Lambda concurrency and throttling in AWS - Commands & Configuration

Introduction

When many requests try to run your AWS Lambda function at the same time, it can get overwhelmed. Lambda concurrency controls how many instances run simultaneously. Throttling happens when requests exceed this limit, causing some to wait or fail.

When you want to limit how many Lambda functions run at once to control costs.

When your backend service can only handle a certain number of requests at a time.

When you want to reserve capacity for critical Lambda functions to avoid delays.

When you want to prevent your Lambda from being overwhelmed by sudden traffic spikes.

When you want to monitor and handle throttled Lambda requests gracefully.

Commands

This command sets a limit of 5 concurrent executions for the Lambda function named 'my-lambda-function'. It helps control how many instances run at the same time.

Terminal

aws lambda put-function-concurrency --function-name my-lambda-function --reserved-concurrent-executions 5

Expected Output

{"ReservedConcurrentExecutions":5}

→

--function-name - Specifies the exact Lambda function to configure.

→

--reserved-concurrent-executions - Sets the maximum number of concurrent executions allowed.

This command checks the current concurrency limit set for the Lambda function to verify the configuration.

Terminal

aws lambda get-function-concurrency --function-name my-lambda-function

Expected Output

{"ReservedConcurrentExecutions":5}

→

--function-name - Specifies the Lambda function to query.

This command retrieves the number of throttled requests for the Lambda function in the last hour to monitor if throttling is happening.

Terminal

aws cloudwatch get-metric-statistics --namespace AWS/Lambda --metric-name Throttles --dimensions Name=FunctionName,Value=my-lambda-function --start-time 2024-06-01T00:00:00Z --end-time 2024-06-01T01:00:00Z --period 60 --statistics Sum

Expected Output

{"Label":"Throttles","Datapoints":[{"Timestamp":"2024-06-01T00:05:00Z","Sum":2}],"ResponseMetadata":{"RequestId":"1234abcd-5678-efgh-ijkl-9012mnopqrst"}}

→

--namespace - Specifies the AWS service namespace.

→

--metric-name - Specifies the metric to retrieve.

→

--dimensions - Filters the metric for the specific Lambda function.

Key Concept

If you remember nothing else from this pattern, remember: setting concurrency limits controls how many Lambda instances run at once, and throttling happens when requests exceed that limit.

Common Mistakes

Not setting any concurrency limit and expecting unlimited Lambda executions.

This can cause unexpected high costs or overwhelm downstream services.

Set a reserved concurrency limit to control usage and protect resources.

Setting concurrency too low without monitoring throttling metrics.

This causes many requests to be throttled and fail silently.

Monitor throttling metrics in CloudWatch and adjust concurrency accordingly.

Confusing reserved concurrency with unreserved concurrency pool.

Reserved concurrency guarantees capacity but reduces the shared pool for other functions.

Understand reserved concurrency reserves capacity and plan limits for all functions.

Summary

Use 'aws lambda put-function-concurrency' to set how many Lambda instances can run at the same time.

Check your concurrency settings with 'aws lambda get-function-concurrency'.

Monitor throttling with CloudWatch metrics to ensure your Lambda is not overwhelmed.

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