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What is AWS Lambda and how does the serverless computing model differ from traditional server based applications? What event sources can trigger a Lambda function to execute, and how does this support building event driven applications? What is a cold start in Lambda, and what strategies help reduce its impact on application performance? How do Lambda layers help share common code and dependencies across multiple functions without duplicating that code in every deployment package? How does Lambda support running functions within a VPC, and what performance considerations does this introduce compared to functions that do not need VPC access? How should an organization approach designing observability and error handling for a complex, multi function serverless architecture built on Lambda?
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AWS › Lambda & Serverless

How does Lambda concurrency work, and what is the difference between reserved concurrency and the account level concurrency limit?

Intermediate

Answer

Lambda's concurrency represents the number of simultaneous executions of your function that can run at any given moment, with every AWS account having an overall regional concurrency limit shared across all functions, and reserved concurrency lets you guarantee a specific portion of that overall limit exclusively for a particular function, both protecting that function from being starved of capacity by other functions and also capping how much that specific function can scale, which can be useful to prevent it from overwhelming a downstream resource like a database.
aws lambda put-function-concurrency --function-name critical-function --reserved-concurrent-executions 50
Real-world example A company reserves fifty concurrent executions specifically for its critical payment processing Lambda function, ensuring it always has guaranteed capacity available even if other, less critical functions in the same account experience an unexpected traffic spike.

Common follow-ups: What happens to a Lambda invocation if the concurrency limit is reached?;How do you request an increase to your account's overall Lambda concurrency limit?

RDS & Databases;Auto Scaling Groups
All 7 Lambda & Serverless questions