AWS Lambda concurrency settings control how many instances of your function can run simultaneously to handle incoming requests. Understanding these settings is crucial for the AWS Certified Developer - Associate exam.
**Types of Concurrency:**
1. **Unreserved Concurrency**: By default, all Lambda…AWS Lambda concurrency settings control how many instances of your function can run simultaneously to handle incoming requests. Understanding these settings is crucial for the AWS Certified Developer - Associate exam.
**Types of Concurrency:**
1. **Unreserved Concurrency**: By default, all Lambda functions in an account share a pool of 1,000 concurrent executions (this limit varies by region). Functions compete for available capacity on a first-come, first-served basis.
2. **Reserved Concurrency**: You can allocate a specific number of concurrent executions to a function. This guarantees that the function always has capacity available and prevents it from consuming all account-level concurrency. Setting reserved concurrency to zero effectively disables the function.
3. **Provisioned Concurrency**: This feature pre-initializes a specified number of execution environments. It eliminates cold starts by keeping functions warm and ready to respond, which is ideal for latency-sensitive applications.
**Key Considerations:**
- **Throttling**: When a function exceeds its concurrency limit, Lambda throttles additional requests. For synchronous invocations, this returns a 429 error. For asynchronous invocations, Lambda retries automatically.
- **Scaling Behavior**: Lambda scales by adding execution environments. The initial burst capacity varies by region (500-3000), then scales by 500 instances per minute until reaching the concurrency limit.
- **Cost Implications**: Provisioned concurrency incurs additional charges since you pay for keeping environments warm, regardless of actual usage.
**Best Practices:**
- Use reserved concurrency to protect downstream resources from being overwhelmed
- Implement provisioned concurrency for production workloads requiring consistent low latency
- Monitor concurrency metrics through CloudWatch to optimize settings
- Consider function timeout values when calculating needed concurrency
Properly configuring concurrency settings ensures your Lambda functions perform reliably while managing costs and protecting other functions in your account from resource starvation.
Lambda Concurrency Settings
Why Lambda Concurrency Settings Are Important
Lambda concurrency settings are crucial for controlling how your serverless applications scale and perform under load. Understanding these settings helps you manage costs, prevent downstream service overload, and ensure predictable application behavior. For the AWS Developer Associate exam, this topic frequently appears in questions about performance optimization, throttling, and application architecture.
What Are Lambda Concurrency Settings?
Concurrency in AWS Lambda refers to the number of function instances serving requests at any given time. AWS provides two main types of concurrency controls:
1. Reserved Concurrency This sets a maximum limit on the number of concurrent executions for a specific function. When you reserve concurrency for a function, that capacity is dedicated exclusively to that function and cannot be used by other functions in your account. This also acts as a maximum limit - the function will never exceed this number of concurrent executions.
2. Provisioned Concurrency This keeps a specified number of function instances initialized and ready to respond. Unlike reserved concurrency, provisioned concurrency eliminates cold starts by keeping execution environments warm. You pay for this capacity whether it is used or not.
How Lambda Concurrency Works
AWS Lambda has an account-level concurrency limit (default 1,000 concurrent executions per region, which can be increased). When a function is invoked:
- If an idle instance exists, Lambda routes the request to it - If no idle instance exists and concurrency limits allow, Lambda creates a new instance - If concurrency limits are reached, the request is throttled
Throttling Behavior: - Synchronous invocations return a 429 error (TooManyRequestsException) - Asynchronous invocations are retried automatically and then sent to a dead-letter queue if configured - Event source mappings (like SQS, Kinesis) retry based on the source configuration
Unreserved Concurrency: Lambda always keeps a pool of unreserved concurrency (minimum 100) available for functions that do not have reserved concurrency set. This ensures that functions can still execute even when other functions consume their reserved capacity.
Key Differences Between Reserved and Provisioned Concurrency
Reserved Concurrency: - Free to configure - Sets a maximum execution limit - Guarantees capacity is available - Does not prevent cold starts - Subtracts from the account pool
Provisioned Concurrency: - Incurs additional costs - Keeps instances pre-initialized - Eliminates cold starts - Ideal for latency-sensitive applications - Can be configured with Application Auto Scaling
Exam Tips: Answering Questions on Lambda Concurrency Settings
Tip 1: Identify the Problem Being Solved - Cold start issues → Think Provisioned Concurrency - Protecting downstream resources → Think Reserved Concurrency - Function consuming too many resources → Think Reserved Concurrency as a throttle - Cost optimization with predictable load → Think Provisioned Concurrency with auto-scaling
Tip 2: Remember the Throttling Responses - 429 errors indicate throttling - Synchronous calls fail when throttled - Asynchronous calls retry automatically (up to 2 times)
Tip 3: Understand the Account-Level Impact - Reserved concurrency for one function reduces available concurrency for others - A minimum of 100 unreserved concurrency is always maintained - Account limits can be increased through AWS Support
Tip 4: Cost Considerations - Reserved concurrency has no additional cost - Provisioned concurrency charges apply even when not actively processing requests - Use provisioned concurrency only when cold start latency is unacceptable
Tip 5: Common Exam Scenarios - A function is overwhelming a database → Set reserved concurrency to limit parallel connections - Users experience slow first responses → Configure provisioned concurrency - Critical function must always have capacity → Reserve concurrency to guarantee availability - Spiky traffic patterns → Use provisioned concurrency with auto-scaling based on schedule or utilization
Summary
Lambda concurrency settings give you fine-grained control over function scaling. Reserved concurrency limits maximum executions and guarantees capacity, while provisioned concurrency eliminates cold starts for latency-sensitive workloads. Understanding when to apply each setting is essential for both real-world applications and the AWS Developer Associate exam.