Crypto Savings Platform Development
User wants to deposit USDC and earn interest without understanding Aave or Compound. This is yield strategy aggregation behind a simple interface. Complexity isn't UI, it's correct liquidity management under the hood: don't lose user funds on strategy switches and calculate interest properly without rounding errors.
Interest accrual: where user money gets lost
Continuous compounding vs discrete accrual
Aave V3 uses liquidityIndex — a cumulative multiplier growing continuously. User balance is scaledBalance * currentLiquidityIndex. On any action (deposit, withdraw) scaledBalance is recalculated via current index.
Custom interest calculation we reviewed calculated interest differently: balance += balance * rate * blocks_elapsed / YEAR_IN_BLOCKS. Seemed equivalent. Problem appeared on deposits: if user added funds, old balance was recalculated at new rate, not the rate at first deposit moment. Over 6 months accumulated ~0.3% error — small in percentage but significant at $5M TVL.
Right approach: store scaledBalance and global growthIndex. All operations go through the index. This is exactly what ERC-4626 does.
ERC-4626 as foundation
ERC-4626 (Tokenized Vault Standard) elegantly solves crypto savings: user deposits assets (USDC), gets shares (sUSDC). convertToAssets(shares) always returns current value with accumulated yield. No need to reinvent.
Critical moment: previewRedeem must return exactly the amount user gets on redeem. Discrepancy between preview and actual violates the standard and enables front-running vector. EIP-4626 specifically requires previewRedeem to be pessimistic (round down for user) so protocol never goes underwater.
Yield strategies — aggregation and rebalancing
Aave V3 integration
Aave Pool accepts supply(address asset, uint256 amount, address onBehalfOf, uint16 referralCode). In return user gets aToken (aUSDC), automatically earning interest via increasing liquidityIndex. On withdraw — withdraw(asset, amount, to).
For savings platform, vault contract acts as onBehalfOf — holds all aTokens, issues shares to users.
Rebalancing between protocols
If Compound yield exceeds Aave — protocol should move liquidity. Sounds simple but has nuances:
- Aave withdraw can take time if utilization high (no free liquidity)
- Rebalancing gas eats part of yield for small deposits
- Flash rebalancing via flash loan — move entire deposit atomically: take flash loan USDC → supply to new protocol → withdraw from old → repay flash loan. Pay 0.09% flash loan fee instead of two separate transactions.
Rebalancing threshold: yield difference must cover gas + flash loan fee with margin. Otherwise unprofitable. Typical threshold — 0.5-1% annual difference.
Chainlink for price calculations
For multi-currency savings (deposit ETH, yield in USDC) need price feeds for correct balance display in base currency. Chainlink Data Feeds on Ethereum mainnet and L2 are standard. Important: check feed updatedAt timestamp to not use stale price. If block.timestamp - updatedAt > heartbeat * 2 — price is stale, need fallback.
Flexible conditions: fixed vs flexible terms
Flexible deposits (like normal savings account): withdraw anytime, variable rate. ERC-4626 vault — standard implementation.
Fixed-term deposits (like bank deposit): user locks funds for 30/90/180 days, gets higher rate. Implementation: lockedUntil timestamp in mapping, withdraw checks it. Rate — locked on deposit via yieldIndex snapshot.
Fixed-term problem: if user needs liquidity early they lose accumulated yield and/or pay penalty. Must explicitly state in UI and contract — earlyWithdrawPenalty in basis points.
Security
Rug pull protection: admin can't withdraw user funds arbitrarily. onlyOwner on withdraw is a red flag for auditors. Timelock on strategy changes (minimum 24-48 hours) via Gnosis Safe + TimelockController.
Pausability: on discovering protocol issue where funds are invested (Aave exploit) — need quick pause on new deposits. Pausable from OpenZeppelin + Emergency withdrawal mode letting users withdraw even when paused (only withdrawals, no new deposits).
Max deposit per user: protects from concentration risk and whale domination blocking other withdrawals.
Working process
Analytics (2-3 days). Choose base protocols, tokenomics (any protocol reward token), multi-chain or single chain, KYC/compliance requirements.
Development (1-2 weeks). ERC-4626 vault → strategy integration → rebalancer → timelock + governance.
Testing. Fork tests on Ethereum mainnet: real Aave rates, real USDC. Scenarios: deposit → yield → rebalance → withdraw. Fuzz on deposit/withdraw/redeem with random amounts and time parameters.
Timeline estimates
Simple vault on one protocol (Aave) with ERC-4626 — 1 week. Multi-strategy platform with rebalancing, fixed-term deposits and governance — 4-6 weeks.
Cost calculated individually.