DeFi protocol development is fundamentally different from regular software development. The code is immutable once deployed, handles real financial assets, and is open-source for anyone to analyze — including attackers.
Pattern 1: Checks-Effects-Interactions. Every external call follows this order: validate inputs (checks), update state (effects), then make external calls (interactions). This prevents reentrancy — the attack that drained $60M from the original DAO.
Pattern 2: Upgradeable Proxy with Time-Lock. Use UUPS or Transparent Proxy for upgradeability, but always pair it with a time-lock (24-48 hour delay) so users can exit before malicious upgrades take effect. Governance multi-sig controls the upgrade path.
Pattern 3: Flash Loan Resistance. Price oracles must be manipulation-resistant. Use TWAP (Time-Weighted Average Price) from Uniswap V3 or Chainlink price feeds — never spot prices from a single DEX pool. Flash loan attacks exploit single-block price manipulation.
Pattern 4: Invariant Testing. Define mathematical invariants your protocol must always satisfy (e.g., "total supply equals sum of all balances") and write fuzz tests that verify these across millions of random transaction sequences.