How do you implement logging with correlation IDs in a microservice?

Question

Accepted Answer

**Correlation IDs** — track requests across multiple services.

```javascript
const { v4: uuidv4 } = require("uuid");
const pino = require("pino");

// Middleware to generate/extract correlation ID
function correlationMiddleware(req, res, next) {
    req.correlationId = req.headers["x-correlation-id"] || uuidv4();
    res.setHeader("x-correlation-id", req.correlationId);
    req.log = logger.child({ correlationId: req.correlationId });
    next();
}

// Pass correlation ID to downstream services
async function callService(url, correlationId) {
    return fetch(url, {
        headers: { "x-correlation-id": correlationId },
    });
}

// Express middleware
app.use(correlationMiddleware);

app.get("/api/orders", async (req, res) => {
    req.log.info("Fetching orders");

// Call another service with correlation ID
    const user = await callService(
        `http://user-service/users/${req.userId}`,
        req.correlationId
    );

req.log.info({ userId: user.id }, "Orders fetched");
    res.json({ orders: [] });
});
```

**Benefits:**
- Trace requests across services.
- Debug production issues.
- Filter logs by request.

Node.js: How do you implement logging with correlation IDs in a microservice?