Nothing against mongo, but even with no extra context postgres is the right choice. More on reddit.com

Depends, dynamodb is just basically a fancy serverless key-value store (with possibility to add secondary indexes and what not). If your app knows the primary key of the things it needs to fetch. Then I'm sure dynamodb would be a good fit. If you need to scan for data, then perhaps not. More on reddit.com

If a relational database is the correct solution, then use a relational database. If he wants to cut costs, go with a smaller instance or something. edit: Another thing... don't let the customer prescribe a solution. The customer will provide the requirements, and based on those requirements, you should design the best solution to meet those requirements. If one of the requirements is a specific cost threshold, the cost requirement can be incorporated into the solutioning. But any time someone says "build it using this specific technology", tune them out and follow the requirements to reach a technology solution. More on reddit.com

I shared my opinion on this in r/flask a few days ago. If you need to run sophisticated queries on your data or provide users the ability to query based upon a large number of different attributes (like a contact management system where users need to search by name, government ID number, address, telephone number and possibly do various wildcard searches), then an RDBMS will be the best choice (RDS) If you need to simply retrieve a large dataset based on a known unique identifier with low latency, a NoSQL DB like DynamoDB is the preferred choice Between RDS and DynamoDB, the latter has the benefits of being a fully manged service where you don't have to manage instance types, storage types, security groups, subnets etc. You simply configure your tables, keys and indexes as well as what type of throughput they should have (and even then, you can just let DynamoDB handle that with on-demand capacity if you don't really care about the cost). DynamoDB also includes streaming options that allow you to push table events to other services and in-memory caching with DAX (though this is usually very expensive). I find access control with DynamoDB more straightforward as it's all managed through IAM permissions. RDS is better if your usage patterns are constant (applications are reading from/writing to your DB 24/7) as your charged per instance-hour and provisioned storage. Serverless is typically more expensive than managing instances yourself if the usage pattern is constant as the main benefit of serverless is pay-per-use. More on reddit.com