1/ Let's try to dispel the biggest confusion about Optimistic Rollups and their 7-day fraud proof window. Imagine you have 1000$ DAI and send it to Alice. Then you send it again to Bob. Obviously you expect that first tx confirms, the second reverts with "out-of-balance" error 👇
2/ Clearly Alice should now have 1000$ DAI and not Bob, provided that the *sequence* of txs are as above. Now, Sequencer posts both transactions to L1 in a batch and also posts a state root (a magic number). L1 does not do anything fancy, except recording these.
3/ If Sequencer is honest, the state root "confirms" to L1 that it's Alice that has $DAI on L2 and not Bob. So Alice can withdraw that $DAI to L1, i.e. release them from L1 deposit contract. Not Bob. But how L1 can be sure that L2 Sequencer is honest ?
4/ What if the posted state root attests that it's actually Bob that has the funds ? With Optimistic Rollups L1 simply waits 7 days (the fraud proof window) and after that - if nobody claims otherwise - it assumes that it is correct. And releases funds to Alice.
5/ Let's go back to your two transactions. When they are final ? When you should get "hard" confirmation from Metamask ? As soon as they are posted to L1 by Sequencer. You don't care about the state root. Your txs. cannot be "reverted", "removed" or otherwise during the 7 days
6/ The 7-day fraud proof window does not affect you. It affects only Alice, Bob, or in general, anyone that does any L2 --> L1 transaction (such as e.g. funds withdrawal). During this 7-days the *effect* of the tx is uncertain from the PoV of L1, but txs themselves are final
7/ So, what is then the "soft" confirmation ? It's simply a promise from the Sequencer that txs will be posted (eventually) to L1 in a sequence that you observe in your UI. But you have to trust the Sequencer to do that. Until it's actually posted to L1, it's not guaranteed
8/ ORs post tx batches to L1 roughly every 5 min. They can theoretically do it in each L1 block, so every 15 seconds (unrealistic of course if the block is full of txs minting digital porcupines)
9/ But once they are posted to L1, they are confirmed, they cannot be changed, in a way they receive L1 "attestation", stamp of approval. 7-day fraud proof window has nothing to do with it

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More from @bkiepuszewski

23 Sep
1/ The second, after censoring, aspect of L2s is what users need to do if L2 operator stops pushing new state roots on L1 @l2beatcom 🧵👇
2/ You can read about the censoring in the previous thread here:
3/ Pushing state roots is crucial so that users can withdraw their funds from the bridge. And to be able to do it, you need to provide Merkle Proof of your balance against that state root on L1
Read 12 tweets
22 Sep
1/ How important is Censorship Resistance for L2s ? Applications can censor individual users based on where they come from, or whole dApps could be rendered useless if Rollups censored all incoming txs to them. What are the possible solutions right now ? 👇@l2beatcom
2/ @arbitrum and @optimismPBC allow users to "force" their transactions via L1. This is perhaps inconvenient for the end users, but it is a very generic mechanism that works for all dApps on a Rollup. And dApps don't need to do anything, it's just there. And it works
3/ @StarkWareLtd based L2s (@SorareHQ, @dydxprotocol, @deversifi, @Immutable) also have force transaction mechanism, but they are application specific. Namely, they allow you to force only certain tx (typically withdrawal request). It's as if each dApp had to implement it on L1
Read 7 tweets
16 Sep
1/ With @arbitrum Sequencer down for an hour, Arbitrum chain effectively reduced itself to L1 Ethereum wrt to cost and speed, but it never stopped working. How is it possible ? And why @l2beatcom claims that users should propose blocks when operator is down ? 🧵👇
2/ Arbitrum, as any L2 Optimistic Rollup, posts on Ethereum L2 transaction batches and, periodically, state roots of L2. Anyone can confirm that the latest state root of L2 is indeed the result of executing L2 transactions previously posted. Verify, don't trust !
3/ So, who is posting L2 txs on Ethereum ? Typically sequencer, but if Sequencer is down, users can do it themselves. This is what we mean by "transacting using L1". Users can do it if Sequencer is down, or if the Sequencer is censoring them
Read 10 tweets
18 Aug
After long discussions we, at @l2beatcom ended up with the series of questions that we hope each Rollup provider will be able to answer so that users will be able to properly assess the risk of their funds deposited into the Rollup: 👇
1. Let's start with the obvious - are you using Validity Proofs or Fraud Proofs on L1 to secure the state ? What are the details ? How can users be sure that you are proving what you claim to be proving ?
2. Transaction data will be needed for Fraud Proofs, Emergency Exits, etc... where is the data stored ? If on L1, how users can be sure that they will be able to decode it if needed ? If not, where is it and what entity(s) are securing it ?
Read 8 tweets
11 Jun
1/ Now that @0xPolygon data is available on BigQuery, a deeper look into smart contract usage, gas usage, etc... is available - as an example, have a look at the cumulative gas usage (on a log scale), compared to Ethereum:
h/t @piotrklis_ 👇
2/ It looks like soon enough Polygon users will spend more gas then Ethereum users in the entire Ethereum history 🤯 You may think this is amazing, but how much of that gas is spent on creating new state ?
3/ If Polygon continues their exponential growth, will the state tree also grow exponentially ? And if so, how the geth nodes that they are using will handle that ?
Read 5 tweets
16 May
1/ The easiest way to understand the difference between L2 Rollup and a sidechain such as @0xPolygon is to inspect closer the exit procedure. Below is tx withdrawing 450,000 USDC from @0xPolygon child chain: ethtx.info/0x5c5f80a7dab5… 👇
2/ First thing to notice is that to perform the exit user needs to submit the chunk of data (input data) containing, among other info, merkle proof for the exit. This data can only be obtained from Matic nodes, it is impossible to construct it just by observing L1 Image
3/ On a Rollup data would be available on L1, so even if all Matic nodes were down, users would still be able to exit their tokens. That's not the case here - you need to get that data from Matic nodes
Read 6 tweets

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