A large real-estate brokerage is exploring the option of adding a cost-effective location-based alert to their existing mobile application. The application backend infrastructure currently runs on AWS. Users who opt for this service will receive alerts on their mobile devices regarding real-estate offers in proximity to their location. For the alerts to be relevant, delivery time needs to be in the low minute count. The existing mobile app has 5 million users across the US. Which one of the following architectural suggestions would you make to the customer?

Answer options:

A.The mobile application will submit its location details to a web service endpoint using ELB and EC2 instances. DynamoDB will be used to store and retrieve the relevant offers from EC2 instances, which would then communicate with mobile carriers or device providers to push alerts back to the mobile application.
B.Use AWS DirectConnect or VPN to establish connectivity with the mobile carrier. EC2 instances will receive the mobile application`s location details through the mobile carrier. RDS will be used to store and retrieve the relevant offers. EC2 instances will communicate with the mobile carrier to push alerts back to the mobile application.
C.SQS ( buffer storage ) would be used to capture the device location details sent from the Mobile application. EC2 instances will process the messages from the SQS queue and retrieve the relevant offers from DynamoDB. SNS Mobile Push will be used to send offers to the mobile application.
D.The mobile application will send the device location`s details using the SNS Mobile Push. EC2 instances will retrieve the relevant offers from DynamoDB. EC2 instances will communicate with the mobile carrier or the device provider to push alerts back to the mobile application.

Answer – C
The scenario has the following architectural considerations: (1) the users should get notifications about the real estate in the area near to their location, (2) only subscribed users should get the notification, (3) the notification delivery should be fast, (4) the architecture should be scalable, and (5) it should be cost-effective.
When the question has considerations for scalability, always think about DynamoDB as it is the most recommended database solution to handle the huge amount of data/records. For automated notifications, always think about SNS. 
Option A is incorrect because sending notifications via mobile earners/device providers as alerts is neither feasible nor cost-effective. 
Option B is incorrect because receiving location via Direct Connect and carrier connection is not cost-effective. It also does not deal with subscriptions. Sending notifications via mobile carriers as the alert is not cost-effective as well.
Option C is CORRECT because (a) SQS is a highly scalable, cost-effective solution for carrying out utility tasks such as holding the location of millions of users, (b) it uses highly scalable DynamoDB, and (c) it uses the cost-effective AWS SNS Mobile Push service to send push notification messages directly to apps on mobile devices.
Option D is incorrect because the AWS SNS Mobile Push service is used to send push notification messages to the mobile devices, not to get the mobile devices` location.
For more information on AWS SNS Mobile Push, please see the diagram and link given below:
https://docs.aws.amazon.com/sns/latest/dg/SNSMobilePush.html
Note:
Option C says that the mobile application will send the device location to the processing EC2 instances using SQS. Then the instances would look at the DynamoDB database for offers relevant to the location. Then finally, SNS Mobile Push, which is part of SNS, will be used to send offers to the mobile application. So it leverages both SQS as well as SNS functionality for different parts of the architecture. This is the correct solution to this problem.