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Hydra plugins

Hydra's behavior and features can be extended through plugins, allowing different Hydra services or plugins to easily take advantage of shared functionalities.

Overview

Hydra plugins extend the HydraPlugin class.

Plugins should be registered before Hydra is initialized via hydra.init.

E.g.

const YourPlugin = require('./your-plugin');
hydra.use(new YourPlugin());

Hydra will automatically call several hooks defined by the plugin class:

Hook Description
setHydra(hydra) called during plugin registration
setConfig(config) called before hydra initialization
updateConfig(serviceConfig) when the service-level config changes, will call configChanged if this plugin's options have changed
configChanged(options) when the plugin-level options change
onServiceReady() when the service has initialized but before the hydra.init Promise resolves

Hook return values

setHydra, setConfig, and onServiceReady can return a value or a Promise.

The actual return value isn't important; if the hook returns a value, success is assumed. If an error in plugin initialization should result in the service failing to start, the plugin hook should throw an Error.

Similarly if a Promise is returned and resolves, success is assumed; the resolve() value is ignored. Fatal errors should reject().

Quick Tutorial

Set up a plugin in five easy steps.

1. Set up a hydra service:

$ yo fwsp-hydra
? Name of the service (`-service` will be appended automatically) pingpong
? Host the service runs on?
? Port the service runs on? 0
? What does this service do? demo
? Does this service need auth? No
? Is this a hydra-express service? No
? Set up logging? No
? Run npm install? Yes

$ cd pingpong-service

2. Create pong-plugin.js:

Tip: On OS X, you can copy this snippet and then pbpaste > pong-plugin.js

// whenever a 'ping' event is emitted, a 'pong' event is emitted after a user-defined delay
const Promise = require('bluebird');
const HydraPlugin = require('hydra/plugin');

class PongPlugin extends HydraPlugin {
  constructor() {
    super('example'); // unique identifier for the plugin
  }
  
  // called at the beginning of hydra.init
  // the parent class will locate the plugin config and set this.opts
  // can return a Promise or a value
  // in this case, there's no return statement, so that value is undefined
  setConfig(hydraConfig) {
    super.setConfig(hydraConfig);
    this.configChanged(this.opts);
    this.hydra.on('ping', () => {
      Promise.delay(this.opts.pongDelay).then(() => {
        this.hydra.emit('pong');
      });
    })
  }
  
  // called when the config for this plugin has changed (via HydraEvent.CONFIG_UPDATE_EVENT)
  // if you need access to the full service config, override updateConfig(serviceConfig)
  configChanged(opts) {
    if (this.opts.pongDelay === "random") {
      this.opts.pongDelay = Math.floor(Math.random() * 3000);
      console.log(`Random delay = ${this.opts.pongDelay}`);
    }
  }
  
  // called after hydra has initialized but before hydra.init Promise resolves
  // can return a Promise or a value
  // this will delay by the port number in ms for demonstration of Promise
  onServiceReady() {
    console.log(`[example plugin] hydra service running on ${this.hydra.config.servicePort}`);
    console.log('[example plugin] delaying serviceReady...');
    return new Promise((resolve, reject) => {
      Promise.delay(this.hydra.config.servicePort)
        .then(() => {
          console.log('[example plugin] delayed serviceReady, pinging...');
          this.hydra.emit('ping');
          resolve();
        });
      });
  }
}

module.exports = PongPlugin;

3. Update hydra section of config.json to pass the plugin configuration:

{
  "hydra": {
    "plugins": {
      "example": {
        "pongDelay": 2000
      }
    }
  }
}

4. Set up hydra service entry-point script:

const version = require('./package.json').version;
const hydra = require('fwsp-hydra');

// install plugin
const PongPlugin = require('./pong-plugin');
hydra.use(new PongPlugin());

// add some console.logs so we can see the events happen
hydra.on('ping', () => console.log('PING!'));
hydra.on('pong', () => {
  console.log('PONG!');
  // send a ping back, after a random delay of up to 2s, to keep the rally going
  setTimeout(() => hydra.emit('ping'), Math.floor(Math.random() * 2000));
});


let config = require('fwsp-config');
config.init('./config/config.json')
  .then(() => {
    config.version = version;
      config.hydra.serviceVersion = version;
      hydra.init(config.hydra)
        .then(() => hydra.registerService())
        .then(serviceInfo => {

          console.log(serviceInfo); // so we see when the serviceInfo resolves

          let logEntry = `Starting ${config.hydra.serviceName} (v.${config.version})`;
          hydra.sendToHealthLog('info', logEntry);
        })
        .catch(err => {
          console.log('Error initializing hydra', err);
        });
  });

5. Try it out!

Run npm start. After an initial delay, you should start seeing PING!s and PONG!s.

6. Learn more from others:

You may want to also learn from the implementation of the following hydra plugins as a reference: