Microcosm is a state management tool for React (and similar libraries). Keep track of user actions, cancel requests, and perform optimistic updates with ease.

Heads up

As of July 2019, Microcosm is no longer in active development. We will continue to maintain it for existing clients, and would be happy to accept contributions to continue support in the future.

It's been a great journey, thanks for taking it with us.

What you get

• A central place to track all application data
• Schedule work with actions
• Actions understand Promises out of the box and move through predefined states.
• Keep loading states out of the data layer. Track action progress using status callbacks.
• Split up application state in large apps while still sharing common data
• Painless optimistic updates
• Track changes and handle business logic with Presenter components
• 5.5kb gzipped (~18kb minified)

At a glance

import Microcosm, { get, set } from 'microcosm'
import axios from 'axios'

let repo = new Microcosm()

function getUser (id) {
  // This will return a promise. Microcosm automatically handles promises.
  // See http://code.viget.com/microcosm/api/actions.html
  return axios(`/users/${id}`)
}

// Domains define how a Microcosm should turn actions into new state
repo.addDomain('users', {
  getInitialState () {
    return {}
  },
  addUser (users, record) {
    // The set helper non-destructively assigns keys to an object
    return set(users, record.id, record)
  },
  register () {
    return {
      [getUser]: {
        done: this.addUser
      }
    }
  }
})

// Push an action, a request to perform some kind of work
let action = repo.push(getUser, 2)

action.onDone(function () {
  let user = get(repo.state, ['users', '2'])

  console.log(user) // { id: 2, name: "Bob" }
})

// You could also handle errors in a domain's register method
// by hooking into `getUser.error`
action.onError(function () {
  alert("Something went terribly wrong!")
})

Why?

Other Flux implementations treat actions as static events; the result of calling a dispatch method or resolving some sort of data structure like a Promise.

But what if a user gets tired of waiting for a file to upload, or switches pages before a GET request finishes? What if they dip into a subway tunnel and lose connectivity? They might want to retry a request, cancel it, or just see what’s happening.

The burden of this state often falls on data stores (Domains, in Microcosm) or a home-grown solution for tracking outstanding requests and binding them to related action data. Presentation layer requirements leak into the data layer, making it harder to write tests, reuse code, and accommodate unexpected changes.

How Microcosm is different

Microcosm actions are first-class citizens. An action can move from an open to error state if a request fails. Requests that are aborted may move into a cancelled state. As they change, actions resolve within a greater history of every other action.

This means that applications can make a lot of assumptions about user actions:

• Actions resolve in a consistent, predictable order
• Action types are automatically generated
• Actions maintain the same public API, no matter what asynchronous pattern is utilized (or not)

This reduces a lot of boilerplate, however it also makes it easier for the presentation layer to handle use-case specific display requirements, like displaying an error, performing an optimistic update, or tracking file upload progress.

Get started

npm install --save microcosm

Check out our quickstart guide.

Documentation

Comprehensive documentation can be found in the docs section of this repo.

Overview

Microcosm is an evolution of Flux that makes it easy to manage complicated async workflows and unique data modeling requirements of complicated UIs.

Actions take center stage

Microcosm organizes itself around a history of user actions. As those actions move through a set lifecycle, Microcosm reconciles them in the order they were created.

Invoking push() appends to that history, and returns an Action object to represent it:

// axios is an AJAX library
// https://github.com/mzabriskie/axios
import axios from 'axios'

function getPlanet (id) {
  // axios returns a Promise, handled out of the box
  return axios(`/planets/${id}`)
}

let action = repo.push(getPlanet, 'venus')

action.onDone(function (planet) {
  console.log(planet.id) // venus
})

Domains: Stateless Stores

A Domain is a collection of side-effect free operations for manipulating data. As actions update, Microcosm uses domains to determine how state should change. Old state comes in, new state comes out:

const PlanetsDomain = {
  getInitialState () {
    return []
  },

  addPlanet (planets, record) {
    return planets.concat(record)
  },

  register() {
    return {
      [getPlanet]: this.addPlanet
    }
  }
}

repo.addDomain('planets', PlanetsDomain)

By implementing a register method, domains can subscribe to actions. Each action is assigned a unique string identifier. Action type constants are generated automatically.

Pending, failed, and cancelled requests

Microcosm makes it easy to handle pending, loading, cancelled, completed, and failed requests:

const PlanetsDomain = {
  // ...handlers

  register() {
    return {
      [getPlanet] : {
        open   : this.setPending,
        update : this.setProgress,
        done   : this.addPlanet,
        error  : this.setError,
        cancel : this.setCancelled
      }
    }
  }
}

open, loading, done, error and cancelled are action states. In our action creator, we can unlock a deeper level of control by returning a function:

import request from 'superagent'

function getPlanet (id) {

  return function (action) {
    action.open(id)

    let request = request('/planets/' + id)

    request.end(function (error, response) {
      if (error) {
        action.reject(error)
      } else {
        action.resolve(response.body)
      }
    })

    // Cancellation!
    action.onCancel(request.abort)
  }
}

First, the action becomes open. This state is useful when waiting for something to happen, such as loading. When the request finishes, if it fails, we reject the action, otherwise we resolve it.

