Writing Custom Middleware in EventSource or DataSource

Godspeed's plugin architecture allows you to extend behavior by injecting custom middleware into your EventSource (e.g., Express).

This is useful for adding: Logging, Rate limiting, Authentication, Metrics, Request/response mutation, Any side-effect logic before or after processing an api or event.

Where Can You Add Middleware?

Middleware can be added inside:

• src/eventsources/types/<eventsource>.ts

You typically do this by subclassing the base plugin and injecting middleware in the initClient() method.

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Example: Adding Middleware in Express EventSource

Here’s how to add a middleware that:

• Overrides the /metrics endpoint
• Combines Express metrics with Prisma metrics

File: src/eventsources/types/express.ts

import { PlainObject } from "@godspeedsystems/core";
import { EventSource } from "@godspeedsystems/plugins-express-as-http";
import promClient from '@godspeedsystems/metrics';
import { PrismaClient } from "../../datasources/prisma-clients/schema";

class MyEventSource extends EventSource {
  async initClient(): Promise<PlainObject> {
    const client = await super.initClient();

    // Add your custom middleware here
    client.get('/metrics', async (req, res) => {
      try {
        const expressMetrics = await promClient.register.metrics();
        const prismaMetrics = await new PrismaClient().$metrics.prometheus();
        res.set('Content-Type', promClient.register.contentType);
        res.end(`${expressMetrics}\n${prismaMetrics}`);
      } catch (err: any) {
        res.status(500).send({ error: err.message });
      }
    });

    return client;
  }
  // (Optional) override setupMetrics to disable default `/metrics` middleware
  setupMetrics(app: any) {
    promClient.collectDefaultMetrics(); // only collects, doesn't auto-bind endpoint
  }
}
export default MyEventSource;

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General Pattern

1. Subclass the plugin’s EventSource class.
2. Override initClient() to gain access to the underlying client (e.g., express() app, PrismaClient, Axios instance).
3. Inject your middleware using standard APIs (e.g., app.use(), app.get(), or interceptors for Axios).

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Example Use Cases

Logging Every Request

client.use((req, res, next) => {
  console.log(`[${req.method}] ${req.originalUrl}`);
  next();
});

Injecting Custom Headers

client.use((req, res, next) => {
  res.set('X-Godspeed-Version', 'v2.0.0');
  next();
});

Protecting a Route

client.get('/admin', (req, res, next) => {
  if (!req.headers['x-admin-token']) {
    return res.status(403).send("Forbidden");
  }
  next();
}, handler);

⚠️

• If your middleware adds an endpoint like /metrics, ensure it doesn't conflict with auto-registered routes (disable via config).
• Middleware runs on every request unless scoped to a specific route.
• Always call next() in Express-style middleware unless you're sending a response directly.