An "S3-compatible" object storage service exposes the same HTTP API and authentication scheme as Amazon S3, allowing any S3-aware client (the AWS SDK, MinIO mc, rclone, Restic, AWS CLI, hundreds of backup tools) to read and write to it without code changes. Compatibility is usually 90-99% rather than 100% — projects pick the most-used subset of S3 features and skip the rest. The AWS S3 API has become the de-facto standard wire protocol for object storage, in the same way POSIX is the de-facto standard for filesystems.

In a self-hosting context

For self-hosters, S3 compatibility is the cheapest way to swap AWS S3 out of an existing application. MinIO, Garage, and SeaweedFS are the three dominant self-hostable S3-compatible servers tracked in this directory; all three are drop-in for the read/write subset of the API that most SaaS replacements use. Pair one of them with a backup tool and you have an end-to-end self-hosted backup target with predictable monthly cost and zero Egress fees.