# AWS API Gateway - Burst Throttling

## What is the Burst?

In API Gateway, the burst limit corresponds to **the maximum number of concurrent request submissions that API Gateway can fulfill at any moment** without returning `429 Too Many Requests` error responses.

{% hint style="info" %}
The burst quota is determined by the API Gateway service team based on the overall RPS quota for the account in the Region. It is not a quota that a customer can control or request changes to.
{% endhint %}

## What is the API Gateway Rate limits?

The rate limits are calculated in Requests Per Second, or RPS.

If a user sends too many requests, API rate limiting can throttle client connections instead of disconnecting them immediately. Throttling lets clients still use your services while still protecting your API.

## How do the Rate and Burst Throttle work together?

When request submissions exceed the steady-state request rate and burst limits, API Gateway fails the limit-exceeding requests and returns `429 Too Many Requests` error responses to the client. When a client receives these error responses, the client can resubmit the failed requests in a way that limits the rate, while complying with the API Gateway throttling limits.

To help understand these throttling limits, here is a example: Let's assume you set the throttling to **Rate = 1000** (requests per second) and the **Burst = 500** (requests):

* If a caller submits **1000** requests in a one-second period evenly (for example, 1 request every millisecond), API Gateway processes all requests without dropping any.
* If the caller submits **1000** requests in the **first millisecond**, API Gateway serves **500** of those requests **due to the burst setting** and throttles the rest in the one-second period (the remaining 500 requests would get a `429 Too Many Requests` response).
* If the caller submits 500 requests in the first millisecond and then evenly spreads another 500 requests through the remaining 999 milliseconds (for example, about 1 request every millisecond), API Gateway processes all 1000 requests in the one-second period without returning `429 Too Many Requests` error responses.

## Function concurrency limits and throttling in AWS Lambda

In AWS Lambda, a concurrency limit determines how many function invocations can run simultaneously in one region. Each region in your AWS account has a Lambda concurrency limit. The limit applies to all functions in the same region and is set to **1000** **by default**.

If you exceed a concurrency limit, Lambda starts throttling the offending functions by rejecting requests.

## How to increase your AWS Lambda concurrency limits?

Open a [support ticket with AWS](https://console.aws.amazon.com/support/home?#/) to request an increase in your account level concurrency limit. 

1. Create a new support case
2. Set **Regarding** value to **Service Limit Increase**.
3. Choose **Lambda** as the **Limit Type.**
4. Fill out the body of the form.
5. Wait for AWS to respond to your request.

## API Gateway invoke Lambda

The default burst levels (**5000**) on AWS API Gateway is way higher than the default maximum concurrency of Lambda Functions (**1000**), so if you are using API Gateway with Lambda you will want to make sure that you have set a value for the **Burst** throttle setting that makes sense for your Lambda Concurrency level.