You and your colleague are working on a serverless application that involves the registration of 500 users at a time. You have written two functions to support this where one function handles the storing of information to DynamoDB, and the other sends a confirmation email using SES.One of the functions that sends an email when a new user’s information is saved to DynamoDB doesn’t scale. On querying DynamoDB, you realize all 500 user’s information is saved successfully. But CloudWatch shows failed invocations for the second function. How will you address the bottleneck in the least possible time?

Answer options:

A.Use AWS ALB with Lambda function to manage traffic.
B.Refactor code to decouple and create multiple functions to send emails.
C.Reserve concurrency for your second Lambda function.
D.Get your second function to poll DynamoDB for new records.

Answer: C
Option A is incorrect because setting up ALB and routing traffic through ALB can take time.
Option B is incorrect because refactoring code may consume a long time and may address the bottleneck.
Option C is CORRECT given that 500 users are registering simultaneously, concurrency would be extremely important, and reserving it would guarantee invocations without having to use the unreserved pool of concurrency. Each account has a default concurrency limit of 900 as concurrent execution for all the available lambda functions, and 100 must be left for unreserved lambdas. Therefore setting concurrency per function to 500 will meet this requirement. As a developer, it is extremely important to know the constraints of the operating environment.
Option D is incorrect because polling DynamoDB may result in unnecessary invocations that can overload downstream resources such as SES.
Reference:
https://docs.aws.amazon.com/lambda/latest/dg/per-function-concurrency.html
 As mentioned here, Concurrency can be defined per function which reserves capacity for the function.
https://aws.amazon.com/about-aws/whats-new/2017/11/set-concurrency-limits-on-individual-aws-lambda-functions/