[gillesjacobs]

Great write-up, concise and very clear. I just started learning dapp development and was able to easily grasp the different forms of L2-scaling.

I am especially excited about recursive ZK-Rollup smart contracts even though they are complex and computationally expensive. They seem to be more trustworthy than Optimistic Rollups and I can see them becoming the standard eventually.

It is great to follow this dapp tech at its frontier, so much potential and new breakthroughs every few months it seems!

[hhkb]

I just read through the whole thing and have no idea what it says. It sounds smart but it’s pseudo-science to me.

[DennisP]

You can run that pseudoscience yourself with Zokrates. They have really good documentation, which makes it a lot more concrete. I didn't get zksnarks at all but that cleared it up for me.

https://zokrates.github.io/

The basic idea is that you have functions with both public and private parameters. You can do two things:

1) Pass in all parameters, get back the output and a proof that the output is correct.

2) Pass in just the proof, the output, and the public parameters, and get verification that the function ran correctly; i.e. that the public parameters and some private parameters you don't know generated that output.

So on chain, you can skip storing all the private parameters, as long as you store the proof, and have a contract verify the proof before storing the public data.

[dboreham]

Not sure about pseudo-science, but I have found blockchain writings to be on the abstract side. Try this interpretation:

The Ethereum blockchain hosts serious money. However it has poor cost and performance characteristics for many potential use cases. One approach to work around those is to move money temporarily from Ethereum to another blockchain (let's call that the secondary chain) that's cheaper/faster; shuffle it around there as much as you want; cash in the money back on Ethereum when you're done shuffling it. "Rollups" are a way to pull off that trick (there are other ways). Rollup means (to me at least) that you arrange to publish a kind of checkpoint of the secondary chain state at some point in time, to Ethereum. This checkpoint operation is relatively cheap to do because it's just a "put" of some small number of bytes. Where this comes in handy is when someone asks to withdraw their money from the secondary chain to Ethereum. If they were able to just say "give me my money", there would be the potential to steal funds. There has to be a way to know that a) they owned that money on the secondary chain and b) they didn't spend it already. This is achieved by making the withdrawal request of the form "give me my money because ... this rollup says I legit have that money". Absent the rollup, or some equivalent scheme the Ethereum contract deciding whether or not to give them their money would need to look at the entire history of the relevant transactions from the secondary chain. That's hard-to-impossible. Then, having invented the rollup concept there are two subspecies of ways to tell if the withdrawal request is valid or fake : 1) by holding off on giving the money for a while during which anyone who is interested can call foul on the validity (and get a reward for doing so) or 2) by means of ZK-proofs, which are a real thing but may be impractical to use for various reasons so it's always a good idea to ask for running code whenever anyone invokes them.

This paper has some useful background for the problem space: https://arxiv.org/pdf/1904.06441.pdf

[DennisP]

One difference between rollups and sidechains is that with rollups the essential data is stored on the main chain, in compressed form.

[dboreham]

Fwiw I disagree that the "data availability" concern is real. My take is that anyone with an interest in having some data will go ahead and store that data. If data integrity needs to be preserved under consensus then publish a hash to a blockchain. Possibly we're actually agreeing on this in your use of "essential"?

[DennisP]

By "essential" I mean all the data required to recover complete state.

Holding the data you need yourself might work ok for your own personal ETH balance, assuming you have some way to prove the absence of any outgoing transfers. In fact, that's how one variant of Plasma works.

But with smart contracts, it gets a lot more complicated. It's way simpler if you can just know that any data you need will be on the network.

[michaelsbradley]

> it’s pseudo-science to me

Maybe do some background reading?

For example: https://arxiv.org/abs/1906.07221

[nyhc99]

Thanks for dropping that link. The info is very accessible.