Microcosm actions are cancellable. Invoking action.cancel() triggers a cancellation event:

let action = repo.push(getPlanet, 'Pluto')

// Wait, Pluto isn't a planet!
action.cancel()

When action.cancel() is called, the action will move into a cancelled state. If a domain doesn't handle a given state no data operation will occur.

Visit the API documentation for actions to read more.

A historical account of everything that has happened

Whenever an action creator is pushed into a Microcosm, it creates an action to represent it. This gets placed into a tree of all actions that have occurred.

For performance, completed actions are archived and purged from memory, however passing the maxHistory option into Microcosm allows for a compelling debugging story, For example, the time-travelling Microcosm debugger:

let forever = new Microcosm({ maxHistory: Infinity })

Taken from the Chatbot example.

Optimistic updates

Microcosm will never clean up an action that precedes incomplete work When an action moves from open to done, or cancelled, the historical account of actions rolls back to the last state, rolling forward with the new action states. This makes optimistic updates simpler because action states are self cleaning:

import { send } from 'actions/chat'

const Messages = {
  getInitialState () {
    return []
  },

  setPending(messages, item) {
    return messages.concat({ ...item, pending: true })
  },

  setError(messages, item) {
    return messages.concat({ ...item, error: true })
  },

  addMessage(messages, item) {
    return messages.concat(item)
  }

  register () {
    return {
      [send]: {
        open: this.setPending,
        error: this.setError,
        done: this.addMessage
      }
    }
  }
}

In this example, as chat messages are sent, we optimistically update state with the pending message. At this point, the action is in an open state. The request has not finished.

On completion, when the action moves into error or done, Microcosm recalculates state starting from the point prior to the open state update. The message stops being in a loading state because, as far as Microcosm is now concerned, it never occured.

Forks: Global state, local concerns

Global state management reduces the complexity of change propagation tremendously. However it can make application features such as pagination, sorting, and filtering cumbersome.

How do we maintain the current page we are on while keeping in sync with the total pool of known records?

To accommodate this use case, there is Microcosm::fork:

const UsersDomain = {
  getInitialState() {
    return []
  },
  addUsers(users, next) {
    return users.concat(next)
  },
  register() {
    return {
      [getUsers]: this.addUsers
    }
  }
})

const PaginatedUsersDomain {
  getInitialState() {
    return []
  },
  addUsers(users, next) {
    let page = next.map(user => user.id)

    // Reduce the user list down to only what was included
    // in the current request
    return users.filter(user => page.contains(user.id))
  },
  register() {
    return {
      [getUsers]: this.addUsers
    }
  }
})

let roster = new Microcosm()
let pagination = parent.fork()

roster.addDomain('users', UsersDomain)
pagination.addDomain('users', PaginatedUsersDomain)

// Forks share the same history, so you could also do
// `pagination.push(getUsers, ...)`
roster.push(getUsers, { page: 1 }) // 10 users
roster.push(getUsers, { page: 2 }) // 10 users

// when it finishes...
console.log(roster.state.users.length) // 20
console.log(pagination.state.users.length) // 10

fork returns a new Microcosm, however it shares the same action history. Additionally, it inherits state updates from its parent. In this example, we've added special version of the roster repo that only keeps track of the current page.

As getUsers() is called, the roster will add the new users to the total pool of records. Forks dispatch sequentially, so the child pagination repo is able to filter the data set down to only what it needs.

Networks of Microcosms with Presenters

Fork is an important component of the Presenter addon. Presenter is a special React component that can build a view model around a given Microcosm state, sending it to child "passive view" components.

All Microcosms sent into a Presenter are forked, granting them a sandbox for data operations specific to a particular part of an application:

class PaginatedUsers extends Presenter {
  setup (repo, { page }) {
    repo.add('users', PaginatedUsersDomain)

    repo.push(getUsers, page)
  }

  getModel () {
    return {
      page: state => state.users
    }
  }

  render () {
    const { page } = this.model

    return <UsersTable users={page} />
  }
}

const repo = new Microcosm()
repo.addDomain('users', UsersDomain)

ReactDOM.render(<PaginatedUsers repo={repo} page="1" />, el)

Inspiration

• Worlds
• Om
• Elm Language
• Flummox
• But the world is mutable
• Event Sourcing Pattern
• Apache Kafka
• LMAX Architecture
• Redux (Provider/Connect and thunks)
• Retroactive Data Structures

---

Visit code.viget.com to see more projects from Viget.