[javascript] Why do we need middleware for async flow in Redux?

According to the docs, "Without middleware, Redux store only supports synchronous data flow". I don't understand why this is the case. Why can't the container component call the async API, and then dispatch the actions?

For example, imagine a simple UI: a field and a button. When user pushes the button, the field gets populated with data from a remote server.

import * as React from 'react';
import * as Redux from 'redux';
import { Provider, connect } from 'react-redux';

const ActionTypes = {
    STARTED_UPDATING: 'STARTED_UPDATING',
    UPDATED: 'UPDATED'
};

class AsyncApi {
    static getFieldValue() {
        const promise = new Promise((resolve) => {
            setTimeout(() => {
                resolve(Math.floor(Math.random() * 100));
            }, 1000);
        });
        return promise;
    }
}

class App extends React.Component {
    render() {
        return (
            <div>
                <input value={this.props.field}/>
                <button disabled={this.props.isWaiting} onClick={this.props.update}>Fetch</button>
                {this.props.isWaiting && <div>Waiting...</div>}
            </div>
        );
    }
}
App.propTypes = {
    dispatch: React.PropTypes.func,
    field: React.PropTypes.any,
    isWaiting: React.PropTypes.bool
};

const reducer = (state = { field: 'No data', isWaiting: false }, action) => {
    switch (action.type) {
        case ActionTypes.STARTED_UPDATING:
            return { ...state, isWaiting: true };
        case ActionTypes.UPDATED:
            return { ...state, isWaiting: false, field: action.payload };
        default:
            return state;
    }
};
const store = Redux.createStore(reducer);
const ConnectedApp = connect(
    (state) => {
        return { ...state };
    },
    (dispatch) => {
        return {
            update: () => {
                dispatch({
                    type: ActionTypes.STARTED_UPDATING
                });
                AsyncApi.getFieldValue()
                    .then(result => dispatch({
                        type: ActionTypes.UPDATED,
                        payload: result
                    }));
            }
        };
    })(App);
export default class extends React.Component {
    render() {
        return <Provider store={store}><ConnectedApp/></Provider>;
    }
}

When the exported component is rendered, I can click the button and the input is updated correctly.

Note the update function in the connect call. It dispatches an action that tells the App that it is updating, and then performs an async call. After the call finishes, the provided value is dispatched as a payload of another action.

What is wrong with this approach? Why would I want to use Redux Thunk or Redux Promise, as the documentation suggests?

EDIT: I searched the Redux repo for clues, and found that Action Creators were required to be pure functions in the past. For example, here's a user trying to provide a better explanation for async data flow:

The action creator itself is still a pure function, but the thunk function it returns doesn't need to be, and it can do our async calls

Action creators are no longer required to be pure. So, thunk/promise middleware was definitely required in the past, but it seems that this is no longer the case?

When working in an enterprise project, there are many requirements available in middle-ware such as (saga) not available in simple asynchronous flow, below are some:

• Running request in parallel
• Pulling future actions without the need to wait
• Non-blocking calls Race effect, example pickup first
• response to initiate the process Sequencing your tasks (first in first call)
• Composing
• Task cancellation Dynamically forking the task.
• Support Concurrency Running Saga outside the redux middleware.
• Using channels

The list is long just review the advanced section in saga documentation

To use Redux-saga is the best middleware in React-redux implementation.

Ex: store.js

  import createSagaMiddleware from 'redux-saga';
  import { createStore, applyMiddleware } from 'redux';
  import allReducer from '../reducer/allReducer';
  import rootSaga from '../saga';

  const sagaMiddleware = createSagaMiddleware();
  const store = createStore(
     allReducer,
     applyMiddleware(sagaMiddleware)
   )

   sagaMiddleware.run(rootSaga);

 export default store;

And then saga.js

import {takeLatest,delay} from 'redux-saga';
import {call, put, take, select} from 'redux-saga/effects';
import { push } from 'react-router-redux';
import data from './data.json';

export function* updateLesson(){
   try{
       yield put({type:'INITIAL_DATA',payload:data}) // initial data from json
       yield* takeLatest('UPDATE_DETAIL',updateDetail) // listen to your action.js 
   }
   catch(e){
      console.log("error",e)
     }
  }

export function* updateDetail(action) {
  try{
       //To write store update details
   }  
    catch(e){
       console.log("error",e)
    } 
 }

export default function* rootSaga(){
    yield [
        updateLesson()
       ]
    }

