Learning Objectives – after this chapter you will be able to:
- Explain the blockchain scalability trilemma.
- Distinguish Layer-1 and Layer-2 scaling approaches.
- Compare optimistic rollups, ZK rollups, state channels and sidechains.
- Explain the role of blob transactions and data availability.
Why Scalability is Hard – The Trilemma
- Every full node verifies every transaction, so a blockchain can process only as many transactions as an ordinary node can handle. Bitcoin, for example, is commonly estimated at around 7 transactions per second.
- The scalability trilemma (a term popularised by Vitalik Buterin) says it is hard for a blockchain to achieve all three at once:
- Decentralization – many independent nodes, affordable to run;
- Security – very expensive to attack;
- Scalability – high throughput and low fees.
- Simply making blocks bigger increases throughput but raises hardware requirements, so fewer people can run nodes – reducing decentralization.
Layer-1 vs Layer-2 Scaling
| Approach | Idea | Examples |
|---|---|---|
| Layer-1 | Change the base protocol itself: bigger or faster blocks, more efficient data, sharding. | Ethereum’s blob space (EIP-4844) and its long-term “danksharding” roadmap |
| Layer-2 | Execute transactions off the main chain and post compressed data or proofs back to Layer-1, inheriting its security. | Rollups, state channels (Lightning Network) |
Layer-2 Solutions Compared
| Solution | How it works | Strengths | Limitations |
|---|---|---|---|
| Optimistic rollup | Batches many transactions off-chain and posts their data to L1, assuming they are valid. Anyone can submit a fraud proof during a challenge window. | EVM-compatible, general purpose. | Withdrawals to L1 wait for the challenge period (commonly about 7 days). |
| ZK rollup | Batches transactions and posts a validity proof (zero-knowledge proof) that L1 verifies mathematically. | Fast finality on L1; no challenge period. | Proof generation is computationally heavy and complex. |
| State channels | Participants lock funds on-chain, exchange signed updates off-chain, and settle the final state on-chain. | Instant, near-free payments between parties. | Funds locked; participants must be online; best for repeated interactions. |
| Sidechains | Separate blockchain with its own consensus, connected by a bridge. | High throughput, flexible design. | Security depends on the sidechain’s own validators, not on L1 – so not a true Layer-2. |
Data Availability and Blobs
- Rollups must publish their transaction data so that anyone can reconstruct the state and verify or challenge it – this is called data availability.
- Ethereum’s Dencun upgrade (March 2024) introduced EIP-4844 blob transactions (“proto-danksharding”): large, cheap data “blobs” that are kept by nodes only temporarily (about 18 days) – long enough for rollups to be verified.
- This reduced rollup data costs significantly and is a step towards full danksharding.
- Bridges move assets between L1, L2s and other chains. They hold large amounts of value and have been frequent targets of attacks, so their security design matters.
Key Takeaways
- The trilemma: balancing decentralization, security and scalability.
- Rollups execute off-chain and post data/proofs to L1: optimistic (fraud proofs) vs ZK (validity proofs).
- EIP-4844 blobs make rollup data cheaper; sidechains have their own security.
Question / Answer (15 questions)
Answer : Every full node must download, verify and store every transaction. To keep nodes affordable for many participants, block size and frequency are limited, which caps the number of transactions per second.
Answer : It is the observation that it is difficult for a blockchain to simultaneously maximise decentralization, security and scalability; improving one often weakens another.
Answer : Larger blocks require more bandwidth, storage and processing power, so fewer people can run full nodes. This reduces decentralization and can weaken security.
Answer : Layer-1 scaling changes the base blockchain protocol itself, such as adding blob space or sharding. Layer-2 scaling processes transactions off the main chain and posts compressed data or proofs back to Layer-1, inheriting its security.
Answer : A rollup is a Layer-2 solution that executes many transactions off-chain, then posts their data and a summary of the new state to the Layer-1 chain, so Layer-1 can secure and verify them.
Answer : They assume batches are valid but allow anyone to submit a fraud proof during a challenge window. If fraud is proven, the invalid batch is reverted and the dishonest party is penalised.
Answer : Funds can be released to Layer-1 only after the challenge period – commonly about seven days – has passed without a successful fraud proof.
Answer : ZK rollups post a cryptographic validity proof with each batch, which Layer-1 verifies immediately. There is no need for a challenge period, but generating proofs is computationally intensive.
Answer : A state channel lets participants lock funds on-chain, exchange many signed updates off-chain and settle only the final state on-chain. Bitcoin’s Lightning Network is a network of payment channels.
Answer : A sidechain has its own consensus and validators. Its security does not come from the main chain, so if the sidechain validators misbehave, Layer-1 cannot protect users’ funds.
Answer : Data availability means the transaction data of a rollup batch is published so anyone can reconstruct the state and verify or challenge it. Without it, users could not prove fraud or withdraw safely.
Answer : EIP-4844, activated in the Dencun upgrade in March 2024, introduced blob-carrying transactions that provide large, cheap, temporarily stored data space for rollups, lowering their costs. It is also called proto-danksharding.
Answer : Blob data is kept by consensus nodes for a limited period of about 18 days, long enough for rollups to be verified and challenged, after which it can be pruned.
Answer : A bridge moves assets or messages between chains, often by locking tokens on one chain and minting representations on another. Bridges hold large pooled funds and complex logic, making them attractive targets for attacks.
Answer : Sharding splits the work of a blockchain so that not every node must process or store all data. Ethereum’s current approach focuses on data sharding (danksharding) to provide more data space for rollups.
MCQ Test : Check Your Understanding (5 questions)
Select an option and press Check Answer, or answer all questions and press Submit Test to see your score. Each answer comes with an explanation.
Bibliography/References
The notes, questions and MCQs on this page are original explanations written for this course. The works listed below were consulted and are cited for attribution and further reading. Product names, logos and trademarks belong to their respective owners. Short quotations, where used, are marked and attributed.
- ethereum.org. (n.d.). Scaling. Ethereum Developer Documentation. https://ethereum.org/en/developers/docs/scaling/
- Ethereum Improvement Proposals. (2022). EIP-4844: Shard Blob Transactions. https://eips.ethereum.org/EIPS/eip-4844
- Buterin, V. (2013). Ethereum Whitepaper: A Next-Generation Smart Contract and Decentralized Application Platform. ethereum.org. https://ethereum.org/en/whitepaper/
- Antonopoulos, A. M. (2017). Mastering Bitcoin: Programming the Open Blockchain (2nd ed.). O’Reilly Media.
- ethereum.org. (n.d.). Networks. Ethereum Developer Documentation. https://ethereum.org/en/developers/docs/networks/