Chapter · Master Base
What a rollup is
Base is a type of Layer 2 called a rollup. The name is a good hint: it takes many transactions, rolls them up into one batch, and posts a summary of that batch to Ethereum. Instead of Ethereum processing each transaction one by one, it just needs to record the compressed batch. That sharing of one cost across hundreds of transactions is a big reason fees drop so much. The important part is that the transaction data still ends up on Ethereum, so anyone can check the work.
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What this lesson covers
Optimistic rollups
Base is an optimistic rollup. Optimistic means the network assumes each batch of transactions is honest by default and does not re-check every one right away. To keep this safe, there is a window of time when anyone can challenge a batch they believe is wrong by submitting proof. If a challenge is valid, the bad batch is thrown out. This trust-but-verify approach is why they are called optimistic. It keeps things fast and cheap while still giving people a way to catch cheating.
What you get asked
What does a rollup actually do?
A rollup batches many transactions together and posts the compressed data to Ethereum, spreading one cost across the whole batch.
Base is an optimistic rollup, which assumes batches are honest but lets anyone ___ a batch they think is wrong.
Optimistic rollups trust batches by default but keep a challenge window. If someone proves a batch is invalid, it gets rejected.
Why do rollups make transactions cheaper?
Posting one batch to Ethereum costs far less per transaction than posting each one alone, so the savings are shared across the batch.
Match each rollup idea to its meaning.
Rollups batch transactions, assume they are honest, allow a challenge window, and settle the data down to Ethereum for security.
The rest of this chapter
Learn how Base works as Coinbase's Ethereum Layer 2, from rollups and the OP Stack to gas in ETH, bridging, the sequencer, and staying safe.