An impromptu chat with Arthur Breitman about Rollups both ZK and Optimistic

This exchange took place on July 27th in the Tezos element chat. It has been formatted and edited to make it more readable.

Jarrod Woodard:

ZkRollups can only contain transactions, not contract calls, right?

Murbard:

They can contain contracts call
But it’s expensive
Complexity inside zkrollups incurs a high cost when generating proofs
So typically zkrollups are designed for simple txs, or a given use case like a cfmm
As opposed to supporting generic smart contracts

Jarrod Woodard:

Thank you! I am updating the wiki.

I am using rollup · master · Nomadic Labs / privacy-team · GitLab as source material.

Is there any other good sources you would recommend?

Thanks again!

Jarrod Woodard:

With non-optimistic rollups, do all nodes have to have the greater hardware or can a network of
specialized roll-up nodes service the chain?
( I’m re-listening to Arthur’s recent YouTube video In case this was explained and I missed it )

Murbard:

No, just one node
Bakers sequence the transactions on the chain and as few as one single node can maintain the rollup
state and post proofs

Jarrod Woodard:

Wow!

Entities other than that node can verify it’s claims?

Murbard:

With a zk rollup, they just verify a tiny proof which takes milliseconds and yes, the claims are verified
With an optimistic rollup they rely on the fact that someone will invalidate false claims

Jarrod Woodard:

Both approaches seem amazing, Thanks for taking the time to respond.

Could (would) you have competing rollup nodes?

Even if that single node were entirely competent and honest, local hazards could take it offline.

Never mind, that was answered in the video… “…Anyone can post a batch of transactions for a rollup
on the chain and anyone can run a validator for a roll up…”

What is the proper way to refer to non-optimistic roll-ups?

Murbard:

Well the only known kind of non optimistic rollup is a zk rollup
Yes you can have competing rollup nodes

Jarrod Woodard:

I thought that might be the case, thank you. I will refer to them as standard rollups (SRU) In order to
avoid confusion.

Would it be possible for the protocol to host both standard rollups and optimistic rollups?

Jarrod Woodard:

From The YouTube video:

“This is a very small cryptographic proof, which can be verified in a fraction of a second by bakers.
And proves that the state of the roll-up is valid after a long series of transactions.”

Does this mean that a standard rollup requires a certain number of confirmations?

If so, is that the same for optimistic rollups?

Murbard

Yes you can have both
ZKRU and ORU
There’s no need for confirmations in a ZKRU
In an ORU you want to leave some time window to give people a chance to challenge invalid
assertions

Jarrod Woodard:

Makes sense, thanks for the corrections!

Murbard, do you mind if I post a summary of our back and forth in the Tezos agora forum?

In regards to OMU “…The validator places a stake when making the assertion…”

Is a stake placed for every rollup they include in a block, or is this left open for various design
approaches?

Murbard:

Jarrod Woodard
“murbard Do you mind if I post a summary of our back and forth in the Tezos agora forum?”
not at all, go ahead

Jarrod Woodard:

Thank you!
Is a stake placed for every ORU a validator includes? …Or is it one stake for all ORUs in a single block?

Murbard:

Jarrod Woodard
“Is a stake placed for every ORU a validator includes? …Or is it one stake for all ORUs in a single
block?”

What does it mean to “include” an ORU?
Blocks can include two different things, including ORU operations (which are generally batches of
transactions) and ORU assertions

Jarrod Woodard:

I had not learned to distinguish between the batches and the assertions. Let me better learn the
difference before responding. Thank you!

Murbard:

The separation of the two is critical for decentralization

End

I have not gotten around to following up with him.

Do you have any questions about Rollups or the convo? Please ask them here!

Thanks for reading and any feedback!

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