Getting Started with Opossum in Node.js Applications

Summary

• Introduction
• What is the Circuit Breaker Pattern?
• What is Opossum?
• Hands on
• Conclusion

Introduction

Initially, when we are building distributed systems, applications often rely on external services such as APIs, databases, and third-party integrations. However, these dependencies can fail, causing cascading failures in your system. The Circuit Breaker pattern helps prevent such failures from bringing down your entire application.

In this post, I will explain about Opossum, a Node.js library that implements the Circuit Breaker pattern, and see how it can improve the resilience of your applications.

What is the Circuit Breaker Pattern?

The Circuit Breaker pattern is a fault-tolerance mechanism that prevents an application from repeatedly trying to execute an operation that is likely to fail. It works similarly to an electrical circuit breaker:

1. Closed State: Requests are allowed to pass through normally.
2. Open State: If failures exceed a defined threshold, the circuit "opens," preventing further requests.
3. Half-Open State: After a timeout, a few requests are allowed to test if the dependency has recovered.

This helps prevent overwhelming failing services and allows the system to recover gracefully.

What is Opossum?

Opossum is a lightweight implementation of the Circuit Breaker pattern for Node.js. It helps monitor failures and fallback mechanisms, ensuring better resilience in applications.

Hands on

Step 1: Install

You can install Opossum using npm:

npm install opossum

Step 2: Creating a Function to Wrap

We will use Node.js and Axios to make an API request.

const axios = require('axios');

async function fetchData() {
  console.log('Requesting...');
  const response = await axios.get('$URL');
  return response.data;
}

Step 3: Implementing Opossum

Now, let's wrap fetchData with Opossum.

const CircuitBreaker = require('opossum');

const options = {
  timeout: 3000, // 3 seconds timeout
  errorThresholdPercentage: 50, // Open circuit if 50% of requests fail
  resetTimeout: 5000 // Attempt recovery after 5 seconds
};

const breaker = new CircuitBreaker(fetchData, options);

breaker.fallback(() => ({ message: 'Fallback: Service unavailable' }));

breaker.on('open', () => console.log('Circuit is open!'));  
breaker.on('close', () => console.log('Circuit is closed!'));  
breaker.on('halfOpen', () => console.log('Circuit is half-open!'));

Step 4: Create the request method

Create a method to perform the request:

function performRequest(breaker) {
  breaker.fire()
    .then(data => console.log('Data:', data))
    .catch(err => console.error('Error:', err));
}

Step 5: Calling the Circuit Breaker

Now, execute the function in a defined interval:

setInterval(() => performRequest(breaker), 1000);

If the API fails repeatedly, Opossum will open the circuit, preventing further requests until recovery.

Step 6: Check information about the Circuit Breaker

To check the stats about the requests performed:

function showStats(breaker) {
  console.log(breaker.stats)
}

Step 7: Calling the Circuit Breaker Stats

Execute the function in a defined interval:

setInterval(() => showStats(breaker), 5000);

Conclusion

Opossum is a powerful library that helps Node.js applications become more fault-tolerant by implementing the Circuit Breaker pattern. This technique prevents cascading failures and improves system stability, especially when working with unreliable external services.

Further Reading

• Project example