Blockchain scaling has been a problem for a long time as it required developers to trade off a bit of either security or decentralization. Ethereum co-founder Vitalik Buterin famously referred to the issue as the blockchain scaling trilemma.
In a bid to address it, developers have started moving to a modular system that offloads some of the work from the base chain. That’s what led to the term rollups, and you can either use an optimistic rollup or a zero-knowledge (ZK) rollup in your application or network.
But what exactly are these terms, and how do they differ? That’s what I’ll walk you through in this guide.
What Rollups Are Trying To Solve
Older blockchains like Bitcoin and Ethereum 1.0 were designed to be secure, decentralized, and transparent, with all activities happening within the main chain. However, that comes at a cost.
As blockchains like Ethereum became popular beyond cryptocurrency, you could have too many users trying to send transactions or interact with smart contracts at the same time. This would squeeze the network’s capacity, slowing down applications and increasing gas fees. Both can be extrememly costs for tasks and processes like DeFi and gaming.
Rollups aim to improve scalability by offloading some of the work from the main chain. So, instead of building an app on Ethereum itself, you can build on a separate chain that’s still connected to Ethereum. The app will execute off-chain, then publish summaries or proofs back to the main Ethereum network.
This way, you get execution speed but still use the base chain to ensure everything is secure and decentralized. Essentially, it’s like letting blockchains have their cake and eat it too. You get higher throughput and lower fees without giving up the main guarantees that make everything reliable.
This leads us to the two main types of rollups, optimistic rollups and ZK rollups.
How Optimistic Rollups Work
An optimistic rollup is actually quite optimistic as it assumes that all the transactions submitted off-chain are valid, unless someone proves otherwise.
This type of rollup doesn’t bother with immediate verifications from the main chain. It executes transactions and then posts the summaries later. These are then subjected to a challenge period that usually lasts 7 days. During this time, withdrawals are delayed, and finality has not yet been achieved. It’s a chance for anyone to dispute the activity as fraudulent.
The technique extends the waiting period, but it keeps the system secure without forcing immediate on-chain verification for every transaction.
An example here is Arbitrium One, the leading rollup on Ethereum. It makes dApp transactions much faster while also saving about 90-95% of the gas fees.
These types of rollups are also EVM-compatible, meaning developers can deploy smart contracts that work almost identically to those on Ethereum.
How Zero-Knowledge Rollups Work
Zero-knowledge (ZK) rollups take a very different approach that doesn’t involve a waiting period. Instead, they use cryptographic proof to show the validity of transactions. The proof is what is sent to Ethereum, and it’s immediately accepted if the proof verifies.
The major advantage of this type of rollup is the speed of withdrawals and finality since every transaction is sent to Ethereum with its proof. End users benefit from this directly — faster confirmations mean a bitcoin wallet app can reflect settled transactions in seconds rather than days. The fact that validation has already occurred also means Ethereum has much less work to do, keeping fees lower.
However, the rollups are also complex as cryptographic proof means complex mathematics and specialized tools. The user experience also tends to vary from the regular Ethereum one, although rollups like zkSync are bridging that gap.
Key Differences Between Optimistic And Zero-Knowledge Rollups
| Feature | Optimistic Rollups | ZK Rollups |
| Validation method | Assumes transactions are valid but fraud can be challenged | Uses cryptographic proofs (immediate) |
| Finality & withdrawals | Delayed due to challenge period (usually 7 days) | Instant |
| Complexity | Relatively simple, easier to implement | More complex due to cryptography |
| EVM compatibility | High. Similar experience as Ethereum. | Improving. Some are more Ethereum-like, but most aren’t. |
Which Model Fits Which Use Case?
The best way to know the type of rollup that fits your application is to consider your priorities in terms of speed, security, and compatibility.
If you are building a DeFi project that looks a bit familiar to users but can tolerate withdrawal delays, go for an optimistic rollup. It also works much better if you already have an existing Ethereum app you need to scale.
ZK rollups, on the other hand, are more suitable when speed and proof of correctness are crucial, like in payment systems or gaming platforms. Since every transaction is final once it’s posted, you’ll avoid lots of errors and disputes.
