rest subscriber

README.md

Automatically retries until all three checks are solved

It starts out with:

const initialState = {
  checks: {
    one: undefined,
    two: undefined,
    three: undefined
  }
}

…and the idea is to get to:

const initialState = {
  checks: {
    one: Symbol.for('solved'),
    two: Symbol.for('solved'),
    three: Symbol.for('solved')
  }
}

In order to do that, three actions are dispatched when starting up the app, setting each of the marks in status Symbol.for('pending')

If a fetch call fails, the check will be set to Symbol.for('failed') via the FAILED action. Since the state is updated, the fetch effect will be called again. The fetch effect will find the updated check (from pending to failed) and trigger fetch again

client.js

const initialState = {
  checks: {
    one: undefined,
    two: undefined,
    three: undefined
  }
}

const reducer = (state, action) => {
  switch (action.type) {
    case 'LOAD':
      return {
        ...state,
        checks: {
          ...state.checks,
          [action.payload.checkName]: Symbol.for('pending')
        }
      }
    
    case 'FAILED':
      return {
        ...state,
        checks: {
          ...state.checks,
          [action.payload.checkName]: Symbol.for('failed')
        }
      }

    case 'SOLVED':
      return {
        ...state,
        checks: {
          ...state.checks,
          [action.payload.checkName]: Symbol.for('solved')
        }
      }
  }
}

const fetchEffect = (prevState, cachedNext) => (state, next) => {
  cachedNext = next
  Object.keys(state.checked)
    .filter((key) => 
      state.checked[key] !== prevState.checked[key] && 
      (
        state.checked[key] === Symbol.for('pending') ||
        state.checked[key] === Symbol.for('failed')
      )
    )
    .forEach((key) => fetch(`/${key}`)
      .then(() => cachedNext({ type: 'SOLVED', payload: { checkName: key } }))
      .catch(() => cachedNext({ type: 'FAILED', payload: { checkName: key } }))
    )
}

const app = (reducer, initialState, actions, effects) => {
  const newState = actions.reduce(reducer, initialState)
  
  const next = (action) => app(reducer, newState, [action], effects)
  
  effects.forEach((effect) => effect(newState, next))
  
  return app
}

app(
  reducer, 
  initialState, 
  [
    { type: 'LOAD', payload: { checkName: 'one' } },
    { type: 'LOAD', payload: { checkName: 'two' } },
    { type: 'LOAD', payload: { checkName: 'three' } }
  ],
  [
    fetchEffect(initialState, () => {}),
    (state) => console.log(state)
  ]
)

server.js

const express = require('express')

const app = express()

const {random} = Math

app.get('*', (_, res) => {
  random() < 0.2
    ? res.sendStatus(404)
    : res.sendStatus(200)
})