Exam AWS Certified Solutions Architect - Professional topic 1 question 903 discussion

BD is right, 2 instances must run always so get RIs for that + spot fleet to compensate increased traffic

A,B is optimal for costs, as D is fixing the autoscale to always have 2 other spots... so the reserved instances are for what?

spot fleets allow you to select on demand capacity and set target tracking using CW metrics. "A" also specifies that target tracking is set to an acceptable backlog. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/on-demand-in-spot.html https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-fleet-target-tracking.html D is incorrect as you are setting the minimum value for spot to 2 rather than 0. It also states that scaling will be set to the size of the queue which I interpret as 1 instance to 1 message which seems a bit excessive and may be costly unless processing time per message is large.

AC gives the option of going for spot instances and the facility to launch EC2 incase of spot not being available. Purchasing reserved instances won't save cost in this case

Correct ab

BD is right, 2 instances must run always so get RIs for that + spot fleet to compensate increased traffic

D is wrong. If you choose B,D, then you have two reserved instances and auto scaling group which manages spot instances. So why you need set minimum value to 2 for the spot instances taking account that you already have two reserved instances. it should set to 0. So A and B are right

BD better than AB

BD https://docs.aws.amazon.com/autoscaling/ec2/userguide/ec2-auto-scaling-mixed-instances-groups.html#capacity-optimized-aws-cli

2 always available rest spot. D vrs A: D uses SQS for scaling which makes sense. We end up with 2 RI + Spot as needed and available.

I'll go with A and B. RI for 2 + Spots B and D make no sense. RI for nothing + Spots

The key word is "combination of actions" and "cost effectively". RIs for the 2 you must have and spot for the rest.

BD 100%

AB because D is all spot which doesn't ensure there are always 2 instances are running. Also you need to actually have 2 on-demand instances running for the reserved instance discount in B to kick in - purchasing reserved instances alone doesn't result in anything running. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-reserved-instances.html Reserved Instances are not physical instances, but rather a billing discount applied to the use of On-Demand Instances in your account.

D is not correct because the instance must be available anytime.. so A B is the preferable

My choice is A&B

Both A and D will allow you to have 2 on-demand instances running full time (covered by the RIs bought in 'B') and scale with spot instances. The difference between the answers is the Spot fleet says it scales from the instance backlog while the ASG is scaling from the SQS queue. It's better to scale from the SQS queue, so I would choose B & D.