Crypto Defi Patterns

Cryptocurrency & DeFi Development Patterns

When to Use This Skill

• Building cryptocurrency trading systems
• Integrating with DeFi protocols
• Interacting with smart contracts
• Developing crypto wallets
• Building blockchain applications

Target Agents

• crypto-api-developer - Primary user
• backend-developer - Crypto backend systems
• financial-systems-expert - Crypto trading

Core Patterns

1. Web3 Integration with ethers.js

import { ethers } from 'ethers'

class Web3Service {
  private provider: ethers.providers.Provider
  private signer: ethers.Signer

  constructor(rpcUrl: string, privateKey: string) {
    this.provider = new ethers.providers.JsonRpcProvider(rpcUrl)
    this.signer = new ethers.Wallet(privateKey, this.provider)
  }

  async getBalance(address: string): Promise<string> {
    const balance = await this.provider.getBalance(address)
    return ethers.utils.formatEther(balance)
  }

  async sendTransaction(to: string, amount: string) {
    const tx = await this.signer.sendTransaction({
      to,
      value: ethers.utils.parseEther(amount)
    })

    const receipt = await tx.wait()
    return receipt
  }
}

2. Smart Contract Interaction

interface IUniswapV2Router {
  swapExactTokensForTokens(
    amountIn: BigNumber,
    amountOutMin: BigNumber,
    path: string[],
    to: string,
    deadline: number
  ): Promise<ContractTransaction>
}

class DeFiService {
  private router: IUniswapV2Router

  constructor(routerAddress: string, signer: Signer) {
    this.router = new ethers.Contract(
      routerAddress,
      UNISWAP_ROUTER_ABI,
      signer
    ) as IUniswapV2Router
  }

  async swapTokens(
    tokenIn: string,
    tokenOut: string,
    amountIn: string,
    slippage: number = 0.5
  ) {
    const path = [tokenIn, tokenOut]
    const amountInWei = ethers.utils.parseEther(amountIn)

    // Calculate minimum output with slippage
    const amountOut = await this.getAmountOut(amountInWei, path)
    const amountOutMin = amountOut.mul(100 - slippage).div(100)

    const deadline = Math.floor(Date.now() / 1000) + 60 * 20 // 20 minutes

    const tx = await this.router.swapExactTokensForTokens(
      amountInWei,
      amountOutMin,
      path,
      await this.signer.getAddress(),
      deadline
    )

    return await tx.wait()
  }
}

3. Wallet Management

import { HDNode } from 'ethers/lib/utils'

class WalletService {
  generateMnemonic(): string {
    return ethers.Wallet.createRandom().mnemonic.phrase
  }

  deriveWallet(mnemonic: string, index: number = 0): ethers.Wallet {
    const path = `m/44'/60'/0'/0/${index}` // BIP44 Ethereum path
    return ethers.Wallet.fromMnemonic(mnemonic, path)
  }

  async signMessage(wallet: ethers.Wallet, message: string): Promise<string> {
    return await wallet.signMessage(message)
  }
}

4. Exchange API Integration

import ccxt from 'ccxt'

class ExchangeService {
  private exchange: ccxt.Exchange

  constructor(exchangeName: string, apiKey: string, secret: string) {
    const ExchangeClass = ccxt[exchangeName]
    this.exchange = new ExchangeClass({
      apiKey,
      secret,
    })
  }

  async fetchBalance() {
    return await this.exchange.fetchBalance()
  }

  async placeOrder(
    symbol: string,
    type: 'market' | 'limit',
    side: 'buy' | 'sell',
    amount: number,
    price?: number
  ) {
    return await this.exchange.createOrder(symbol, type, side, amount, price)
  }

  async fetchOrderBook(symbol: string, limit: number = 20) {
    return await this.exchange.fetchOrderBook(symbol, limit)
  }
}

Best Practices

1. Use testnet for development
2. Implement proper key management
3. Handle gas fees appropriately
4. Implement transaction retry logic
5. Monitor blockchain confirmations
6. Validate addresses before transactions
7. Implement rate limiting for APIs
8. Use hardware wallets for production
9. Implement proper error handling
10. Audit smart contract interactions