Reserved concurrency in AWS Lambda is a feature that guarantees a specific number of concurrent executions for a particular Lambda function. This is crucial for troubleshooting and optimization in serverless architectures.
When you set reserved concurrency for a function, you allocate a portion of…Reserved concurrency in AWS Lambda is a feature that guarantees a specific number of concurrent executions for a particular Lambda function. This is crucial for troubleshooting and optimization in serverless architectures.
When you set reserved concurrency for a function, you allocate a portion of your account's total concurrency limit exclusively to that function. For example, if your account has a default limit of 1000 concurrent executions and you reserve 100 for a critical function, that function will always have access to those 100 execution slots.
Key benefits for optimization include:
1. **Guaranteed Availability**: Critical functions won't be throttled due to other functions consuming all available concurrency. This ensures consistent performance for essential business processes.
2. **Cost Control**: By limiting concurrency, you can prevent runaway costs from unexpected traffic spikes or recursive invocations.
3. **Downstream Protection**: Reserved concurrency acts as a throttle, protecting downstream resources like databases from being overwhelmed by too many simultaneous requests.
4. **Isolation**: Functions with reserved concurrency operate independently, preventing noisy neighbor problems within your account.
For troubleshooting, understanding reserved concurrency helps identify throttling issues. When a function reaches its reserved concurrency limit, additional invocations receive a 429 error (TooManyRequestsException). CloudWatch metrics like ConcurrentExecutions and Throttles help monitor these scenarios.
Configuration considerations:
- Setting reserved concurrency to 0 effectively disables the function
- The sum of all reserved concurrency cannot exceed your account limit minus 100 (kept for unreserved functions)
- Unreserved functions share the remaining concurrency pool
Best practices include setting appropriate reserved concurrency based on expected load patterns, monitoring throttle metrics, and adjusting values during performance testing. This feature is essential for building reliable, predictable serverless applications that maintain performance under varying load conditions.
Reserved Concurrency in AWS Lambda
What is Reserved Concurrency?
Reserved concurrency is a Lambda feature that guarantees a specific number of concurrent executions will always be available for a particular function. When you set reserved concurrency for a function, you are essentially carving out a portion of your account's total concurrency limit exclusively for that function.
Why is Reserved Concurrency Important?
Reserved concurrency serves two critical purposes:
1. Guaranteed Capacity: It ensures your critical functions always have execution capacity available, even during traffic spikes from other functions in your account.
2. Throttling Control: It acts as a maximum concurrency limit for a function, preventing it from consuming too many resources or overwhelming downstream systems like databases.
How Reserved Concurrency Works
AWS Lambda accounts have a default concurrency limit (typically 1,000 concurrent executions per region). When you reserve concurrency:
- The reserved amount is subtracted from your account's unreserved concurrency pool - Other functions cannot use the reserved concurrency - The function with reserved concurrency cannot exceed that limit - Setting reserved concurrency to 0 effectively disables the function (throttles all invocations)
Example: If your account has 1,000 concurrent executions and you reserve 100 for Function A, then Function A is guaranteed 100 executions, and all other functions share the remaining 900.
Reserved vs Provisioned Concurrency
Reserved Concurrency: Sets a maximum limit and guarantees availability from the pool. Functions still experience cold starts.
Provisioned Concurrency: Pre-initializes a specified number of execution environments to eliminate cold starts. This has additional costs.
Key Configuration Points
- Reserved concurrency is free to configure - Minimum value is 0 (disables the function) - Maximum is your account's unreserved concurrency minus 100 (AWS reserves 100 for other functions) - Can be set via AWS Console, CLI, or CloudFormation
Exam Tips: Answering Questions on Reserved Concurrency
Tip 1: When a question mentions protecting downstream resources (databases, APIs) from being overwhelmed, reserved concurrency is often the answer because it caps the maximum concurrent executions.
Tip 2: If a scenario describes critical functions that must always be able to execute even when other functions are consuming resources, think reserved concurrency.
Tip 3: Questions about temporarily disabling a function often have setting reserved concurrency to 0 as the correct answer.
Tip 4: Remember that reserved concurrency does NOT eliminate cold starts - only provisioned concurrency does that. If the question focuses on latency reduction or cold start elimination, provisioned concurrency is likely the answer.
Tip 5: Reserved concurrency is a free feature, while provisioned concurrency incurs additional charges. Cost-related questions may hinge on this distinction.
Tip 6: Watch for questions about throttling - if a function is being throttled and has reserved concurrency set, increasing the reserved concurrency value or removing the limit may be the solution.
Tip 7: Account-level concurrency limits affect all functions. If you see questions about functions competing for resources, reserved concurrency helps isolate and guarantee capacity for specific functions.