The rumors are true: Metis is leading innovation among all L2s.

Metis Andromeda has revolutionized the Layer 2 landscape forever, as it became the cheapest Layer 2 in existence. By tackling one of the 7-not-yet-fully-solved challenges shared by Vitalik in early 2021, Andromeda network is achieving tx fees never seen before in the Layer 2 space. The most recent proof of Metis being at the forefront of L2 innovation is a whole new classification being created for this novel technology, as it keeps breaking paradigms: Smart L2s.

Originally, L2Beat reclassified us as an “Optimistic Chain”. Even though we do think the term sounds pretty catchy, it’s not accurate for our innovative infrastructure. The word “chain” would imply that Metis Andromeda has its own consensus mechanism, which is most definitely not the case, given Metis Andromeda still relies on Ethereum’s security. Smart L2s are Layer 2s with an innovative approach of submitting only what is needed, and having the ability to bring back the rest if needed: pretty much an On-Demand Rollup.

Innovation

Every time there’s innovation, there will be critics to come with it. Human beings have always been, and still are, afraid of what they aren’t capable of understand. Some great examples of inventions that were criticized, and in some cases even ridiculed at first, include:

• The Electric Lightbulb: “a conspicuous failure”, “good enough for our transatlantic friends … but unworthy of the attention of practical or scientific men.” (British Parliament)
• Alternating current —“fooling around with alternating current is just a waste of time. Nobody will use it, ever.” (George Westinghouse)
• The telephone — “obvious limitations of his device, which is hardly more than a toy.” (Western Union Company)
• Automobiles: “The prices will never be sufficiently low”, “the horse is here to stay but the automobile is only a novelty — a fad,” (president of the Michigan Savings Bank)
• Smart L2s: “Metis is not a real L2” (?)

To keep it short… We know how these end.

ELI5: So, what is a Smart L2 after all?

A Smart L2 is the newest and most recently updated form of traditional optimistic rollups.

Smart L2s are a Layer 2 infrastructures that:

1. Collect or, roll up a batch of transactions
2. Post Transaction data to Memolabs with its Merkle Tree Roots (think of MTRs as identifier tags) and post the Tx Batch MTR and the state roots on Mainnet
3. Whenever needed (fraud proof posted / data needing to be verified on L1): the rest of the data gets brought back on-chain via the indentifier tag and everything is resolved in Layer 1.

We consider our new model to have the optimal balance, since it enjoys enhanced scalability, enhanced economic benefits, and Ethereum’s L1 consensus mechanism / underlying security.

Hold up! If Metis rolls up transactions, how is it not a rollup?!

Purely semantics. Quoted by Vitalik Buterin: “Rollups move computation (and state storage) off-chain, but keep some data per transaction on-chain. To improve efficiency, they use a whole host of fancy compression tricks to replace data with computation wherever possible.”

Even though Metis does keep the txs MerkleTreeRoot and stateroot in mainchain, these are not considered “data” and therefore Metis don’t qualify for a “pure rollup”. Like we said: purely semantics. We are not here to argue, though: we are here to build, to innovate and to keep leading the exploration of the Layer 2 space.

“But what about the Fisherman’s Dilemma previously mentioned by Vitalik”

Metis gets around the fisher dilemma because under this model, the sequencer is not a permanent role. So the cost of kicking out a bad sequencer is finite, so Metis’s security again is backed by L1’s security through fraud proofing. Regarding other situations when the sequencer stops the entire peer network and withholds the data: Metis has all these situations covered by making the cost of dealing with those situations finite and feasible.

Metis still relies on the security of Ethereum Mainnet as the last line of defense for its L2. Metis does have a peer network to catch block production issues even sooner, but calling Metis a sidechain would completely ignore the fact that our Layer 2 security is ultimately based on L1’s. We’re not getting rid of Ethereum Mainnet’s security; we’re adding an extra layer to it.

How are Smart L2s different from other solutions?

Let’s briefly define each and then see how Metis, and Smart L2s, differ from the rest:

Sidechain: A sidechain is a separate blockchain which runs in parallel to Ethereum Mainnet and operates independently. It has its own consensus algorithm. It is connected to Mainnet by a two-way bridge.

Plasma: A plasma chain is a separate blockchain that is anchored to the main Ethereum chain, and uses fraud proofs (like optimistic rollups) to arbitrate disputes. These chains are sometimes referred to as “child” chains as they are essentially smaller copies of the Ethereum Mainnet. Plasma does not support general computation. Only basic token transfers, swaps, and a few other transaction types are supported via predicate logic.

Validium: Uses validity proofs like ZK-rollups but data is not stored on the main layer 1 Ethereum chain. Limited support for general computation/smart contracts; specialized languages required.

Rollups: Perform transaction execution outside layer 1 and then the data is posted to layer 1 where consensus is reached. As transaction data is included in layer 1 blocks, this allows rollups to be secured by native Ethereum security.

• Optimistic Rollup: assumes transactions are valid by default and only runs computation, via a fraud proof, in the event of a challenge.
• Zero-Knowledge Rollup: runs computation off-chain and submits a validity proof to the chain.

So, based on the definitions found on Ethereum.org, we can conclude that:

1. Metis is not a sidechain, because Metis doesn’t have its own consensus protocol.
2. Metis is not a Plasma nor Validium, because neither Plasma or Validium support general computation, and Metis is EVM-Equivalent (the complete opposite).
3. Metis is not a zk-Rollup, as it doesn’t use validity proofs.
4. Metis is very similar to Optimistic Rollups, but with enhanced efficiency.

Deep Dive and Comparison: Optimistic Rollups vs Smart L2s

How an Optimistic Rollup Works

Structure

Receiving Transaction Data

• Individual transactions go to the Sequencer.

Posting Transaction and State Data

• The Sequencer posts the transaction data to Ethereum to the Canonical Transaction Chain (CTC). The Sequencer also executes the transaction data and posts the resulting state in the State Commitment Chain (SCC) as a state root.

Retrieving Transaction and State Data

• The Verifier then downloads the data from the CTC on Layer 1 and executes the transaction batch locally.

Comparing Results

• If the transaction execution results in the same state as the sequencer has submitted on the SCC, then nothing else happens.
• If the transaction execution results in a mismatch, the Verifier posts a Fraud Proof to Layer 1, which is executed by Ethereum. The Sequencer is then slashed and the Verifier who has submitted the fraud proof is rewarded with a portion of the slashed fee.

Actors

Sequencer/Block Producer

• Posting transaction data on L1 — Canotical Transaction Chain
• Posting state root on L1 — State Commitment Chain

Verifier

• Posting Fraud Proofs on L1

How Metis’ Smart L2 Works

Receiving Transaction Data

• Individual transactions go to the Block Producer. The Block Producer will take the transaction and propagate it to the Peer Network. The peers are there to guarantee that the transaction is valid, if it is not, it will get rejected. The Sequencer is part of the Peer Network and downloads the data from there (peer network). This mechanism is described as Temporary Finality.

Posting Transaction and State Data

• The Sequencer then posts the transaction data to Memolabs storage with the identifier from the merkle tree root (MTR) of the transaction bundle and submits the MTR of the transaction bundle to Layer 1. The Sequencer also posts the resulting state in the State Commitment Chain (SCC) as a state root.

Retrieving Transaction and State Data

• The Validator retrieves the transaction data from the Peer Network. They are also able to retrieve the data from the Memolabs storage.

Comparing Results

• If the MTR of the transaction bundle matches the locally-calculated MTR from the downloaded transaction bundle, then the local results are compared with the MTR submitted by the Sequencer. If both the MTR for the transaction data and the state root equal the local copy that the verifier has, then nothing happens.
• If either the MTR of the transactions or the state root does not match, the data is attempted to be retrieved by the Sequencer.

Ensuring Data Availability

• Because there is no transaction data on Layer 1, the Sequencer that has posted the MTR of the transaction data has the responsibility to post the data. A request is made by the Validator to the Sequencer to post the data to Layer 1. This request is combined with an amount of ETH required to post the batch. This amount must be paid by the Validator. The Sequencer has 24 hours to post the transaction data on-chain. The Sequencer will post all of the transaction data on Layer 1 and Ethereum will verify the validity of the data with a Fraud Proof posted by the Validator.
• If the Sequencer does not submit the data after the 24 hour period, the Sequencer is slashed and the Validator who has paid the ETH will be reimbursed. A new Sequencer is randomly chosen and the new Sequencer posts the MTR of the transaction bundle and the resulting state. The data is retrieved from the Peer Network or Memolabs.

Actors

Sequencer

• Posting merkle tree root of the transaction

Block Producer

• Propagating transactions
• Maintaining ordering

Peer Network

• Sharing transaction data information
• Peers verify the data that the Block Producer submits

Memolabs

• Permanent storage of transaction data

Validator

• Makes requests to the sequencer to post the data

Conclusion

There is no definite answer on which layer 2 solution is better, and there isn’t supposed to be one. Even though sometimes innovation is perceived as competition, we’re all here for the same reason: to make Ethereum better and to help Web3.0 achieve the mass adoption that we know is coming. We already tackled and are solving one of the 7 not-yet-fully-solved challenges in rollups, and you know this is just the beginning: Metis will keep knocking down one domino after another. We’re eager to share our vision with the world and very excited for the 2nd half of this 2022. So… buckle up, anon.

Sources

• https://mirror.xyz/0x64A156A617C4226665c79F6cC8cCF078A4650E26/ek16XHkNd_kK0aVR1HWuaHX_n1dhPpmtTm7nNA-5i_w
• https://metisdao.medium.com/the-tech-journey-lower-gas-costs-storage-layer-on-metis-867ddcf6d381
• https://medium.com/omgpool/plasma-vs-optimistic-rollups-9808c2f64975
• https://ethereum.org/en/developers/docs/scaling/sidechains/