Thread by @mt_1466

In today’s rollup world, the #L222 teams building rollups @zksync, @StarkWareLtd, @arbitrum, @optimismFND, etc.) still play a critical and centralizing role in the L2 process.

To what extent?
What can be done?
And #wen??

A thread on the entities and roles in a rollup tx

At a high level, there are three entities involved in a rollup transaction:
- the user on the rollup
- the rollup operator
- #Ethereum L1.

The rollup operator that sits in between the user and mainnet has tremendous responsibility and also some power.

Within this framework, there are also three crucial actors in the collection, execution, and finalization of a ZKR block:
- sequencers
- provers
- validators (verifier)

Sequencers are a small set of nodes that run this specialized hardware, receive, order, and compute the transactions.

Typically, they are not very transparent and/or auditable and are run by the L2 team.

Provers are the nodes, again generally run by the L2 team, that generate the validity proof.

Sequencers and provers can generally be lumped together in these early days of rollups.

Validators are a much larger set of L1 nodes that verify the validity of the zk-proofs submitted by the provers.

This group serves to hold the provers accountable and ensure censorship resistance.

To describe the process in further detail:
1) A highly-compressed batch of L2 transactions is combined with the current state root (sequencers)
2) The combination is sent to an off-chain prover
3) The prover computes the txs, generating a validity proof of the results

...

4) The prover sends this validity proof to on-chain validators (Ethereum L1 nodes)
5) The validators confirm the validity proof
6) The smart contract on Ethereum's L1 that maintains the state of the rollup is updated to the new state

Due to the complex computation involved in the validity proofs, special-purpose hardware may be needed to run a node, creating a centralizing effect on the L2.

Worry not, though!

Even though the ZKR chain may be quite centralized when compared mainnet Ethereum, L2 transactions are always ultimately verified and finalized by the L1.

This gives L2 transactions the same security guarantees as L1 transactions.

This is one of the key benefits of using a rollup rather than some other L2 or alt-L1.

It’s important to remember that while rollup technology can be quite technical, at its core, it is simply a smart contract on mainnet Ethereum.

Rollups connect with Ethereum via this smart contract. They are “trust-minimized bridges” because the smart contract on the L1 acts as a light client receiving block headers and validating by fraud/validity proofs.

Ethereum has access to a rollup's data and conducts its on-chain fraud and validity proofs. This proof process is why rollups can have as little as one node but still maintain the same trust assumptions as the Ethereum base layer.

Additionally, rollups that share a settlement layer can build trust-minimized bridges between them because their state transitions can be easily verified through the settlement layer via full nodes.

That's a wrap!

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