What Actually Happens When You Switch Currency Data Providers: A Migration and Total Cost of Ownership Guide

// TypeScript — Currency Provider Adapter
// Abstracts provider-specific logic so switching
// providers requires changing only this file.

interface RateResult {
  from: string;
  to: string;
  exchangeRate: number;
  rateTime: string;
  source: string;
}

interface CurrencyProvider {
  name: string;
  getRate(from: string, to: string): Promise<RateResult>;
  convert(from: string, to: string, amount: number): Promise<RateResult>;
  healthCheck(): Promise<boolean>;
}

// --- New Provider Implementation ---
class CurrencyExchangeAppProvider implements CurrencyProvider {
  name = 'currency-exchange.app';
  private readonly baseUrl = 'https://currency-exchange.app/api';
  private readonly apiKey: string;

  constructor(apiKey: string) {
    this.apiKey = apiKey;
  }

  async getRate(from: string, to: string): Promise<RateResult> {
    const url = new URL(this.baseUrl + '/v1-get-currency-exchange-rate');
    url.searchParams.set('from', from);
    url.searchParams.set('to', to);
    url.searchParams.set('x-api-key', this.apiKey);

    const response = await fetch(url.toString());
    if (!response.ok) throw new Error(`API error: ${response.status}`);
    const data = await response.json();

    return {
      from: data.from,
      to: data.to,
      exchangeRate: data.exchangeRate,
      rateTime: data.rateTime,
      source: this.name,
    };
  }

  async convert(from: string, to: string, amount: number): Promise<RateResult> {
    const url = new URL(this.baseUrl + '/v1-convert-currency');
    url.searchParams.set('from', from);
    url.searchParams.set('to', to);
    url.searchParams.set('amount', String(amount));
    url.searchParams.set('x-api-key', this.apiKey);

    const response = await fetch(url.toString());
    if (!response.ok) throw new Error(`API error: ${response.status}`);
    const data = await response.json();

    return {
      from: data.from,
      to: data.to,
      exchangeRate: data.exchangeRate,
      rateTime: data.rateTime,
      source: this.name,
    };
  }

  async healthCheck(): Promise<boolean> {
    try {
      const result = await this.getRate('USD', 'EUR');
      return result.exchangeRate > 0;
    } catch {
      return false;
    }
  }
}

// --- Usage: swap provider in one place ---
const provider: CurrencyProvider = new CurrencyExchangeAppProvider(
  process.env.FX_API_KEY ?? '',
);

async function getConversionRate(from: string, to: string) {
  return provider.getRate(from, to);
}

// TypeScript — Parallel Rate Comparison Logger
// Runs both old and new providers, logs deviations

interface ComparisonEntry {
  timestamp: string;
  from: string;
  to: string;
  oldRate: number;
  newRate: number;
  deviation: number;
  deviationPercent: number;
}

const PAIRS = ['USD/EUR', 'USD/GBP', 'USD/JPY', 'EUR/GBP', 'AUD/USD'];
const TOLERANCE_PERCENT = 0.5; // 0.5% threshold

async function logComparison() {
  const results: ComparisonEntry[] = [];

  for (const pair of PAIRS) {
    const [from, to] = pair.split('/');
    const [oldRate, newRate] = await Promise.all([
      oldProvider.getRate(from, to),
      newProvider.getRate(from, to),
    ]);

    const deviation = Math.abs(oldRate.exchangeRate - newRate.exchangeRate);
    const deviationPercent = (deviation / oldRate.exchangeRate) * 100;

    results.push({
      timestamp: new Date().toISOString(),
      from,
      to,
      oldRate: oldRate.exchangeRate,
      newRate: newRate.exchangeRate,
      deviation,
      deviationPercent: Number(deviationPercent.toFixed(4)),
    });

    if (deviationPercent > TOLERANCE_PERCENT) {
      console.warn(`[ALERT] ${pair} deviation: ${deviationPercent}%`);
    }
  }

  // Append to CSV or database for analysis
  console.table(results);
}

// Run on a schedule (e.g., every 15 min for 5 business days)
// This gives you ~1,600 data points per pair to analyze