7 · The spec & its versions

Status: stub.

The Zarr specification defines the data model and the on-store layout. There are two versions in active use, and knowing which one you are looking at explains a lot of apparent inconsistency.

What this chapter will cover

• What "the spec" actually specifies: the data model, metadata documents, and the key layout — not an API.
• V2 vs V3: where they differ (metadata files, key structure, extension points), why V3 exists, and how to tell them apart at a glance.
• Forwards- vs backwards-compatibility: what a V3 reader can and cannot do with V2 data, kept explicit rather than smoothed over.
• Where the spec text lives and how to read it.