Chapter 3. Example application using the circuit breaker add-on

This example application consists of two microservices:

greeting-service imports the circuit breaker add-on. greeting-service protects calls to the remote name-service by wrapping these calls in a circuit breaker object.

greeting-service exposes the following endpoints:

The following code example shows how to develop greeting-service:

'use strict';
const path = require('path');
const http = require('http');
const express = require('express');
const bodyParser = require('body-parser');

// Import the circuit breaker add-on
const Opossum = require('@redhat/opossum');

const probe = require('kube-probe');
const nameService = require('./lib/name-service-client');

const app = express();
const server = http.createServer(app);

// Add basic health check endpoints
probe(app);

const nameServiceHost = process.env.NAME_SERVICE_HOST || 'http://nodejs-circuit-breaker-redhat-name:8080';

// Set some circuit breaker options
const circuitOptions = {
  timeout: 3000, // If name service takes longer than 0.3 seconds,
                 // trigger a failure
  errorThresholdPercentage: 50, // When 50% of requests fail,
                                // trip the circuit
  resetTimeout: 10000 // After 10 seconds, try again.
};

// Create a new circuit breaker instance and pass the remote nameService
// as its first parameter
const circuit = new Opossum(nameService, circuitOptions);

// Create the app with an initial websocket endpoint
require('./lib/web-socket')(server, circuit);

// Serve index.html from the file system
app.use(express.static(path.join(__dirname, 'public')));
// Expose the license.html at http[s]://[host]:[port]/licences/licenses.html
app.use('/licenses', express.static(path.join(__dirname, 'licenses')));

// Send and receive json
app.use(bodyParser.json());

// Greeting API
app.get('/api/greeting', (request, response) => {
  // Use the circuit breaker’s fire method to execute the call
  // to the name service
  circuit.fire(`${nameServiceHost}/api/name`).then(name => {
    response.send({ content: `Hello, ${name}`, time: new Date() });
  }).catch(console.error);
});

// Circuit breaker state API
app.get('/api/cb-state', (request, response) => {
  response.send({ state: circuit.opened ? 'open' : 'closed' });
});

app.get('/api/name-service-host', (request, response) => {
  response.send({ host: nameServiceHost });
});

module.exports = server;

name-service exposes the following endpoints:

The following code example shows how to develop name-service:

'use strict';

const path = require('path');
const http = require('http');
const express = require('express');
const bodyParser = require('body-parser');
const cors = require('cors');
const probe = require('kube-probe');

const app = express();
const server = http.createServer(app);

// Adds basic health-check endpoints
probe(app);

let isOn = true;
const { update, sendMessage } = require('./lib/web-socket')(server, _ => isOn);

// Send and receive json
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));

// CORS support
app.use(cors());

// Name service API
app.get('/api/name', (request, response) => {
  if (isOn) {
    response.send('World!');
  } else {
    response.status(500).send('Name service down');
  }
  sendMessage(`${new Date()} ${isOn ? 'OK' : 'FAIL'}`);
});

// Current state of service
app.put('/api/state', (request, response) => {
  isOn = request.body.state === 'ok';
  response.send({ state: isOn });
  update();
});

app.get('/api/info',
  (request, response) => response.send({ state: isOn ? 'ok' : 'fail' }));

// Expose the license.html at http[s]://[host]:[port]/licenses/licenses.html
app.use('/licenses', express.static(path.join(__dirname, 'licenses')));

module.exports = server;