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Node.js Design Patterns

Implementing a Basic Failsafe Socket

Explore how to implement a basic failsafe TCP socket in Node.js using the State pattern. This lesson guides you through creating a client that queues data while offline and resends it once the server reconnects, improving reliability in networked applications.

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Let’s now work on a more concrete example so that we can apply what we’ve learned about the State pattern.

Let’s build a TCP client socket that doesn’t fail when the connection with the server is lost; instead, we want to queue all the data sent during the time in which the server is offline and then try to send it again as soon as the connection is reestablished. We want to leverage this socket in the context of a simple monitoring system, where a set of machines sends some statistics about their resource utilization at regular intervals. If the server that collects these resources goes down, our socket will continue to queue the data locally until the server comes back online.

Let’s start by creating a new module called failsafeSocket.js that defines our context object.

Node.js
import { OfflineState } from './offlineState.js'
import { OnlineState } from './onlineState.js'
export class FailsafeSocket {
  constructor (options) {                         // (1)
    this.options = options
    this.queue = []
    this.currentState = null
    this.socket = null
    this.states = {
      offline: new OfflineState(this),
      online: new OnlineState(this)
    }
    this.changeState('offline')
  }
  changeState (state) {                           // (2)
    console.log(`Activating state: ${state}`)
    this.currentState = this.states[state]
    this.currentState.activate()
  }
  send (data) {
    this.currentState.send(data)                  // (3)
  }
}

The FailsafeSocket class is made of three main elements:

  1. The constructor initializes various data structures, ...