---
name: data-service-d74586bd0c98
description: Create data fetching services with circuit breaker pattern for API resilience. Services handle fetch, cache, retry, and expose typed data to panel components.
---

# Data Service Pattern

Each panel's data comes from a dedicated **service module** in `src/services/`. Services handle API calls, caching, error handling with circuit breaker pattern.

## Circuit Breaker

The circuit breaker prevents cascading failures when an API is down. After N consecutive failures, it enters "cooldown" mode and serves cached data instead of hitting the API.

Create `src/utils/circuit-breaker.ts`:

```typescript
interface CircuitState {
  failures: number;
  cooldownUntil: number;
}

interface CacheEntry<T> {
  data: T;
  timestamp: number;
}

export interface CircuitBreakerOptions {
  name: string;
  maxFailures?: number;
  cooldownMs?: number;
  cacheTtlMs?: number;
}

export class CircuitBreaker<T> {
  private state: CircuitState = { failures: 0, cooldownUntil: 0 };
  private cache: CacheEntry<T> | null = null;
  private name: string;
  private maxFailures: number;
  private cooldownMs: number;
  private cacheTtlMs: number;

  constructor(options: CircuitBreakerOptions) {
    this.name = options.name;
    this.maxFailures = options.maxFailures ?? 2;
    this.cooldownMs = options.cooldownMs ?? 5 * 60 * 1000;  // 5 minutes
    this.cacheTtlMs = options.cacheTtlMs ?? 10 * 60 * 1000; // 10 minutes
  }

  isOnCooldown(): boolean {
    if (Date.now() < this.state.cooldownUntil) return true;
    if (this.state.cooldownUntil > 0) {
      this.state = { failures: 0, cooldownUntil: 0 };
    }
    return false;
  }

  getCached(): T | null {
    if (this.cache && Date.now() - this.cache.timestamp < this.cacheTtlMs) {
      return this.cache.data;
    }
    return null;
  }

  recordSuccess(data: T): void {
    this.state = { failures: 0, cooldownUntil: 0 };
    this.cache = { data, timestamp: Date.now() };
  }

  recordFailure(error?: string): void {
    this.state.failures++;
    if (this.state.failures >= this.maxFailures) {
      this.state.cooldownUntil = Date.now() + this.cooldownMs;
      console.warn(`[${this.name}] Cooldown for ${this.cooldownMs / 1000}s`);
    }
  }

  async execute<R extends T>(fn: () => Promise<R>, defaultValue: R): Promise<R> {
    if (this.isOnCooldown()) {
      const cached = this.getCached();
      return (cached as R) ?? defaultValue;
    }
    const cached = this.getCached();
    if (cached !== null) return cached as R;
    try {
      const result = await fn();
      this.recordSuccess(result);
      return result;
    } catch (e) {
      console.error(`[${this.name}] Failed:`, e);
      this.recordFailure(String(e));
      return defaultValue;
    }
  }
}

export function createCircuitBreaker<T>(options: CircuitBreakerOptions): CircuitBreaker<T> {
  return new CircuitBreaker<T>(options);
}
```

## Service Module Pattern

Each service is a TypeScript module that exports async data-fetching functions:

```typescript
// src/services/stock-market.ts
import { createCircuitBreaker } from '../utils/circuit-breaker';

export interface StockQuote {
  symbol: string;
  name: string;
  price: number | null;
  change: number | null;
  sparkline?: number[];
}

const breaker = createCircuitBreaker<StockQuote[]>({
  name: 'StockMarket',
  cacheTtlMs: 60_000, // 1 minute cache
});

export async function fetchStockQuotes(symbols: string[]): Promise<StockQuote[]> {
  return breaker.execute(async () => {
    const params = new URLSearchParams({ symbols: symbols.join(',') });
    const resp = await fetch(`/api/stocks?${params}`);
    if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
    return resp.json();
  }, []);
}
```

## Key Patterns

1. **One circuit breaker per API endpoint** — create at module level
2. **Export typed interfaces** for data shapes
3. **Wrap fetch calls** in `breaker.execute(fn, defaultValue)`
4. **Default values** are empty arrays `[]` or empty objects `{}`
5. **Services are stateless modules** — no class instances, just exported functions
6. **API proxy**: Frontend calls `/api/...` which proxies to external APIs (hides API keys)

## API Proxy Pattern

For APIs requiring keys, create server-side proxy endpoints:

```typescript
// api/stocks.ts (Vercel serverless function)
export default async function handler(req, res) {
  const { symbols } = req.query;
  const apiKey = process.env.FINNHUB_API_KEY;
  const data = await fetch(`https://finnhub.io/api/v1/quote?symbol=${symbols}&token=${apiKey}`);
  res.json(await data.json());
}
```

This keeps API keys server-side and provides a single endpoint for the frontend.



---
Source attribution: AG064
Original project: https://github.com/AG064/OpenSpace/blob/ff671ef61ed6587c49bd54c81cdc751c0b99e363/showcase/skills/data-service/SKILL.md
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Dave AI.

## Upstream license notice

MIT License

Copyright (c) 2026 ✨Data Intelligence Lab@HKU✨

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Dave AI resource metadata

Creator / publisher: AG064
Original project: https://github.com/AG064/OpenSpace/blob/ff671ef61ed6587c49bd54c81cdc751c0b99e363/showcase/skills/data-service/SKILL.md
Attribution basis: Source repository record

Fields of work: Development & Engineering
Roles: Backend Developer, Data Engineer, Full-Stack Developer
Specialties: API, Backend, Cross Platform
Tools: Typescript
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: a9c5e52d8841c69c09505303e1b993438bd618d8e654591b20deb5633356c998