Circuit.js

'use strict';

const EventEmitter = require('events').EventEmitter;
const Promise = require('bluebird');
const promisifyIfFunction = require('./utils').promisifyIfFunction;
const TimeOutError = require('./TimeOutError');
const CircuitBrokenError = require('../lib/CircuitBrokenError');
const consts = require('./consts');

const defaultOptions = {
  isFailure: () => true
};

/**
 * Class that can sit on top of a Brakes. It's basically just a pair of primary and fallback Promises you can put on
 * top of a Brake that monitors a common Service (eg: ).
 */
class Circuit extends EventEmitter {
  constructor(brakes, main, fallback, options) {
    super();

    if (!(brakes instanceof EventEmitter)) {
      throw new Error(consts.NO_BRAKES);
    }
    this._brakes = brakes;

    if (!main || typeof main !== 'function') {
      throw new Error(consts.NO_FUNCTION);
    }
    else if (fallback) {
      if (typeof fallback !== 'function') {
        if (options) {
          throw new Error(consts.NO_FUNCTION);
        }
        options = fallback;
        fallback = undefined;
      }
    }
    this._opts = Object.assign({}, defaultOptions, options);
    this._this = this._opts.this || this;

    this._serviceCall = promisifyIfFunction(main, this._opts.isPromise, this._opts.isFunction);

    if (fallback) {
      this.fallback(fallback, this._opts.isPromise, this._opts.isFunction);
    }
  }

  exec() {
    this._brakes.emit('exec');

    // Save circuit generation to scope so we can compare it
    // to the current generation when a request fails.
    // This prevents failures from bleeding between circuit generations.
    const execGeneration = this._brakes._circuitGeneration;

    if (this._brakes._circuitOpen) {
      this._brakes._stats.shortCircuit();
      if (this._fallback) {
        return this._fallback.apply(this, arguments);
      }
      else if (this._brakes._fallback) {
        return this._brakes._fallback.apply(this, arguments);
      }
      return Promise.reject(new CircuitBrokenError(this._brakes.name, this._brakes._stats._totals, this._brakes._opts.threshold));
    }

    const startTime = Date.now();

    // we use _execPromise() wrapper on the service call promise
    // to allow us to more easily hook in stats reporting
    return this._execPromise
      .apply(this, arguments)
      .tap(() => this._brakes.emit('success', Date.now() - startTime))
      .catch(err => {
        const endTime = Date.now() - startTime;

        // trigger hook listeners
        if (err instanceof TimeOutError) {
          this._brakes.emit('timeout', endTime, err, execGeneration);
        }
        else if (this._opts.isFailure(err)) {
          this._brakes.emit('failure', endTime, err, execGeneration);
        }
        // if fallback exists, call it upon failure
        // there are no listeners or stats collection for
        // the fallback function. The function is fire-and-forget
        // as far as `Brakes` is concerned
        if (this._fallback) {
          return this._fallback.apply(this, arguments);
        }
        else if (this._brakes._fallback) {
          return this._brakes._fallback.apply(this, arguments);
        }

        if (err && err.message && this._brakes.name && this._brakes._opts.modifyError) {
          err.message = `[Breaker: ${this._brakes.name}] ${err.message}`;
        }

        return Promise.reject(err);
      });
  }

  /*
   Execute main service call
   */
  _execPromise() {
    return new Promise((resolve, reject) => {
      // start timeout timer
      const timeoutTimer = setTimeout(() => {
        reject(new TimeOutError(consts.TIMEOUT));
      }, this._opts.timeout || this._brakes._opts.timeout);

      this._serviceCall.apply(this._this, arguments).then(result => {
        clearTimeout(timeoutTimer);
        resolve(result);
      }).catch(err => {
        clearTimeout(timeoutTimer);
        reject(err);
      });

      timeoutTimer.unref();
    });
  }

  fallback(func, isPromise, isFunction) {
    this._fallback = promisifyIfFunction(func, isPromise, isFunction);
    return this._fallback;
  }
}

module.exports = Circuit;