Microcosm is Flux with first-class actions and state sandboxing.
The source of truth in Microcosm is a historical record of actions. As they move through a set lifecycle, Microcosm reconciles actions in the order they were created. This makes optimistic updates, cancellation, and loading states much simpler. They self clean.
It also provides strong separation between actions and state. Microcosms can "fork" to accommodate specific use cases. Keep global state global, while easily supporting pagination, filtering, and other secondary data processing.
- Batteries included. Just install
microcosm
. Plugins and middleware should not be required to immediately be productive. - Easy to use. Less boilerplate. More expressive. The Microcosm interface is easy to use, easy to understand, and easy to maintain.
- Powerful: Microcosm's transactional state management, action statuses, and state sandboxing provide exceptional tools for building applications.
Comprehensive documentation can be found in the docs section of this repo.
If you'd rather look at working code, head over to the example apps or checkout out the quickstart guide.
npm install --save microcosm
Microcosm is an evolution of Flux that makes it easy to manage complicated async workflows and unique data modeling requirements of complicated UIs.
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:
function getPlanet (id) {
// Fetch returns a Promise, handled out of the box
return fetch('/planets/' + id).then(response => response.json())
}
let action = repo.push(getPlanet, 'venus')
action.onDone(function (planet) {
console.log(planet.id) // venus
})
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.
Microcosm makes it easy to handle pending, loading, cancelled, completed, and failed requests:
const PlanetsDomain = {
// ...handlers
register() {
return {
[getPlanet.open] : this.setPending,
[getPlanet.done] : this.addPlanet,
[getPlanet.error] : this.setError,
[getPlanet.loading] : this.setProgress,
[getPlanet.cancelled] : 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.
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.
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 {
[action.open] : this.setPending,
[action.error] : this.setError,
[action.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.
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.
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)
}
model () {
return {
page: state => state.users
}
}
view ({ page }) {
return <UsersTable users={page} />
}
}
const repo = new Microcosm()
repo.addDomain('users', UsersDomain)
ReactDOM.render(<PaginatedUsers repo={repo} page="1" />, el)
Good question! Other popular implementations of Flux treat actions as static events. The result of calling a dispatch method or resolving some sort of data structure like a Promise.
But what about everything before that point? A user might get tired of waiting for a file to upload, or dip into a subway tunnel and lose connectivity. They might want to retry an 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.
While manageable, we’ve found that this can be cumbersome. That it can lead to interface-specific requirements leaking into the data layer, resulting in complicated code, and unexpected bugs as requirements change.
Microcosm thinks of actions as stories. They go through different states as they move from start to completion. Actions have a common public interface, regardless of what data structures or asynchronous patterns are utilized. An interface that is easy to query from the presentation layer in order to handle use-case specific display requirements.
This makes it easier to handle complicated behaviors such as optimistic updates, dialog windows, or long running processes.
- 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.