And then action.js

 export default function updateFruit(props,fruit) {
    return (
       {
         type:"UPDATE_DETAIL",
         payload:fruit,
         props:props
       }
     )
  }

And then reducer.js

import {combineReducers} from 'redux';

const fetchInitialData = (state=[],action) => {
    switch(action.type){
      case "INITIAL_DATA":
          return ({type:action.type, payload:action.payload});
          break;
      }
     return state;
  }
 const updateDetailsData = (state=[],action) => {
    switch(action.type){
      case "INITIAL_DATA":
          return ({type:action.type, payload:action.payload});
          break;
      }
     return state;
  }
const allReducers =combineReducers({
   data:fetchInitialData,
   updateDetailsData
 })
export default allReducers; 

And then main.js

import React from 'react';
import ReactDOM from 'react-dom';
import App from './app/components/App.jsx';
import {Provider} from 'react-redux';
import store from './app/store';
import createRoutes from './app/routes';

const initialState = {};
const store = configureStore(initialState, browserHistory);

ReactDOM.render(
       <Provider store={store}>
          <App />  /*is your Component*/
       </Provider>, 
document.getElementById('app'));

try this.. is working

The short answer: seems like a totally reasonable approach to the asynchrony problem to me. With a couple caveats.

I had a very similar line of thought when working on a new project we just started at my job. I was a big fan of vanilla Redux's elegant system for updating the store and rerendering components in a way that stays out of the guts of a React component tree. It seemed weird to me to hook into that elegant dispatch mechanism to handle asynchrony.

I ended up going with a really similar approach to what you have there in a library I factored out of our project, which we called react-redux-controller.

I ended up not going with the exact approach you have above for a couple reasons:

1. The way you have it written, those dispatching functions don't have access to the store. You can somewhat get around that by having your UI components pass in all of the info the dispatching function needs. But I'd argue that this couples those UI components to the dispatching logic unnecessarily. And more problematically, there's no obvious way for the dispatching function to access updated state in async continuations.
2. The dispatching functions have access to dispatch itself via lexical scope. This limits the options for refactoring once that connect statement gets out of hand -- and it's looking pretty unwieldy with just that one update method. So you need some system for letting you compose those dispatcher functions if you break them up into separate modules.

Take together, you have to rig up some system to allow dispatch and the store to be injected into your dispatching functions, along with the parameters of the event. I know of three reasonable approaches to this dependency injection:

• redux-thunk does this in a functional way, by passing them into your thunks (making them not exactly thunks at all, by dome definitions). I haven't worked with the other dispatch middleware approaches, but I assume they're basically the same.
• react-redux-controller does this with a coroutine. As a bonus, it also gives you access to the "selectors", which are the functions you may have passed in as the first argument to connect, rather than having to work directly with the raw, normalized store.
• You could also do it the object-oriented way by injecting them into the this context, through a variety of possible mechanisms.

Update

It occurs to me that part of this conundrum is a limitation of react-redux. The first argument to connect gets a state snapshot, but not dispatch. The second argument gets dispatch but not the state. Neither argument gets a thunk that closes over the current state, for being able to see updated state at the time of a continuation/callback.

OK, let's start to see how middleware working first, that quite answer the question, this is the source code applyMiddleWare function in Redux:

function applyMiddleware() {
  for (var _len = arguments.length, middlewares = Array(_len), _key = 0; _key < _len; _key++) {
    middlewares[_key] = arguments[_key];
  }

  return function (createStore) {
    return function (reducer, preloadedState, enhancer) {
      var store = createStore(reducer, preloadedState, enhancer);
      var _dispatch = store.dispatch;
      var chain = [];

      var middlewareAPI = {
        getState: store.getState,
        dispatch: function dispatch(action) {
          return _dispatch(action);
        }
      };
      chain = middlewares.map(function (middleware) {
        return middleware(middlewareAPI);
      });
      _dispatch = compose.apply(undefined, chain)(store.dispatch);

      return _extends({}, store, {
        dispatch: _dispatch
      });
    };
  };
}

Look at this part, see how our dispatch become a function.

