Putting all resources in one stack is a bad idea, since different tiers have different life cycles and frequencies of change. For additional guidance about organizing your stacks, you can use two common frameworks: a multi-layered architecture and service-oriented architecture (SOA).
http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/best-
practices.html#organizingstack s

Spot instances are the cheapest option, and you can use minimum run duration if your AMI takes more than a few minutes to create.
Spot instances are also available to run for a predefined duration - in hourly increments up to six hours in length - at a significant discount (30-45%) compared to On-Demand pricing plus an additional 5% during off-peak times1 for a total of up to 50% savings.
https://aws.amazon.com/ec2/spot/pricing/

Explanation
The AWS Documentation mentions the following
After you add lifecycle hooks to your Auto Scaling group, they work as follows:
1. Auto Scaling responds to scale out events by launching instances and scale in events by terminating instances.
2. Auto Scaling puts the instance into a wait state (Pending:Wait orTerminating: Wait). The instance is paused until either you tell Auto Scaling to continue or the timeout period ends.
For more information on Autoscaling Lifecycle hooks, please visit the below URL: * http://docs.aws.a mazon.com/autoscaling/latest/userguide/lifecycle-hooks.htm I

Explanation
The AWS Documentation mentions the following on package updates for the Clastic beanstalk environment
You can configure your environment to apply minor and patch version updates automatically during a
configurable weekly maintenance window with Managed
Platform Updates. Elastic Beanstalk applies managed updates with no downtime or reduction in capacity, and
cancels the update immediately if instances running
your application on the new version fail health checks.
For more information on Elastic beanstalk managed updates please refer to the URL:
* https://docs.aws.a mazon.com/elasticbeanstalk/latest/dg/environment- platform -update-managed, html
* http://docs.aws.amazon.com/elasticbeanstalk/latest/dg/using-features.platform.upgrade.html