One important difference between shards and rollups that I did not get into is *upgradeability*: shards would upgrade their EVM automatically, rollups need their own governance. But could we change that? (thread)

I made one proposal a few months ago: https://notes.ethereum.org/@vbuterin/enshrined_zk_evm

Basically, do it in a highly flexible way by putting the proof system outside of consensus and letting individual clients prove in their own way. But the more I think about it, the more that seems overly complex and brittle.

It's easier if you agree on one, or N, proof systems. Then, in each hard fork, we would have to also agree on the verification key of the ZK-EVM (rough analogy: "hash of the ZK-EVM interpreter") for each proof system. We then expose those keys through a precompile.

Rollups that wish to be "EVM-compatible" could point to the ZK-EVM v-keys provided by the precompile. Rollups that wish to do their own thing could stay with their own v-keys.

Potentially, rollups could even be "almost compatible": the v-keys could provide a standard interface for extending the EVM with other features

If we go "full scifi", you could make it work with arbitrary proof systems: you could provide a formal proof (eg. in coq or lean) that a new proof system is valid, and that the ZK-EVM v-key you provide for the new system corresponds to the same EVM as the existing v-keys. But that's far away.

Another benefit of this approach of focusing on specific proof systems and v-keys, is that it supports not just rollups, but also validiums and other off-chain systems, where data is potentially unavailable and so re-proving under a different system is not always possible.

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Rollups that want to be (fully or partially) EVM-compatible would, at least for the EVM bits, piggyback on ethereum's off-chain governance.

What would governance look like in this scenario?

Exploring ways to streamline the upgrade process for rollups, perhaps through standardized governance frameworks or interoperability protocols, could bridge this gap. This could enhance the efficiency and cohesiveness of the Ethereum ecosystem while maintaining the decentralization and autonomy that rollups offer.

Great point, Vitalik! The distinction between shards and rollups in terms of upgradeability is indeed crucial. The automatic upgrade of EVM in shards offers seamless advancements, whereas rollups requiring their own governance introduces both flexibility and complexity.

Excellent point! While shards automatically upgrade their EVM, rollups rely on their own governance. What if rollups adopted a hybrid model: using shard-like automatic upgrades for routine changes, but retaining governance for major decisions? This could combine efficiency with community control. Thoughts?

One thing I didn't see addressed in your post is whether both shards and rollups can exist at the same time. E.g. could you not have rollups as currently set up and then also shards that work as you described as alternatives to L2?