> Can you specify in which scenario you think Redis is cheaper than caching things in, say, dynamodb.
When not hosted on AWS? Who says we have to compare dynamodb to AWS managed Redis? Redis the company has paid hosted versions. You can run it as part of your k8s cluster too.
> High read/write and low-ish size. Also it's faster
You posted a vague and meaningless assertion. If you do not have latency numbers and cost differences, you have absolutely nothing to show for, and you failed to provide any rationale that justified even whether any cache is required at all.
> DynamoDB On-Demand: Typical latency is single-digit milliseconds (usually between 1–10 milliseconds for standard requests)
I know very little use cases where that difference is meaningful. Unless you have to do this many times sequentially in which case optimizing that would be much more interesting than a single read being .5 ms versus the typical 3 to 4 for dynamo (that last number is based on experience)
> At 10k RPS you'll see a significant cost savings with Redis over DynamoDB.
You need to be more specific than that. Depending on your read/write patterns and how much memory you need to allocate to Redis, back of the napkin calculations still point to the fact that Redis can still cost >$1k/month more than DynamoDB.
Did you actually do the math on what it costs to run Redis?
Can you specify in which scenario you think Redis is cheaper than caching things in, say, dynamodb.