  ...
  getState: store.getState,
  dispatch: function dispatch(action) {
  return _dispatch(action);
}

• Note that each middleware will be given the dispatch and getState functions as named arguments.

OK, this is how Redux-thunk as one of the most used middlewares for Redux introduce itself:

Redux Thunk middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods dispatch and getState as parameters.

So as you see, it will return a function instead an action, means you can wait and call it anytime you want as it's a function...

So what the heck is thunk? That's how it's introduced in Wikipedia:

In computer programming, a thunk is a subroutine used to inject an additional calculation into another subroutine. Thunks are primarily used to delay a calculation until it is needed, or to insert operations at the beginning or end of the other subroutine. They have a variety of other applications to compiler code generation and in modular programming.

The term originated as a jocular derivative of "think".

A thunk is a function that wraps an expression to delay its evaluation.

//calculation of 1 + 2 is immediate 
//x === 3 
let x = 1 + 2;

//calculation of 1 + 2 is delayed 
//foo can be called later to perform the calculation 
//foo is a thunk! 
let foo = () => 1 + 2;

So see how easy the concept is and how it can help you manage your async actions...

That's something you can live without it, but remember in programming there are always better, neater and proper ways to do things...

There are synchronous action creators and then there are asynchronous action creators.

A synchronous action creator is one that when we call it, it immediately returns an Action object with all the relevant data attached to that object and its ready to be processed by our reducers.

Asynchronous action creators is one in which it will require a little bit of time before it is ready to eventually dispatch an action.

By definition, anytime you have an action creator that makes a network request, it is always going to qualify as an async action creator.

If you want to have asynchronous action creators inside of a Redux application you have to install something called a middleware that is going to allow you to deal with those asynchronous action creators.

You can verify this in the error message that tells us use custom middleware for async actions.

So what is a middleware and why do we need it for async flow in Redux?

In the context of redux middleware such as redux-thunk, a middleware helps us deal with asynchronous action creators as that is something that Redux cannot handle out of the box.

With a middleware integrated into the Redux cycle, we are still calling action creators, that is going to return an action that will be dispatched but now when we dispatch an action, rather than sending it directly off to all of our reducers, we are going to say that an action will be sent through all the different middleware inside the application.

Inside of a single Redux app, we can have as many or as few middleware as we want. For the most part, in the projects we work on we will have one or two middleware hooked up to our Redux store.

A middleware is a plain JavaScript function that will be called with every single action that we dispatch. Inside of that function a middleware has the opportunity to stop an action from being dispatched to any of the reducers, it can modify an action or just mess around with an action in any way you which for example, we could create a middleware that console logs every action you dispatch just for your viewing pleasure.

There are a tremendous number of open source middleware you can install as dependencies into your project.

You are not limited to only making use of open source middleware or installing them as dependencies. You can write your own custom middleware and use it inside of your Redux store.

One of the more popular uses of middleware (and getting to your answer) is for dealing with asynchronous action creators, probably the most popular middleware out there is redux-thunk and it is about helping you deal with asynchronous action creators.

There are many other types of middleware that also help you in dealing with asynchronous action creators.

You don't.

But... you should use redux-saga :)

Dan Abramov's answer is right about redux-thunk but I will talk a bit more about redux-saga that is quite similar but more powerful.

Imperative VS declarative

• DOM: jQuery is imperative / React is declarative
• Monads: IO is imperative / Free is declarative
• Redux effects: redux-thunk is imperative / redux-saga is declarative

When you have a thunk in your hands, like an IO monad or a promise, you can't easily know what it will do once you execute. The only way to test a thunk is to execute it, and mock the dispatcher (or the whole outside world if it interacts with more stuff...).

If you are using mocks, then you are not doing functional programming.

Seen through the lens of side-effects, mocks are a flag that your code is impure, and in the functional programmer's eye, proof that something is wrong. Instead of downloading a library to help us check the iceberg is intact, we should be sailing around it. A hardcore TDD/Java guy once asked me how you do mocking in Clojure. The answer is, we usually don't. We usually see it as a sign we need to refactor our code.

