SeaweedFS and this project have different purposes. This project is intended to show off how to configure NGINX to act as a S3 proxying gateway by using [njs](https://nginx.org/en/docs/njs/). If you look at the github for it, you will see it is just a collection of nginx config and javascript files. This all works will standard open source NGINX. All it does is proxy files like a L7 load balancer, but in this case, it adds AWS v2/v4 headers to the upstream requests.
As for caching, that is totally configurable to whatever you want; the example configuration is set to 1 hour but that is arbitrary. In fact, one of the interesting this is all of the additional functionality that can be enabled because the proxying is being done by NGINX.
Regarding read and write, that can be enabled for AWSv2 signatures, but it is more difficult to do in AWSv4 signatures. I have an idea about how to accomplish it with v4 signatures, but it will take some time to prototype it.
What is "asynchronous write back"?