REPR Design Pattern: What is it and how does it work?

📒 Introduction

In the world of software design, patterns serve as reusable solutions to common problems, ensuring that developers can create maintainable, efficient, and scalable applications. One such emerging pattern is the REPR Design Pattern, which stands for Request, Evaluate, Process, and Respond. This pattern focuses on managing the lifecycle of operations in modern, event-driven systems.

This article delves into the REPR Design Pattern, exploring its core components, practical applications, and implementation examples using TypeScript and Node.js.

🤨 What is the REPR Design Pattern?

The REPR Design Pattern provides a structured way to handle workflows in a system, especially in scenarios where requests trigger a series of actions, evaluations, and responses. It ensures that each phase of the operation is well-defined, independent, and easy to extend or maintain.

Core Phases:

1️⃣ Request:
Capture and validate the incoming request or event.
Example: An HTTP API endpoint receives a POST request.

2️⃣ Evaluate:
Analyze the request to determine the appropriate course of action.
Example: Validate inputs, check permissions, or identify the operation type.

3️⃣ Process:
Execute the business logic or main operation.
Example: Query a database, call external services, or perform computations.

4️⃣ Respond:
Provide feedback to the requester, whether it’s the result, an error, or a status update.
Example: Return an HTTP response, log the outcome, or send an event notification.

🧙 Benefits of the REPR Pattern

✓ Clear Separation of Concerns: Each phase has a distinct responsibility, making the code easier to understand and test.

✓ Reusability: Modular components in each phase can be reused across different workflows.

✓ Scalability: Handles both synchronous and asynchronous workflows effectively.

✓ Extensibility: Additional phases or steps can be added without disrupting the existing logic.

✓ Implementing REPR in Node.js and TypeScript

Below is an example of implementing the REPR pattern for a user registration system.

Example

// utils/validateRequest.ts
export function validateRequest(data: any): void {
  if (!data.email || !data.password) {
    throw new Error("Invalid request: Missing email or password.");
  }
}

// services/userService.ts
export async function createUser(data: { email: string; password: string }): Promise<{ id: string }> {
  // Simulate database operation
  return { id: "12345" };
}

// handlers/registerUser.ts
import { validateRequest } from "../utils/validateRequest";
import { createUser } from "../services/userService";

export async function registerUser(request: any): Promise<any> {
  try {
    // Request: Validate input
    validateRequest(request.body);

    // Evaluate: Perform additional checks (e.g., user already exists)
    if (request.body.email === "[e-mail]") {
      throw new Error("User already exists.");
    }

    // Process: Create the user
    const user = await createUser(request.body);

    // Respond: Return success response
    return { status: 201, data: { userId: user.id } };
  } catch (error) {
    // Respond: Return error response
    return { status: 400, error: error.message };
  }
}

Example

import express from "express";
import { registerUser } from "./handlers/registerUser";

const app = express();
app.use(express.json());

app.post("/register", async (req, res) => {
  const response = await registerUser(req);
  res.status(response.status).json(response);
});

app.listen(3000, () => console.log("Server is running on port 3000"));

🌎 ## Real-World Use Cases for REPR

🌉 API Gateways: Handle incoming requests, validate headers, evaluate routing logic, and respond with aggregated results.

🚚 Event-Driven Systems: Process events like user actions, evaluate their impact, and respond with notifications or state changes.

⚙️ Workflow Engines: Manage multi-step workflows such as order processing or ticket management.

Best Practices

✓ Modular Design: Break each phase into small, reusable functions or modules.

✓ Error Handling: Ensure each phase has robust error handling to prevent cascading failures.

✓ Logging and Monitoring: Log operations and outcomes at each phase for debugging and auditing.

✓ Performance Optimization: Optimize bottleneck phases, such as database queries in the "Process" phase.

Conclusion

The REPR Design Pattern is a versatile and powerful approach to managing workflows in modern software systems. By structuring operations into clear phases—Request, Evaluate, Process, and Respond—developers can create applications that are maintainable, scalable, and resilient.

Whether you're building an API, processing events, or handling complex workflows, adopting the REPR pattern can help streamline your development process and improve system reliability.

Start implementing REPR today, and see the difference it makes in your projects!