s3proxy

S3Proxy puts an S3-compatible API in front of storage that isn't S3: Google Cloud Storage, Azure Blob, OpenStack Swift, SFTP, or a plain local filesystem. Your app keeps speaking S3 while the backend becomes swappable. One Java process, Apache 2.0, free.

Running it is a Docker container or an executable jar with a properties file, and reference Kubernetes manifests ship in the repo. The killer development use: point it at the local filesystem and integration-test your S3 code with no AWS bill and no network dependency.

Use it for tests, for migrating between clouds without rewriting storage code, or for bolting S3 semantics onto legacy storage. MinIO (minio/minio) is the answer when you want an actual self-hosted object store rather than a translation layer in front of one.

The catch: it's a proxy, so every request pays an extra hop, and S3 API coverage is broad but not total. Newer S3 calls and some multipart edge cases may not translate to every backend. Test your specific access patterns first.