Container Object Storage Interface (COSI)
A Kubernetes API standard for provisioning and managing object storage buckets as native Kubernetes resources, analogous to CSI (Container Storage Interface) for block and file storage.
Summary
A Kubernetes API standard for provisioning and managing object storage buckets as native Kubernetes resources, analogous to CSI (Container Storage Interface) for block and file storage.
COSI standardizes how Kubernetes workloads request and consume object storage. Instead of manually creating S3 buckets and distributing credentials, teams define BucketClaim resources and COSI drivers provision buckets from any S3-compatible backend.
- COSI is not yet GA in Kubernetes. It is an evolving standard under kubernetes-sigs. Production adoption requires evaluating driver maturity for your specific storage backend.
- COSI provisions buckets and access credentials — it does not manage data inside buckets. Data lifecycle, retention, and organization remain application concerns.
scoped_toKubernetes Object Provisioning & Policy — the K8s-native bucket provisioning standardenablesS3 API interoperability — standardized bucket provisioning across S3-compatible backendssolvesPolicy Sprawl — declarative, RBAC-controlled bucket provisioning
Definition
A Kubernetes-native API specification for provisioning and managing object storage buckets and access credentials as declarative Kubernetes resources (BucketClaim, BucketAccess, BucketClass).
Kubernetes has standards for block (CSI) and file storage but lacks one for object storage. COSI fills this gap, enabling Kubernetes workloads to request S3-compatible buckets through the same declarative model used for all other resources.
Kubernetes-native bucket provisioning, declarative S3 access management, GitOps-driven object storage lifecycle.
Recent developments
container-object-storage-interface-spec/spec.md at master. COSI spec with RPC Interface, Identity and Provisioner services, protocol definitions for S3/AzureBlob/GCS, and security recommendations for UNIX Domain Socket communication. Per GitHub (kubernetes-sigs/container-object-storage-interface-spec) (2024-12-03).
COSI advances toward v1alpha2, with Beta promotion targeted. The Kubernetes SIG Storage roadmap has COSI moving to the v1alpha2 API, targeted for Kubernetes 1.36, with plans for eventual promotion to Beta. Notable v1alpha2 changes: a single BucketAccess can request access to multiple BucketClaims, new BucketAccessModes (ReadWrite/WriteOnly/ReadOnly), and separate access grants for ObjectData/ObjectMetadata/BucketMetadata; a Backup Repository-COSI project is also active. Per SIG Storage Spotlight 2026 and 2026 KubeCon EU – Data Protection WG Deep Dive (tertiary, two independent sources agree).
Vendor driver ecosystem keeps widening. Beyond the original Ceph/Rook driver, COSI now has production drivers from Scality (open-sourced 2025), HPE (Alletra Storage MP X10000), Dell, and Huawei (driver updated June 2026) — real enterprise adoption of the spec beyond the CNCF reference implementation, which itself remains actively maintained (last release April 2026). Per Scality COSI driver release, HPE COSI writeup, and Huawei COSI driver repo (tertiary/secondary). Sources: GitHub (kubernetes-sigs/container-object-storage-interface-spec)
Connections4
Outbound4
Resources3
COSI project homepage documenting the Kubernetes-native standard for provisioning and consuming object storage buckets.
COSI specification repository defining the gRPC protocol between Kubernetes and object storage providers.
COSI API implementation repository with Kubernetes custom resource definitions for BucketClaim, BucketAccess, and BucketClass.