You have a stock market trading application. It sends real-time data to AWS Kinesis which is then connected to a Lambda function to process and sort the information before it saves to a DynamoDB table. Customers then consume this table via a dashboard. As soon as the market opens, your customers complain that not all data is delivered to them. Which Lambda CloudWatch metric should you look at first to resolve this problem?

Answer options:

A.Throttles
B.Dwell time
C.ConcurrentExecutions
D.IteratorAge

Answer: D
Option A is incorrect as throttles will measure the number of Lambda function invocation attempts that were throttled when the invocation rates are more than concurrency limits. In this case, this is not the first metric to look at, as failed invocations will often trigger retry attempts.
Option B is incorrect as this option is not a CloudWatch metric but a metric that can be observed via X-Ray.
Option C is incorrect as this metric is an aggregated value for all Lambda functions making it difficult to find data per function. 
Option D is CORRECT as iterator age will determine the age of stream records processed by functions. Amazon says, “Measures the age of the last record for each batch of records processed. Age is the difference between the time Lambda received the batch and when the last record in the batch was written to the stream. “This then leads to exceeding retention period for Kinesis. This should be the first to look at as data streams are not processed as fast as the stream records are getting generated. When the stock market opens, it`s generally one of the busiest times of the day and could cause a spike in records.
Reference:
https://docs.aws.amazon.com/lambda/latest/dg/monitoring-functions-metrics.html
https://aws.amazon.com/premiumsupport/knowledge-center/lambda-iterator-age/