Today I put together some slides summarising the current state of exploratory work on the Zarr v3 protocol spec. The purpose of this blog post is to share those slides more widely, and to provide some context explaining why work has started on a v3 spec.

Why work on a v3 spec?

The current (v2) Zarr spec is implemented in a number of software libraries, and is a stable and robust protocol that is used in production in a number of different scientific communities. If you need to store and compute in parallel against large array-like data, it’s a good solution. So why start thinking about a new protocol version?


One reason is that the v2 protocol is somewhat Python-centric, and includes some features which are not straightforward to implement in other languages. This has meant that implementations do not all support the same feature set. It would be good to have a minimal v3 protocol spec that could be fully implemented in any language, so all implementations have parity around a core feature set.

Unifying Zarr and N5

Another reason is that we would like to merge development efforts between the Zarr and N5 communities, and so a goal for the v3 spec is to unify the two approaches and provide a common implementation target.


A third reason is that a number of different groups have started experimenting and extending the Zarr protocol in interesting ways, but it’s not always clear how to extend the v2 protocol to support new features. It would be good if the v3 spec provided a variety of clear extension points and extension mechanisms.

Cloud storage

Finally, while the v2 spec can be used very effectively with distributed storage systems like Amazon S3 or Google Cloud Storage, there is room for improvement, particularly regarding how metadata is stored and organised.

Zarr v3 design update

I you are interested in knowing more about the current status of work on the v3 spec, please take a look at the v3 design update slides. The slides use reveal.js and have both horizontal and vertical navigation - if you haven’t seen that before, then navigate downwards first wherever you can, before navigating to the right.

As I mention in the slides, the current v3 spec is just a straw man, meant to illustrate some ideas and potential solutions, but everything is up for discussion. So if you have any comments or ideas, please do get in touch, anyone is welcome to participate.

Written by Alistair Miles