Source

The sagas (as they got implemented in redux-saga) are declarative and like the Free monad or React components, they are much easier to test without any mock.

See also this article:

in modern FP, we shouldn’t write programs — we should write descriptions of programs, which we can then introspect, transform, and interpret at will.

(Actually, Redux-saga is like a hybrid: the flow is imperative but the effects are declarative)

Confusion: actions/events/commands...

There is a lot of confusion in the frontend world on how some backend concepts like CQRS / EventSourcing and Flux / Redux may be related, mostly because in Flux we use the term "action" which can sometimes represent both imperative code (LOAD_USER) and events (USER_LOADED). I believe that like event-sourcing, you should only dispatch events.

Using sagas in practice

Imagine an app with a link to a user profile. The idiomatic way to handle this with each middleware would be:

redux-thunk

<div onClick={e => dispatch(actions.loadUserProfile(123)}>Robert</div>

function loadUserProfile(userId) {
  return dispatch => fetch(`http://data.com/${userId}`)
    .then(res => res.json())
    .then(
      data => dispatch({ type: 'USER_PROFILE_LOADED', data }),
      err => dispatch({ type: 'USER_PROFILE_LOAD_FAILED', err })
    );
}

redux-saga

<div onClick={e => dispatch({ type: 'USER_NAME_CLICKED', payload: 123 })}>Robert</div>


function* loadUserProfileOnNameClick() {
  yield* takeLatest("USER_NAME_CLICKED", fetchUser);
}

function* fetchUser(action) {
  try {
    const userProfile = yield fetch(`http://data.com/${action.payload.userId }`)
    yield put({ type: 'USER_PROFILE_LOADED', userProfile })
  } 
  catch(err) {
    yield put({ type: 'USER_PROFILE_LOAD_FAILED', err })
  }
}

This saga translates to:

every time a username gets clicked, fetch the user profile and then dispatch an event with the loaded profile.

As you can see, there are some advantages of redux-saga.

The usage of takeLatest permits to express that you are only interested to get the data of the last username clicked (handle concurrency problems in case the user click very fast on a lot of usernames). This kind of stuff is hard with thunks. You could have used takeEvery if you don't want this behavior.

You keep action creators pure. Note it's still useful to keep actionCreators (in sagas put and components dispatch), as it might help you to add action validation (assertions/flow/typescript) in the future.

Your code becomes much more testable as the effects are declarative

You don't need anymore to trigger rpc-like calls like actions.loadUser(). Your UI just needs to dispatch what HAS HAPPENED. We only fire events (always in the past tense!) and not actions anymore. This means that you can create decoupled "ducks" or Bounded Contexts and that the saga can act as the coupling point between these modular components.

This means that your views are more easy to manage because they don't need anymore to contain that translation layer between what has happened and what should happen as an effect

For example imagine an infinite scroll view. CONTAINER_SCROLLED can lead to NEXT_PAGE_LOADED, but is it really the responsibility of the scrollable container to decide whether or not we should load another page? Then he has to be aware of more complicated stuff like whether or not the last page was loaded successfully or if there is already a page that tries to load, or if there is no more items left to load? I don't think so: for maximum reusability the scrollable container should just describe that it has been scrolled. The loading of a page is a "business effect" of that scroll

Some might argue that generators can inherently hide state outside of redux store with local variables, but if you start to orchestrate complex things inside thunks by starting timers etc you would have the same problem anyway. And there's a select effect that now permits to get some state from your Redux store.

Sagas can be time-traveled and also enables complex flow logging and dev-tools that are currently being worked on. Here is some simple async flow logging that is already implemented:

Decoupling

Sagas are not only replacing redux thunks. They come from backend / distributed systems / event-sourcing.

It is a very common misconception that sagas are just here to replace your redux thunks with better testability. Actually this is just an implementation detail of redux-saga. Using declarative effects is better than thunks for testability, but the saga pattern can be implemented on top of imperative or declarative code.

