DynamoDB vs RDS for a counter store?

I have multiple lambda functions, which a file passes through for processing, and I want to store the status of each computation in a counter, for better logging and monitoring.

Current plan is to log it like this:

file-name  Lambda1  Lambda2 Lambda3
Name1      Done     Done    In-process

And, I'll be updating the values as Received, In-process and Done under the respective Lambda functions for the file.

So, two doubts:

• Is this a good practice? If not, then is there a better alternative for the same?

• If it is, then what would make a better/cheaper store? RDS or DynamoDB?

Why not something like Redis? (can use it through ElastiCache). Hard to answer this without knowing the operational requirements of this project. Is this data strictly for logging/research, or is it operational (i.e. this "table" is consulted in order to know which lambda to invoke).

Dynamo vs. RDS - depends how spiky your traffic is. Dynamo is very scalable and will give you a guaranteed throughput. But if you traffic can grow 10x over hours (spiky) Dynamo might be too expensive, since you'll need the maximum throughput provisioned or else you'll get throttled.

RDS is basically managed RDBMS on an instance type that you choose, so it can handle larger spikes, but ultimately not as scalable as Dynamo.

Again, I would consider Redis, for it's simplicity (seems like you just need a KV/counter store), and incredible speed/efficiency. If it's just for logging, it's probably good enough.

StackExchange DevOps Q#261, answer score: 4