02. ETH.BUILD: Blockchain
Overview
Section titled “Overview”Now that you know an account is a key pair, the next question is: a key pair on what? The answer is the blockchain — a shared, append-only ledger that every participant keeps a copy of and agrees on. This lesson builds the mental model of that ledger: what a block is, why blocks form a chain, and why the order of events is the thing the whole system exists to agree on.
Key concepts
Section titled “Key concepts”- Block — a batch of transactions bundled together, plus a header with metadata (a timestamp, a reference to the previous block, and a summary of the resulting state). Blocks are produced at a roughly fixed cadence (about every 12 seconds on Ethereum mainnet).
- Chain — each block header contains the hash of the previous block. That link is what makes it a chain: change any past block and its hash changes, which breaks every block after it. This is why history is effectively tamper-evident.
- Hash — a fixed-length fingerprint of some data. Any change to the input produces a completely different fingerprint, so hashes let the network detect tampering cheaply.
- State — the current snapshot of every account balance and every contract’s stored data. A block does not store the whole world; it stores the changes, and applying blocks in order reconstructs the state.
- Ordering and consensus — the reason blockchains exist is to make a decentralized network agree on one ordered history without a central referee. Order matters: if two transactions both try to spend the same balance, which one comes first decides the outcome.
- Finality / confirmations — a very recent block can still be reorganized away; the more blocks built on top of it (“confirmations”), the more settled it is. This is why apps wait for a few confirmations before treating a payment as done.
Common pitfalls
Section titled “Common pitfalls”- Picturing the chain as storing balances directly. It stores an ordered list of changes; balances are the result of replaying them.
- Assuming a transaction is final the instant it appears. Until it is included in a block and has some confirmations behind it, it can still be dropped or reordered.
- Confusing “decentralized” with “no rules.” The whole point is a strict, shared rule for ordering that no single party controls.
Check yourself
Section titled “Check yourself”- Why does changing one old block invalidate every block after it?
- What does a block actually contain — the full state, or the changes to it?
- Why do applications wait for confirmations instead of trusting a brand-new block?
Watch the Video
Section titled “Watch the Video”
ETH.BUILD: Blockchain
Video credit: Austin Griffith · Duration: 23 minutes