In the first place, the saga is a piece of software that permits to coordinate long running transactions (eventual consistency), and transactions across different bounded contexts (domain driven design jargon).

To simplify this for frontend world, imagine there is widget1 and widget2. When some button on widget1 is clicked, then it should have an effect on widget2. Instead of coupling the 2 widgets together (ie widget1 dispatch an action that targets widget2), widget1 only dispatch that its button was clicked. Then the saga listen for this button click and then update widget2 by dispaching a new event that widget2 is aware of.

This adds a level of indirection that is unnecessary for simple apps, but make it more easy to scale complex applications. You can now publish widget1 and widget2 to different npm repositories so that they never have to know about each others, without having them to share a global registry of actions. The 2 widgets are now bounded contexts that can live separately. They do not need each others to be consistent and can be reused in other apps as well. The saga is the coupling point between the two widgets that coordinate them in a meaningful way for your business.

Some nice articles on how to structure your Redux app, on which you can use Redux-saga for decoupling reasons:

• http://jaysoo.ca/2016/02/28/organizing-redux-application/
• http://marmelab.com/blog/2015/12/17/react-directory-structure.html
• https://github.com/slorber/scalable-frontend-with-elm-or-redux

A concrete usecase: notification system

I want my components to be able to trigger the display of in-app notifications. But I don't want my components to be highly coupled to the notification system that has its own business rules (max 3 notifications displayed at the same time, notification queueing, 4 seconds display-time etc...).

I don't want my JSX components to decide when a notification will show/hide. I just give it the ability to request a notification, and leave the complex rules inside the saga. This kind of stuff is quite hard to implement with thunks or promises.

I've described here how this can be done with saga

Why is it called a Saga?

The term saga comes from the backend world. I initially introduced Yassine (the author of Redux-saga) to that term in a long discussion.

Initially, that term was introduced with a paper, the saga pattern was supposed to be used to handle eventual consistency in distributed transactions, but its usage has been extended to a broader definition by backend developers so that it now also covers the "process manager" pattern (somehow the original saga pattern is a specialized form of process manager).

Today, the term "saga" is confusing as it can describe 2 different things. As it is used in redux-saga, it does not describe a way to handle distributed transactions but rather a way to coordinate actions in your app. redux-saga could also have been called redux-process-manager.

See also:

• Interview of Yassine about Redux-saga history
• Kella Byte: Claryfing the Saga pattern
• Microsoft CQRS Journey: A Saga on Sagas
• Medium response of Yassine

Alternatives

If you don't like the idea of using generators but you are interested by the saga pattern and its decoupling properties, you can also achieve the same with redux-observable which uses the name epic to describe the exact same pattern, but with RxJS. If you're already familiar with Rx, you'll feel right at home.

const loadUserProfileOnNameClickEpic = action$ =>
  action$.ofType('USER_NAME_CLICKED')
    .switchMap(action =>
      Observable.ajax(`http://data.com/${action.payload.userId}`)
        .map(userProfile => ({
          type: 'USER_PROFILE_LOADED',
          userProfile
        }))
        .catch(err => Observable.of({
          type: 'USER_PROFILE_LOAD_FAILED',
          err
        }))
    );

Some redux-saga useful resources

• Redux-saga vs Redux-thunk with async/await
• Managing processes in Redux Saga
• From actionsCreators to Sagas
• Snake game implemented with Redux-saga

2017 advises

• Don't overuse Redux-saga just for the sake of using it. Testable API calls only are not worth it.
• Don't remove thunks from your project for most simple cases.
• Don't hesitate to dispatch thunks in yield put(someActionThunk) if it makes sense.

If you are frightened of using Redux-saga (or Redux-observable) but just need the decoupling pattern, check redux-dispatch-subscribe: it permits to listen to dispatches and trigger new dispatches in listener.

const unsubscribe = store.addDispatchListener(action => {
  if (action.type === 'ping') {
    store.dispatch({ type: 'pong' });
  }
});

Abramov's goal - and everyone's ideally - is simply to encapsulate complexity (and async calls) in the place where it's most appropriate.

