Layer-2 Scaling

Relavant Projects:

Instead of putting all activity on the blockchain directly, users perform the bulk of their activity off-chain in a “layer 2” protocol. There is a smart contract on-chain, which only has two tasks: processing deposits and withdrawals, and verifying proofs that everything happening off-chain is following the rules. There are multiple ways to do these proofs, but they all share the property that verifying the proofs on-chain is much cheaper than doing the original computation off-chain. ~An Incomplete Guide to Rollups

Three Major Types:

  1. State Channels (built atop [sidechain])
  2. Plasma
  3. Rollups


Rollups are fully general-purpose; one can even run an EVM inside a rollup, allowing existing Ethereum applications to migrate to rollups with almost no need to write any new code.

How It Works:

Two Flavours of Roll-ups (to address someone submitting a batch that just takes all the assets):

  1. Optimistic Rollups - uses “fraud proofs”. If anyone discovers one batch had an incorrect post-state root, they can publish a proof. Contract verifies proof and reverses the batch and all batches after it.
    • Issues: 1 week withdrawal period to handle proof publishing. Higher pre-transaction on-chain gas costs.
  2. ZK Rollups - uses “validity proofs”. Every batch includes a proof called [zk-snarks] (e.g. using the PLONK protocol) to prove that the post-state root is the correct result of executing the batch.
    • Issues: 10x more computationally expensive = high gas cost per batch. Complex technology. 1000x more computationally expensive running off-chain.

In general, my own view is that in the short term, optimistic rollups are likely to win out for general-purpose EVM computation and ZK rollups are likely to win out for simple payments, exchange and other application-specific use cases, but in the medium to long term ZK rollups will win out in all use cases as ZK-SNARK technology improves.

Problems with Rollups: