Rollups Optimistic ZK Ethereum Arbitrum Scaling Blockchain

7.75 billion people have internet access. Imagine if all 7.75 billion of them accessed a singular blockchain all at once. That blockchain would become slow, like watching a snail move through molasses slow. This scenario is where rollups come into play.

What are Rollups?

Rollups are one type of mechanism that increases the speed of transitions of a blockchain. Many layer two blockchains use this mechanism, for instance, Optimism, Arbitrum, and Starknet. If the term layer two is unfamiliar, it is essentially a blockchain that takes transactions meant for a layer one blockchain and submits to that layer one chain. You can think of executing a transaction on a layer one as the road and using a layer 2 as an airway. You can both drive and fly to a destination but one is much faster than the other.

In terms of what exactly rollups do, rollups execute a group of transactions outside of the layer one chain and then push the transaction data to the layer one afterwards. Transactions become faster because all of the group’s transaction data does not need to be relayed to the layer one. Hence, the amount of data per transaction is decreased and there are less people trying to commit transactions from the layer two than the layer one, increasing the speed of transaction.

Two Common Types of Rollups

Right now you might be thinking what is stopping a bad actor from submitting a fraudulent transaction from a layer two rollup chain to a layer one. This comes through their validation mechanisms and essentially what makes each type of rollup different.

Below are the two most common types of rollups right now (really the only types used now).

Optimistic Rollups

As the name suggests optimistic rollups are optimistic. Initially they assume that all transactions are valid and add them to layer one.

However, for someone to commit a transaction on an optimistic rollup chain to a layer one, they have to place some bounty or monetary value to declare their transaction as valid. Then, if someone else believes your transaction is invalid, they can put a bounty on it and contest the transaction.

What happens next transaction is replayed. Essentially, that specific transaction is placed again in its totality on the layer one blockchain with its *original state and the layer one’s security mechanism. If that layer one states the transaction as invalid the transaction sender loses their bounty, in the other case the person who believed it was invalid loses their bounty.

Side note on Original State: In this case original state means the state, data and data positioning, of the layer one blockchain at the time that transaction was originally placed.

Zero Knowledge Rollups

Zero Knowledge Rollups or more commonly known as ZK Rollups rely on *zero knowledge validity proofs in order to “prove” that transactions are valid. Prove is in quotation because technically these proofs are not one hundred percent accurate, but the chance of them being wrong is so minimal it is essentially always going to be correct. The benefit of using ZK compared to Optimistic is that there is no waiting period for someone to claim a transaction is invalid. Also, ZK requires less transaction data to be pushed to the layer one.

So why would someone not use ZK?

The downside of ZK come from the fact that it requires a lot of computing power and also it's harder to build an EVM (Ethereum virtual machine) compatible layer two solution using ZK. However, in my opinion ZK rollups are really the future of layer twos on Ethereum and blockchain scaling technology in general.

Side Note on Zero Knowledge Proofs: The idea of zero knowledge proofs may seem extremely counter intuitive and rightfully so. I mean how do we prove something without any prior knowledge. Essentially there is a lot high level cryptography and math going on that I am not the most qualified to explain, so if you are interested in how these proofs can be useful in blockchain check out Vitalik’s blog post.

Wrap Up

Overall, rollups are a concept and mechanism that allow blockchains to scale. They group transactions together and commit them to a layer one chain faster by not needing to put all of the information for each transaction onto the layer one chain. Currently, we are only seeing the beginning of rollups increasing the scalability of blockchains, but I’m sure in the near future rollups will allow ecosystems like Ethereum to hit record high transaction per second speeds.