SimpleDB | Implementation of simpleDB from MIT database systems course | Learning library

I read this answer, which clarified a lot of things, but I'm still confused about how I should go about designing my primary key.

First off I want to clarify the idea of WCUs. I get that WCU is the write capacity of max 1kb per second. Does it mean that if writing a piece of data takes 0.25 seconds, I would need 4 of those to be billed 1 WCU? Or each time I write something it consumes 1 WCU, but I could also write X times within 1 second and still be billed 1 WCU?

Usage

I want to create a table that stores the form data for a set of gyms (95% will be waivers, the rest will be incidents reports). Most of the time, each forms will be accessed directly via its unique ID. I also want to query the forms by date, form, userId, etc..

We can assume an average of 50k forms per gym

Options

• First option is straight forward: having the formId be the partition key. What I don't like about this option is that scan operations will always filter out 90% of the data (i.e. the forms from other gyms), which isn't good for RCUs.

• Second option is that I would make the gymId the partition key, and add a sort key for the date, formId, userId. To implement this option I would need to know more about the implications of having 50k records on one partition key.

• Third option is to have one table per gyms and have the formId as partition key. This seems to be like the best option for now, but I don't really like the idea of having a a large number of tables doing the same thing in my account.

Is there another option? Which one of the three is better?

Edit: I'm assuming another option would be SimpleDB?

I have the following requirements:

• For every deleted record in RDS we need to archive it into somewhere cheaper on AWS.
• Reduce storage cost
• Not using Glacier
• Context oriented (e.g. a file per table)
• re-import is not a requirement

I'm not an experienced user with AWS, so I'm still a bit lost among the amount of options it has to offer and I'd like to know if you have more ideas to help me clear it out.

Initial thoughts:

1. The microservice that deletes the record, might send it to a broker (RabbitMQ for e.g.) and another microservice (let's call it archiver) will listen to it, write into a file, zip and send to S3. This approach has some technical challenges though: in order to make sense create big files, I need to wait the queue to growth a bit, wrap it into a stream and zip inside S3. The transaction control is very weak as well, since file writing and ack on messages are signal based i.e. I'll remove the messages from the broker just after the file is created.
2. Add a new column to the "archiveble" tables as "deleted (bool)" and run a separate job fetching only those records and saving them into S3. Discarded they don't want the new microservice with access to other's databases.
3. Following the same approach as in the first item, but instead of save into S3, save into a cheaper database. SimpleDB?

I have tried all of the solutions I could find and still haven't been able to get around this error. The error occurs in the getNotes() section, in its forth line. Also, the getNote() is currently not doing anything, so you don't need to look at that. I directly copied all code from these youtube tutorials, which I then adapted to my needs.

https://www.youtube.com/watch?v=pa_lghjVQVA&t=933s

Thanks for any help.