AWS Lambda
Developer Guide

Understanding Scaling Behavior

Concurrent executions refers to the number of executions of your function code that are happening at any given time. You can estimate the concurrent execution count, but the concurrent execution count will differ depending on whether or not your Lambda function is processing events from a stream-based event source.

  • Stream-based event sources for Lambda functions that process Kinesis or DynamoDB streams the number of shards is the unit of concurrency. If your stream has 100 active shards, there will be at most 100 Lambda function invocations running concurrently. This is because Lambda processes each shard’s events in sequence.

  • Event sources that aren't stream-based – If you create a Lambda function to process events from event sources that aren't stream-based (for example, Lambda can process every event from other sources, like Amazon S3 or API Gateway), each published event is a unit of work, in parallel, up to your account limits. Therefore, the number of events (or requests) these event sources publish influences the concurrency. You can use the this formula to estimate your concurrent Lambda function invocations:

    events (or requests) per second * function duration

    For example, consider a Lambda function that processes Amazon S3 events. Suppose that the Lambda function takes on average three seconds and Amazon S3 publishes 10 events per second. Then, you will have 30 concurrent executions of your Lambda function.

Request Rate

Request rate refers to the rate at which your Lambda function is invoked. For all services except the stream-based services, the request rate is the rate at which the event sources generate the events. For stream-based services, AWS Lambda calculates the request rate as follow:

request rate = number of concurrent executions / function duration

For example, if there are five active shards on a stream (that is, you have five Lambda functions running in parallel) and your Lambda function takes about two seconds, the request rate is 2.5 requests/second.


AWS Lambda will dynamically scale capacity in response to increased traffic, subject to your account's Account Level Concurrent Execution Limit. To handle any burst in traffic, Lambda will immediately increase your concurrently executing functions by a predetermined amount, dependent on which region it's executed (see table below).

If the default Immediate Concurrency Increase value, as noted in the table below, is not sufficient to accommodate the traffic surge, Lambda will continue to increase the number of concurrent function executions by 500 per minute until your account safety limit has been reached or the number of concurrently executing functions is sufficient to successfully process the increased load.


Because Lambda depends on Amazon EC2 to provide Elastic Network Interfaces for VPC-enabled Lambda functions, these functions are also subject to Amazon EC2's rate limits as they scale. If your Amazon EC2 rate limits prevent VPC-enabled functions from adding 500 concurrent invocations per minute, please request a limit increase by following the instructions in the preceding section under To request a limit increase for concurrent executions.

Beyond this rate (i.e. for applications taking advantage of the full Immediate concurrency increase), your application should handle Amazon EC2 throttling (502 EC2ThrottledException) through client-side retry and backoff. For more details, see Error Retries and Exponential Backoff in AWS.

The table below outlines the immediate concurrency increase per region:

Region Immediate Concurrency Increase (function executions)
Asia Pacific (Tokyo) 1000
Asia Pacific (Seoul) 500
Asia Pacific (Mumbai) 500
Asia Pacific (Singapore) 500
Asia Pacific (Sydney) 500
China (Beijing) 500
Canada (Central) 500
EU (Frankfurt) 1000
EU (London) 500
EU (Ireland) 3000
EU (Paris) 500
AWS GovCloud (US) 500
US East (Ohio) 500
US West (N. California) 500
US West (Oregon) 3000
US East (N. Virginia) 3000
South America (São Paulo) 500
AWS GovCloud (US) 500

To learn how to view and manage the concurrent executions for your function, see Managing Concurrency