection count:', state.rejectionCount)
  console.log('Currently executing:', state.isExecuting)
})

// Unsubscribe when done
unsubscribe()

Available State Properties

The AsyncRateLimiterState includes:

successCount: Number of successful function executions
errorCount: Number of failed function executions
settleCount: Total number of completed executions (success + error)
rejectionCount: Number of rejected executions due to rate limiting
isExecuting: Whether the async function is currently executing
lastResult: Result from the most recent successful execution
executionTimes: Array of timestamps when executions occurred (used for rate limiting calculations)

Helper Methods

The async rate limiter provides helper methods that compute values based on the current state:

const asyncLimiter = new AsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// These methods use the current state to compute values
console.log(asyncLimiter.getRemainingInWindow()) // Number of calls remaining in current window
console.log(asyncLimiter.getMsUntilNextWindow()) // Milliseconds until next window

const asyncLimiter = new AsyncRateLimiter(asyncFn, { limit: 5, window: 1000 })

// These methods use the current state to compute values
console.log(asyncLimiter.getRemainingInWindow()) // Number of calls remaining in current window
console.log(asyncLimiter.getMsUntilNextWindow()) // Milliseconds until next window

These methods are computed values that use the current state and don't need to be accessed through the store.

For core rate limiting concepts and synchronous rate limiting, see the Rate Limiting Guide.

When to Use Async Rate Limiting

Async Rate Limiting in TanStack Pacer

Basic Usage Example

Key Differences from Synchronous Rate Limiting

1. Return Value Handling

2. Error Handling

3. Different Callbacks

4. Sequential Execution

Dynamic Options and Enabling/Disabling

Framework Adapters

State Management

Accessing State

Framework Adapters

Initial State

Subscribing to State Changes

Available State Properties

Helper Methods

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