Lambda concurrency and throttling in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Understanding Lambda Throttling Behavior

You have an AWS Lambda function with a reserved concurrency limit set to 5. If 10 requests arrive simultaneously, what happens to the extra requests beyond the limit?

A The first 5 requests run immediately; the other 5 are throttled and receive a throttling error.

B The first 5 requests are queued and run after the initial 5 finish; no errors occur.

C All 10 requests run concurrently because Lambda automatically scales beyond reserved concurrency.

D Lambda rejects all 10 requests because the concurrency limit is set to 5.

💡 Hint

Think about what reserved concurrency means for Lambda execution limits.

Reserved concurrency limits the number of concurrent executions. Requests beyond this limit are throttled and receive errors immediately; they are not queued.

❓ Architecture

intermediate

Designing for High Lambda Concurrency

You expect a sudden spike of 1000 concurrent Lambda invocations. Your account's default concurrency limit is 100. What is the best architectural approach to handle this spike without throttling?

A Use an SQS queue to buffer requests and trigger Lambda with a controlled concurrency rate.

B Set the function's reserved concurrency to 1000 to allow all invocations.

C Deploy multiple identical Lambda functions and distribute requests evenly among them.

D Request a concurrency limit increase from AWS and use reserved concurrency for the function.

💡 Hint

Think about how to smooth out sudden spikes using AWS services.

Using an SQS queue buffers incoming requests and triggers Lambda at a controlled rate, preventing throttling. Simply increasing concurrency or deploying multiple functions does not solve account limits or sudden spikes effectively.

❓ security

advanced

Impact of Lambda Concurrency on Downstream Services

You have a Lambda function that writes to a database. The function has a reserved concurrency of 50. Suddenly, the function is throttled due to high traffic. What is a potential security risk if throttling is not handled properly?

A Throttled requests automatically bypass authentication checks.

B Throttled Lambda invocations may retry rapidly, causing a denial of service on the database.

C Throttling causes Lambda to expose environment variables to unauthorized users.

D Lambda throttling disables encryption for data sent to the database.

💡 Hint

Consider what happens when clients retry failed requests aggressively.

If throttled Lambda invocations retry too quickly, they can overwhelm downstream services like databases, causing denial of service or degraded performance.

✅ Best Practice

advanced

Configuring Reserved Concurrency for Multiple Lambda Functions

You have three Lambda functions: A, B, and C. Your account concurrency limit is 100. You want to ensure function A always has at least 30 concurrent executions available, but functions B and C can share the remaining concurrency. How should you configure reserved concurrency?

A Do not set reserved concurrency for any function; rely on account limit.

B Set reserved concurrency to 30 for function A; set 35 for B and 35 for C.

C Set reserved concurrency to 30 for function A; leave B and C without reserved concurrency.

D Set reserved concurrency to 100 for function A; no concurrency for B and C.

💡 Hint

Reserved concurrency guarantees capacity for a function.

Setting reserved concurrency for function A guarantees it always has 30 concurrent executions. Functions B and C share the remaining 70 concurrency without reserved limits, allowing flexible usage.

🧠 Conceptual

expert

Analyzing Lambda Throttling Metrics

You monitor your Lambda function's CloudWatch metrics and see a sudden spike in the Throttles metric while Invocations remain steady. What is the most likely explanation?

A The function's timeout is too short, causing throttling errors.

B The function is failing internally, causing retries that increase throttling.

C The function's memory size is too small, causing slow execution and throttling.

D The function's reserved concurrency limit was reached, causing throttling despite steady invocation count.

💡 Hint

Think about what causes throttling independent of invocation count.

Throttling occurs when concurrent executions exceed reserved concurrency limits. Even if invocation count is steady, hitting concurrency limits causes throttling.