What is concurrency in AWS Lambda?

Difficulty: 6/10

reserved concurrency, provisioned concurrency, scaling limits

Concurrency in AWS Lambda refers to the number of instances of a function that can run simultaneously. When multiple events invoke a Lambda function at the same time, AWS Lambda launches multiple instances (up to a limit) to handle the load in parallel. This helps in handling scale and latency effectively for event-driven workloads.

Types of Concurrency in Lambda

1
Unreserved Concurrency: The default concurrency pool shared by all functions in an account.
2
Reserved Concurrency: Dedicates a fixed number of concurrent executions to a specific function.
3
Provisioned Concurrency: Pre-initializes Lambda instances so they’re ready to respond instantly, ideal for low-latency needs.

Important Notes About Lambda Concurrency

1
Each function can scale up to the regional concurrency limit by default (e.g., 1,000 concurrent executions).
2
Setting reserved concurrency for a function prevents other functions from affecting its concurrency.
3
Provisioned concurrency helps eliminate cold starts for high-performance applications.

CLI Command to Set Reserved Concurrency

Scenario Questions

0-2 years experience

2-5 years experience

1During a rollout you observe that a spike in traffic is causing downstream API timeouts. Walk me through how you’d diagnose whether Lambda concurrency is the root cause and what you’d change.

Follow-up Questions