# ZK rollup architecture
zkSync is an L2 protocol based on ZK rollup architecture. ZK rollup is a flavour of a larger "rollup" family. For more background, we recommend Vitalik Buterin's 2021 guide to rollups (opens new window).
All funds are held by a smart contract on the mainchain, while computation and storage are performed off-chain. The main idea is that instead of verifying each transaction separately, transactions are "rolled up" to a single item (the rollup block), which is then being verified, approving all them simultaneously.
Informally, it works as follows:
- Users sign transactions and submit them to validators.
- Validators roll up thousands of transactions together in a single block and submit a cryptographic commitment (the root hash) of the new state to the smart contract on mainnet along with a cryptographic proof (a SNARK) that this new state is indeed the result of the application of some correct transactions to the old state.
- Additionally to the proof, the state ∆ (a small amount of data for every transaction) is published over the mainchain
network as cheap
calldata. This enables anyone to reconstruct the state at any moment.
- The proof and the state ∆ are verified by the smart contract, thus verifying both the validity of all the transactions included in the block and the block data availability.
SNARK verification is much cheaper than verifying every transaction individually, and storing the state off-chain is significantly cheaper than storing it on EVM. Hence enabling a huge boost of scalability (~100-200x mainnet capacity) and tx cost savings.
zkRollup architecture provides the following guarantees:
- Validators can never corrupt the state or steal funds (unlike Sidechains).
- Users can always retrieve the funds from the zkRollup smart contract even if validator(s) stop cooperating, because the data is available (unlike Plasma).
- Neither users nor a single trusted third party needs to be online to monitor zkRollup blocks in order to prevent fraud (unlike fraud-proof systems, such as payment channels or Optimistic Rollups).
In other words, zkRollup strictly inherits the security guarantees of the underlying L1.
Here are some links to explore the topic of zkRollups:
- Vitalik Buterin's guide to rollups (Jan 2021) (opens new window)
- Original idea proposal by Buterin (opens new window)
- Matter Labs' zkRollup talk at Zcon1 (video) (opens new window)
- Awesome Zero-Knowledge Proofs materials (opens new window)
- zkRollup vs. Optimistic Rollup deep dive (opens new window)
- Validity proofs (zkRollup) vs. fraud proofs (opens new window)
# Maximum throughput
Since the upgrade on Feb 9, 2021 (opens new window) that brought support of
recursion to zkSync on mainnet, protocol throughput is limited essentially only by the need to publish state changes
for every transaction via
calldata Ethereum, to ensure data availability. With the current block gas limit of 12.5M,
zkSync can process over 2000 TPS.
# Transaction finality
Transactions in zkSync reach the finality of Ethereum once the SNARK proof of the zkSync block is generated and accepted by the smart contract. The proof time generation is expected to be about 10 minutes, i.e. 10 minutes after submitted, the zkSync (proof) transaction is as final as any L1 Ethereum transaction included in the same Ethereum block as the transaction with the proof. At the moment, when a user sends a transaction, we wait for the block to get filled, and so we don't generate the proof immediately. With higher throughput on the zkSync system, time between blocks will decrease.
In contrast, fraud-based scaling solutions (e.g. optimistic rollup) require at least 2 weeks of a lockout period to operate more-or-less securely, which results in 2 weeks objective* tx finality time.
It should be added that Matter Labs and other companies in the ZKP space are constantly working on improving the prover efficiency, which will result in lower finality times (potentially down to under 1 minute).
* Subjective finality time can be shorter for optimistic rollup users who validate every tx themselves, but this would defeat the purpose of optimistic rollups as a scaling solution.
# Instant confirmations
Even though time to finality is about 10 minutes, it does not affect the usability of the network. Transactions submitted to zkSync by users are instantly confirmed, instantly displayed to the receiving party in the UI and API (although they are marked as unconfirmed), and the transferred assets can immediately be used to make further transfers (which might even end up in the same zkSync block).
At the moment, instant confirmations are pure promise on the side of zkSync validators to include the transaction in the next block. Users who do not trust the validator should await full finality before considering the assets as received.
In the future, a security bond will be added to zkSync by the validators. This will provide instant economic finality guarantees. It will work as follows.
Validators elected to participate in the zkSync block production will have to post a significant security bond to the zkSync smart contract on the mainnet. A consensus run by the validators provides a subsecond confirmation to the user that their transaction will be included in the next zkSync block, signed by a supermajority of (more than) ⅔ of the consensus participants (weighted by stake).
If a new zkSync block is produced and submitted to the mainchain, it cannot be reverted. However, if it doesn’t contain the promised transactions, the security bond of the intersection of the signers of the original receipt and the signers of the new block will be slashed. This intersection is guaranteed to have more than ⅓ of the stake. This guarantees that at least ⅓ of the security bond can be slashed and that only malicious validators will be punished.
A portion of the slashed funds will be used to compensate the tx recipient. The rest will be burned.
# Congested mainnet
Periodically, extraordinary events lead to very high levels of congestion on the Ethereum network (notable examples are the Cryptokitties crisis (opens new window) and the Shanghai DOS attack (opens new window)). During such peak load times, gas prices skyrocket and it might become prohibitively expensive to move crypto assets, rendering some services inoperative or preventing arbitrage opportunities.
Moreover, some systems can generally fail in extreme circumstances, leading to cascading failures (the recent DeFi Black Thursday (opens new window) is an excellent example). This is especially worrisome for fraud-proof-based scaling solutions (payment channels, optimistic rollups), because there is a risk that their automated security bots won't be able to get their fraud-proof transactions mined in case of an attack during high congestion, thus jeopardizing the security of all assets under the control of such systems. What makes the problem for these systems worse, you can never know until the situation occurs (just like it happened (opens new window) for Maker's liquidation bot).
In contrast, zkSync is positioned exceptionally well to thrive in a high-congestion environment.
First and foremost, a congested network (just like a case of targeted DOS attack) can never create any threat to assets in zkSync. Any movement of funds within or out of zkSync requires a zero-knowledge proof of validity, and it's simply unaffected by L1 censorship in any way.
Secondly, the normal operation of zkSync is also unlikely to be disrupted, even for smaller amounts. The validator's node is configured to automatically increase the gas price to over-the-average level to get zkSync blocks mined with high priority. Since the cost per transaction are ~1/100th of the cost of corresponding plain transaction on the L1, zkSync users will be least affected.