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Other Layer 2 Scaling Solutions vs. ZK-Rollups

While rollup protocols and other Layer 2 scaling methods are similar in operation, each is optimized for a distinct use case within the blockchain ecosystem.

Optimistic Rollups vs. ZK-Rollups

Certain Layer 2 solutions may benefit from increased scalability and throughput by avoiding data posting to the Ethereum mainnet. This, however, comes at the cost of decreased security.
Both ZK-Rollups and optimistic rollups improve scalability by off-chain processing of transactions and submitting substantially compressed data to the Ethereum mainnet. Where they diverge is in their techniques of verification. ZK-Rollups authenticate transactions using sophisticated cryptographic validity proofs, with each batch of transactions submitting a proof of validity to the mainnet.
On the other hand, optimistic rollups presume that all transactions are valid and transmit transaction batches without performing any calculations. This results in a significant increase in scalability. However, there is a period during which anyone may challenge the authenticity of any transaction. If an anomaly is detected, the rollup does a fraud proof (a check on transactions for potential fraud) using the Layer 1 data. This waiting period for the challenge phase results in larger delays than with ZK-Rollups.
By paying sequencers, the optimistic rollup protocol incentivizes valid transactions (parties responsible for storing and executing user transactions off-chain). If a sequencer submits a fake transaction to the Ethereum blockchain, their ETH stake is halved.
Because optimistic rollups do not require complex computations, they can boost scalability by up to 100x. However, their withdrawal period is significantly longer than that of ZK-Rollups due to the challenge period. They are also more vulnerable to attacks from quantum computers.

Sidechains vs. ZK-Rollups

Sidechains are blockchains that are connected by bridge contracts to the primary blockchain, such as Ethereum. They are distinguished from ZK-Rollups principally by the fact that they accept data from the mainnet without the requirement to validate it. Rather than that, sidechains rely on a group of parties that manage the bridge contracts to verify that the mainnet is secure and communicate this information to the bridge.
Sidechains, being a distinct entity, have their own security features and consensus procedures for transaction processing. As is the case with ZK-Rollups, sidechain transactions are frequently less expensive than mainchain transactions.
Meanwhile, for ZK-Rollups, sequencers provide evidence of the network's status to the bridge contract, which is responsible for validating the evidence and ensuring that the network has not been corrupted.
ZK-Rollups rely on Ethereum network security, which consumes additional resources. As a result, ZK-Rollups are more expensive than sidechains.
The primary distinction between ZK-Rollups and sidechains is the mechanism by which transaction data is stored. Whereas rollups aggregate Layer 2 transactions and then submit them to the mainchain, sidechains establish whole independent chains — and then submit only the deposit and withdrawal to the mainchain. While ZK-Rollups can rely on the mainchain's security, sidechains are entirely on their own. Additionally, like any sidechain, the operators responsible for forging new blocks may cease production or restrict withdrawals. Sidechains, like mainchains, are vulnerable to 51 percent attacks.

Plasma Chains v. ZK-Rollups

Plasma chains were first proposed as a scalability solution by Joseph Poon and Vitalik Buterin (co-founder of Ethereum).
Plasma is a Layer 2 scalability solution that offloads transactions to a Layer 2 sidechain (or "child chain") and periodically updates the Ethereum mainnet with newly created blocks. Plasma generates blocks via consensus mechanisms, similar to sidechains, but attaches block roots to the mainchain, which serves as a security layer.
Similar to ZK-Rollups, plasma chain transactions are less expensive than mainchain transactions. Additionally, they use fraud proofs to safeguard the plasma chain. These checks and balances enable users to contest the validity of their funds on the mainchain, providing security comparable to rollups.
Both ZK-Rollups and plasma chains scale Ethereum by offloading transactions to a Layer 2 sidechain. However, the fundamental difference between ZK-Rollups and plasma chains is that ZK-Rollups combine hundreds of transfers into a single transaction that is validated using a ZK-proof on the Ethereum mainnet, whereas plasma chains create and submit a single transaction for each transfer.
Plasma's scalability is constrained by the large amount of data required on the mainnet to validate blocks when users request withdrawals from the sidechain. Additionally, the challenge period is lengthy, and users are required to remain online — or risk forfeiting their rewards. ZK-Rollups are more user-friendly, consume fewer resources, and thus offer a more promising scaling solution for Ethereum than plasma in the majority of use cases.
One significant advantage of plasma and rollups is that funds can be withdrawn even if the exchange hosting the scaling solution is unavailable due to the fraud proofs on the main public ledger.

State Channels vs. ZK-Rollups

State channels are scalability solutions that enable secure transactions between two or more parties off the Ethereum blockchain. The parties involved communicate without using the mainchain by exchanging state updates.
When any of the parties wishes to terminate their use of the channels, they initiate a "exit" — that is, they submit the most recent state updates to the mainchain, and the current balances are transferred to the parties that initiated the channel. By verifying final balances and signatures, the mainchain validates this current state, making it difficult for anyone to exit an invalid state.
However, there is a disadvantage: the mainchain has no way of knowing whether additional transactions have been added to the previous ones following the submission of the last state updates. Allowing a period for any of the parties to challenge the exit is one way around this. To avoid the lengthy wait times associated with withdrawals to the mainchain, parties to the transaction can sign a "conclusion proof" that allows a party to exit without waiting for the challenge period to expire.
State channels have the advantage of allowing transactions between two parties without the involvement of a third party, in contrast to ZK-Rollups, which require all transactions to be processed by a rollup operator.
Another distinction between state channels and ZK-Rollups is that state channels have an immediate finality: once the counterparty confirms receipt of an update, the state is final (i.e., the transfer of values is instant).