Add optimistic chains to scaling section

[emmanuel-awosika]

Is your feature request related to a problem? Please describe.

The scaling section has a page on optimistic rollups, but doesn't have a resource on OR-like blockchains that store data off-chain. Current examples are Metis and Arbitrum Nova, although similar projects may appear in future.

While these "optimistic chains" (which is how L2Beat describes them) are clearly different from optimistic rollups, they cannot be accurately described as sidechains, either, for some reasons:

1. Sidechains usually have some form of consensus mechanism and a validator set responsible for processing transactions and advancing the chain's state (and securing it). Optimistic chains (e.g., Nova or Metis) have 1-of-N honesty assumptions (relying on the honesty of a single validator) and use a centralized sequencer to process transactions.

2. Optimistic chains derive their security differently from Ethereum. Like optimistic rollups, you can challenge transactions from an optimistic chain on Ethereum (via fraud proofs). The (critical) difference here is that an honest validator needs access to state data to be able to compute the state and detect fraud.

In the happy case (where off-chain data is available) an optimistic chain is likely more secure than a sidechain. But in the pessimistic case (where off-chain data is unavailable), an optimistic chain loses any security guarantees since it's impossible to prove fraud. Thus, optimistic chains might be theoretically safer than a sidechain, but not quite as secure as an optimistic rollup w/ on-chain data availability.

Also, there are also different qualities that come with off-chain DA (high throughput, low costs, risk of censorship, freezing the chain, etc.) that don't fit into the OR classification.

Describe the solution you'd like

Add a new page specifically for optimistic chains to the scaling section. Similar to how we have a separate page for validiums (even if they're mostly similar to ZK-rollups), a separate page for optimistic chains sounds like a good idea.

Describe alternatives you've considered

Putting optimistic chains as part of the page on sidechains. But, as I've explained, I don't think putting optimistic chains and sidechains in the same conversation is ideal.

Additional context

We could just decide not to classify optimistic chains as L2s (i.e., the status quo) and leave it at that. But people using these protocols are unlikely to read the fine print--all they see is a low-fee blockchain claiming "high security guarantees" w/o knowing what tradeoffs and trust assumptions are involved.

For example, Metis was removed from the L2s list after it started using MEMO (a decentralized storage platform) to store transaction data, but it still describes itself as an L2. Here's from the official website:

Metis has developed an Optimistic Rollup solution to bundle and sequence transactions on the layer above Ethereum and then send that data back as one transaction to the Ethereum Layer 1 main chain. This way, we are able to offer extremely fast transactions that take just a few seconds and cost just a few cents, while still maintaining the security of Ethereum Layer 1.

As I said in an earlier comment, Ethereum.org should be an authority on whatever new scaling tech is out there. I believe it's the least we can do to help the community.

Want to contribute?

I can help out with creating this page (or adding it to the sidechains section, if that's preferable).

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