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.

Practice

1. What does Lambda concurrency control in AWS Lambda functions?

easy

A. The total number of functions in an AWS account

B. The number of function instances that can run at the same time

C. The size of the function's deployment package

D. The amount of memory allocated to the function

Solution

1. Step 1: Understand Lambda concurrency concept

   Concurrency in Lambda means how many copies of your function can run simultaneously.

2. Step 2: Identify what concurrency controls

   It controls the number of function instances running at the same time to handle requests.

3. Final Answer:

   The number of function instances that can run at the same time -> Option B

4. Quick Check:

   Concurrency = simultaneous function runs [OK]

Hint: Concurrency = how many functions run at once [OK]

Common Mistakes:

• Confusing concurrency with memory size
• Thinking concurrency limits total functions
• Mixing concurrency with deployment package size

2. Which of the following is the correct way to set reserved concurrency for a Lambda function using AWS CLI?

easy

A. aws lambda update-function --name MyFunc --concurrency 5

B. aws lambda set-concurrency --function MyFunc --reserved 5

C. aws lambda put-function-concurrency --function-name MyFunc --reserved-concurrent-executions 5

D. aws lambda configure-concurrency --function MyFunc --limit 5

Solution

1. Step 1: Recall AWS CLI command for reserved concurrency

   The correct command is 'put-function-concurrency' with '--reserved-concurrent-executions' option.

2. Step 2: Match the correct syntax

   aws lambda put-function-concurrency --function-name MyFunc --reserved-concurrent-executions 5 matches the exact AWS CLI syntax to set reserved concurrency.

3. Final Answer:

   aws lambda put-function-concurrency --function-name MyFunc --reserved-concurrent-executions 5 -> Option C

4. Quick Check:

   Reserved concurrency CLI = put-function-concurrency [OK]

Hint: Use 'put-function-concurrency' with reserved count [OK]

Common Mistakes:

• Using wrong command names like 'set-concurrency'
• Incorrect option flags
• Confusing update-function with concurrency settings

3. Given a Lambda function with a reserved concurrency of 3, what happens if 5 requests arrive simultaneously?

medium

A. 3 requests run immediately; 2 are throttled and may be retried

B. All 5 requests run simultaneously without delay

C. Only 1 request runs; others are queued

D. All 5 requests are rejected immediately

Solution

1. Step 1: Understand reserved concurrency effect

   Reserved concurrency limits how many instances run at once; here it is 3.

2. Step 2: Analyze request handling

   When 5 requests come, 3 run immediately; the other 2 exceed limit and get throttled (rejected or retried).

3. Final Answer:

   3 requests run immediately; 2 are throttled and may be retried -> Option A

4. Quick Check:

   Requests > concurrency = throttling [OK]

Hint: Requests over concurrency limit get throttled [OK]

Common Mistakes:

• Assuming all requests run simultaneously
• Thinking requests queue automatically
• Believing throttled requests are dropped without retry

4. You set a reserved concurrency of 2 for a Lambda function but notice throttling errors even with only 2 requests at once. What is the likely cause?

medium

A. The reserved concurrency setting was not applied correctly

B. The account-level concurrency limit is lower than 2

C. The function timeout is set too high

D. The function's memory size is too small

Solution

1. Step 1: Check reserved concurrency application

   If throttling occurs at 2 requests, reserved concurrency might not be set or applied properly.

2. Step 2: Rule out other causes

   Memory size and timeout do not cause throttling; account limit lower than 2 is unlikely if 2 requests run.

3. Final Answer:

   The reserved concurrency setting was not applied correctly -> Option A

4. Quick Check:

   Throttling with low requests = misconfigured concurrency [OK]

Hint: Check if reserved concurrency is actually set [OK]

Common Mistakes:

• Blaming memory size for throttling
• Ignoring concurrency setting errors
• Assuming timeout causes throttling

5. You have two Lambda functions, A and B, sharing an account concurrency limit of 10. Function A has reserved concurrency 7. If both functions receive 7 simultaneous requests each, how many requests for function B will be throttled?

hard

A. 3 requests for function B will be throttled

B. 0 requests for function B will be throttled

C. 7 requests for function B will be throttled

D. 4 requests for function B will be throttled

Solution

1. Step 1: Understand reserved concurrency reservation

   Function A reserves 7 concurrency, so it can run up to 7 instances without throttling.

2. Step 2: Calculate remaining concurrency for function B

   Account limit is 10, so 10 - 7 = 3 concurrency left for function B.

3. Step 3: Analyze requests for function B

   Function B gets 7 requests but only 3 can run concurrently; 7 - 3 = 4 requests are throttled.

4. Final Answer:

   4 requests for function B will be throttled -> Option D

5. Quick Check:

   Reserved concurrency reduces available concurrency [OK]

Hint: Reserved concurrency reduces shared limit for others [OK]

Common Mistakes:

• Ignoring reserved concurrency reservation
• Assuming full account concurrency is available to all
• Miscounting throttled requests