The scene:

I needed to process data from an s3 bucket using pyspark. The s3 bucket was owned by a different account. I had done this before. But this time, there was a twist: we needed to encrypt the data because of GDPR requirements. At the end of the processing, I needed to save the results to another s3 bucket for loading into Redshift.

Thus began a weeks-long saga of learning about AWS the hard way.