Where's the best place to do that in the standard Redux dataflow? How about:

• Reducers? No way. They should be pure functions with no side-effects. Updating the store is serious, complicated business. Don't contaminate it.
• Dumb View Components? Definitely No. They have one concern: presentation and user-interaction, and should be as simple as possible.
• Container Components? Possible, but sub-optimal. It makes sense in that the container is a place where we encapsulate some view related complexity and interact with the store, but:
  • Containers do need to be more complex than dumb components, but it's still a single responsibility: providing bindings between view and state/store. Your async logic is a whole separate concern from that.
  • By placing it in a container, you'd be locking your async logic into a single context, for a single view/route. Bad idea. Ideally it's all reusable, and totally decoupled.
• Some other Service Module? Bad idea: you'd need to inject access to the store, which is a maintainability/testability nightmare. Better to go with the grain of Redux and access the store only using the APIs/models provided.
• Actions and the Middlewares that interpret them? Why not?! For starters, it's the only major option we have left. :-) More logically, the action system is decoupled execution logic that you can use from anywhere. It's got access to the store and can dispatch more actions. It has a single responsibility which is to organize the flow of control and data around the application, and most async fits right into that.
  • What about the Action Creators? Why not just do async in there, instead of in the actions themselves, and in Middleware?
    • First and most important, the creators don't have access to the store, as middleware does. That means you can't dispatch new contingent actions, can't read from the store to compose your async, etc.
    • So, keep complexity in a place that's complex of necessity, and keep everything else simple. The creators can then be simple, relatively pure functions that are easy to test.

To Answer the question:

Why can't the container component call the async API, and then dispatch the actions?

I would say for at least two reasons:

The first reason is the separation of concerns, it's not the job of the action creator to call the api and get data back, you have to have to pass two argument to your action creator function, the action type and a payload.

The second reason is because the redux store is waiting for a plain object with mandatory action type and optionally a payload (but here you have to pass the payload too).

The action creator should be a plain object like below:

function addTodo(text) {
  return {
    type: ADD_TODO,
    text
  }
}

And the job of Redux-Thunk midleware to dispache the result of your api call to the appropriate action.

Redux can't return a function instead of an action. It's just a fact. That's why people use Thunk. Read these 14 lines of code to see how it allows the async cycle to work with some added function layering:

function createThunkMiddleware(extraArgument) {
  return ({ dispatch, getState }) => (next) => (action) => {
    if (typeof action === 'function') {
      return action(dispatch, getState, extraArgument);
    }

    return next(action);
  };
}

const thunk = createThunkMiddleware();
thunk.withExtraArgument = createThunkMiddleware;

export default thunk;

https://github.com/reduxjs/redux-thunk

To answer the question that is asked in the beginning:

Why can't the container component call the async API, and then dispatch the actions?

Keep in mind that those docs are for Redux, not Redux plus React. Redux stores hooked up to React components can do exactly what you say, but a Plain Jane Redux store with no middleware doesn't accept arguments to dispatch except plain ol' objects.

Without middleware you could of course still do

const store = createStore(reducer);
MyAPI.doThing().then(resp => store.dispatch(...));

But it's a similar case where the asynchrony is wrapped around Redux rather than handled by Redux. So, middleware allows for asynchrony by modifying what can be passed directly to dispatch.

That said, the spirit of your suggestion is, I think, valid. There are certainly other ways you could handle asynchrony in a Redux + React application.

One benefit of using middleware is that you can continue to use action creators as normal without worrying about exactly how they're hooked up. For example, using redux-thunk, the code you wrote would look a lot like

function updateThing() {
  return dispatch => {
    dispatch({
      type: ActionTypes.STARTED_UPDATING
    });
    AsyncApi.getFieldValue()
      .then(result => dispatch({
        type: ActionTypes.UPDATED,
        payload: result
      }));
  }
}

const ConnectedApp = connect(
  (state) => { ...state },
  { update: updateThing }
)(App);

which doesn't look all that different from the original — it's just shuffled a bit — and connect doesn't know that updateThing is (or needs to be) asynchronous.

If you also wanted to support promises, observables, sagas, or crazy custom and highly declarative action creators, then Redux can do it just by changing what you pass to dispatch (aka, what you return from action creators). No mucking with the React components (or connect calls) necessary.