Highly Available = Multi-AZ Cluster Read-only + Near Real time = readable standby. Read replicas are async whereas readable standby is synchronous. https://stackoverflow.com/questions/70663036/differences-b-w-aws-read-replica-and-the-standby-instances

It's either C or D. To be honest, I find the newest questions to be ridiculously hard (roughly 500+). I agree with @alexandercamachop that Multi Az in Instance mode is cheaper than Cluster. However, with Cluster we have reader endpoint available to use out-of-box, so there is no need to provide read-replicas, which also has its own costs. The ridiculous part is that I'm pretty sure even the AWS support would have troubles to answer which configuration is MOST cost-effective.

Option D: Multi-AZ cluster deployment with two readable standby instances would be more costly and is not necessary if read replicas are sufficient for the data scientists' needs. Thus, Option C is the most cost-effective and operationally efficient solution to meet the company's requirements.

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/multi-az-db-clusters-concepts.html

Not A - Not "highly available" Not B - "Access to the secondary instance" is not possible in Multi-AZ Not C - Multi-AZ + two (!) read replicas is more expensive than cluster D - Provides "readable standby instances"

D https://aws.amazon.com/about-aws/whats-new/2023/01/amazon-rds-multi-az-readable-standbys-rds-postgresql-inbound-replication/

https://aws.amazon.com/blogs/database/choose-the-right-amazon-rds-deployment-option-single-az-instance-multi-az-instance-or-multi-az-database-cluster/ C would mean you are paying for 4 instances (primary, backup, and 2 read instances). D would be 3 (primary, and 2 backup). Difficult to be sure, pricing calculator doesn't even include clusters yet.

Option D is the most cost-effective solution that meets the requirements for this scenario. The key considerations are: Data scientists need read-only access to near real-time production data without affecting performance. High availability is required. Cost should be minimized.

https://aws.amazon.com/blogs/database/choose-the-right-amazon-rds-deployment-option-single-az-instance-multi-az-instance-or-multi-az-database-cluster/ only multi AZ cluster have reader endpoint. multi AZ instance secondary replicate is not allow to access

Support for D: Amazon RDS now offers Multi-AZ deployments with readable standby instances (also called Multi-AZ DB cluster deployments) in preview. You should consider using Multi-AZ DB cluster deployments with two readable DB instances if you need additional read capacity in your Amazon RDS Multi-AZ deployment and if your application workload has strict transaction latency requirements such as single-digit milliseconds transactions. https://aws.amazon.com/blogs/database/readable-standby-instances-in-amazon-rds-multi-az-deployments-a-new-high-availability-option/

Unlike Multi-AZ instance deployment, where the secondary instance can't be accessed for read or writes, Multi-AZ DB cluster deployment consists of primary instance running in one AZ serving read-write traffic and two other standby running in two different AZs serving read traffic.

D. using Multi-AZ DB cluster deployments with two readable DB instances if you need additional read capacity in your Amazon RDS Multi-AZ deployment and if your application workload has strict transaction latency requirements such as single-digit milliseconds transactions. https://aws.amazon.com/blogs/database/readable-standby-instances-in-amazon-rds-multi-az-deployments-a-new-high-availability-option/ while on read replicas, Amazon RDS then uses the asynchronous replication method for the DB engine to update the read replica whenever there is a change to the primary DB instance. https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ReadRepl.html

Why not b. Shouldnt it have less number of instances than both c and d?

D: https://aws.amazon.com/tw/blogs/database/readable-standby-instances-in-amazon-rds-multi-az-deployments-a-new-high-availability-option/

Forgot to vote

I think it's D. C: Multi-AZ instance = active + standby + two read replicas = 4 RDS instances D: Multi-AZ cluster = Active + two standby = 3 RDS instances Single-AZ and Multi-AZ deployments: Pricing is billed per DB instance-hour consumed from the time a DB instance is launched until it is stopped or deleted. https://aws.amazon.com/rds/postgresql/pricing/?pg=pr&loc=3 In the case of a cluster, you will pay less.

Multi-AZ instance: the standby instance doesn’t serve any read or write traffic. Multi-AZ DB cluster: consists of primary instance running in one AZ serving read-write traffic and two other standby running in two different AZs serving read traffic. https://aws.amazon.com/blogs/database/choose-the-right-amazon-rds-deployment-option-single-az-instance-multi-az-instance-or-multi-az-database-cluster/