backend-pool

A module to keep an updated list of healthy backends. It ensures background healthchecking according to the defined options and support great customization.

How to use

const BackendPool = require('backend-pool');

const pool = new BackendPool({
  healthcheck:   '/health',     // default healthcheck path
  healthyAfter:      3,         // successful responses
  unhealthyAfter:    2,         // unsuccessful response
  removeAfter:      10,         // unsuccessful reponses
  checkInterval:  5000,         // ms
  checkTimeout:   1000,         // ms
  isHealthy:      (res, cb) => { cb(true|false) },
});

pool.add('http://whatever.net');
pool.add({
  address: 'http://xpto.etc',
  unhealthyAfter: 1,
  checkInterval: 20000
});

setInterval(() => console.log(pool.getHealthyAddresses()), 10000);

## Supported options

All options can be passed to the constructor as defaults and can also be passed to the .add({...}) method if they are backend-specific settings.

• healthcheckPath: The healthcheck path to be used. Mandatory to be defined at construction time or as .add() option;
• healthyAfter: Number of consecutive successful responses to consider a backend as 'healthy'; (default: 3);
• unhealthyAfter: Number of consecutive unsuccessful responses to consider a backend as 'unhealthy'; (default: 1);
• removeAfter: Number of consecutive unsuccessful responses to remove a backend from the pool; A value of undefined or null means backends are never removed; (default: undefined);
• checkInterval: The time (in ms) between health checks; (default: 10000);
• checkTimeout: The maximum time to wait for a healthcheck endpoint to answer; (default: 1000);
• isHealthy: A function which decides if a response represents a healthy state; The function arguments are (httpResponse, callback); If you need the response data, you'll need to read it from the stream; (default: 200 status code).

Events

Both the backend pool and the individual backend objects (returned by .add() and .get*()) emit events.

• healthy: Notifies when a backend was considered healthy;
• unhealthy: Notifies when a backend was considered unhealthy;
• removed: Notified when a backend is going to be removed.

Methods

A backend pool object supports the following methods:

• get(address): Returns a backend object for a backend with a specific address or null in case a backend isn't found;
• getHealthy(): Returns a list of backend objects for the backends in healthy state;
• getByState(state): Returns a list of backend objects for the backends in a specific state;
• getHealthyAddresses(): Returns a list of backend addresses for the backends in healthy state;
• add(address|options): Adds a backend to the pool (in NEW state) by its address (URL) or object containing all the backend-specific options (and an address extra options)`; Supports all options defined defined above for the constructor;
• remove(address|backendObject): Removes a backend from the pool;