Lambda with S3 event triggers in AWS - Time & Space Complexity

Time Complexity: Lambda with S3 event triggers

O(n)

Understanding Time Complexity

We want to understand how the time to process files grows when using Lambda triggered by S3 events.

Specifically, how does the number of files affect the number of Lambda executions?

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.

// S3 bucket configured to trigger Lambda on object creation
// Lambda function processes each new object

exports.handler = async (event) => {
  for (const record of event.Records) {
    const bucket = record.s3.bucket.name;
    const key = record.s3.object.key;
    // Process the object identified by bucket and key
  }
};
    

This sequence shows Lambda triggered by S3 when new files are added, processing each file individually.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: Lambda function invocation triggered by each S3 object creation event.
• How many times: Once per new object uploaded to the S3 bucket.

How Execution Grows With Input

Each new file uploaded causes one Lambda invocation, so the total executions grow directly with the number of files.

Input Size (n)	Approx. API Calls/Operations
10	10 Lambda invocations
100	100 Lambda invocations
1000	1000 Lambda invocations

Pattern observation: The number of Lambda calls grows linearly as more files are added.

Final Time Complexity

Time Complexity: O(n)

This means the total Lambda executions increase directly in proportion to the number of files uploaded.

Common Mistake

[X] Wrong: "One Lambda invocation can process all files at once, so time stays the same regardless of file count."

[OK] Correct: Each file upload triggers a separate event and Lambda invocation; they do not batch automatically.

Interview Connect

Understanding how event-driven functions scale with input size helps you design systems that handle growth smoothly and predict costs.

Self-Check

"What if the Lambda function was triggered by a batch of S3 events instead of one per file? How would the time complexity change?"

Practice

1. What happens when you configure an AWS Lambda function with an S3 event trigger for object creation?

easy

A. The Lambda function runs manually and is not triggered by S3 events.

B. The Lambda function runs only when the S3 bucket is deleted.

C. The Lambda function runs automatically whenever a new file is uploaded to the S3 bucket.

D. The Lambda function runs only when an object is deleted from the S3 bucket.

Solution

1. Step 1: Understand S3 event triggers

   S3 event triggers can invoke Lambda functions automatically on specific events like object creation.

2. Step 2: Identify the event type for object creation

   When a new file is uploaded, the event type is 'ObjectCreated', which triggers the Lambda function.

3. Final Answer:

   The Lambda function runs automatically whenever a new file is uploaded to the S3 bucket. -> Option C

4. Quick Check:

   S3 ObjectCreated event triggers Lambda automatically [OK]

Hint: Remember: S3 triggers Lambda on file uploads automatically [OK]

Common Mistakes:

• Confusing object deletion with creation events
• Thinking Lambda runs manually without triggers
• Assuming triggers only work on bucket deletion

2. Which of the following is the correct JSON snippet to configure an S3 event trigger for a Lambda function on object creation?

easy

A. { "Events": ["s3:ObjectCreated:*"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" }

B. { "Events": ["s3:ObjectRemoved:*"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" }

C. { "Events": ["lambda:InvokeFunction"], "LambdaFunctionArn": "arn:aws:s3:::myBucket" }

D. { "Events": ["s3:ObjectCreated:Delete"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" }

Solution

1. Step 1: Identify correct event type for object creation

   The event type for object creation is "s3:ObjectCreated:*" which covers all create events.

2. Step 2: Check Lambda function ARN placement

   The LambdaFunctionArn must point to the Lambda function's ARN, not the bucket ARN.

3. Final Answer:

   { "Events": ["s3:ObjectCreated:*"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" } -> Option A

4. Quick Check:

   Use "s3:ObjectCreated:*" for creation triggers [OK]

Hint: Use "s3:ObjectCreated:*" for all object creation events [OK]

Common Mistakes:

• Using ObjectRemoved event instead of ObjectCreated
• Putting bucket ARN instead of Lambda ARN
• Using invalid event names like ObjectCreated:Delete

3. Given this Lambda function code triggered by S3 object creation:

def lambda_handler(event, context):
    bucket = event['Records'][0]['s3']['bucket']['name']
    key = event['Records'][0]['s3']['object']['key']
    return f"File {key} uploaded to {bucket}"

What will be the output if a file named 'photo.jpg' is uploaded to bucket 'my-images'?

medium

A. "File uploaded to my-images"

B. "File photo.jpg uploaded to my-images"

C. "File my-images uploaded to photo.jpg"

D. Runtime error due to missing event keys

Solution

1. Step 1: Extract bucket and key from event

   The code accesses event['Records'][0]['s3']['bucket']['name'] as bucket name and event['Records'][0]['s3']['object']['key'] as file key.

2. Step 2: Substitute values from the upload event

   For file 'photo.jpg' uploaded to 'my-images', bucket='my-images' and key='photo.jpg'.

3. Final Answer:

   "File photo.jpg uploaded to my-images" -> Option B

4. Quick Check:

   Event keys map file and bucket correctly [OK]

Hint: Check event['Records'][0]['s3'] keys for bucket and object [OK]

Common Mistakes:

• Mixing bucket name and object key positions
• Assuming event keys are missing causing errors
• Confusing string formatting output

4. You configured an S3 bucket to trigger a Lambda function on object creation, but the Lambda is not invoked. What is the most likely cause?

medium

A. The S3 bucket notification configuration is missing permission for S3 to invoke Lambda.

B. The S3 bucket is in a different AWS region than the Lambda function.

C. The Lambda function code has a syntax error.

D. The Lambda function's execution role lacks permission to read from S3.

Solution

1. Step 1: Check permissions for S3 to invoke Lambda

   S3 must have permission to invoke the Lambda function via a resource-based policy on Lambda.

2. Step 2: Understand common misconfigurations

   Missing this permission causes Lambda not to trigger even if bucket notifications are set.

3. Final Answer:

   The S3 bucket notification configuration is missing permission for S3 to invoke Lambda. -> Option A

4. Quick Check:

   S3 needs invoke permission on Lambda [OK]

Hint: Grant S3 permission to invoke Lambda in Lambda's resource policy [OK]

Common Mistakes:

• Confusing Lambda execution role with S3 invoke permission
• Assuming code errors prevent invocation
• Thinking region mismatch blocks invocation

5. You want a Lambda function to process only PNG files uploaded to an S3 bucket. How should you configure the S3 event trigger to achieve this?

hard

A. Set the event type to 's3:ObjectCreated:Put' without any filters.

B. Set the event type to 's3:ObjectRemoved:*' and add a prefix filter '.png'.

C. Configure the Lambda function code to ignore non-PNG files without using filters.

D. Set the event type to 's3:ObjectCreated:*' and add a filter with suffix '.png' in the bucket notification configuration.

Solution

1. Step 1: Use event type for object creation

   Use 's3:ObjectCreated:*' to capture all create events including uploads.

2. Step 2: Apply suffix filter for '.png'

   Configure the bucket notification with a filter to trigger Lambda only for files ending with '.png'.

3. Final Answer:

   Set the event type to 's3:ObjectCreated:*' and add a filter with suffix '.png' in the bucket notification configuration. -> Option D

4. Quick Check:

   Use event filters to limit Lambda triggers by file type [OK]

Hint: Use suffix filter '.png' in S3 event trigger for file type filtering [OK]

Common Mistakes:

• Using ObjectRemoved event instead of ObjectCreated
• Relying only on Lambda code filtering without event filters
• Using prefix filter with file extension instead of suffix