Order of reducer and saga

Question

When dispatching an action is the order when it arrives to a reducer and a saga guaranteed?

Can I rely on that it

first enters the reducer
then the saga?

Reducer:

 function reducer(state, action) {

    switch (action.type) {
       case 'MY_ACTION':
       // decorate action so that an epic doesn't have to take data from store
       action.ports = state.itemsModified;                 
       return state;
     }
    }

Saga:

export function* sagaUpdatePorts() {
    yield* ReduxSaga.takeEvery(actions.GRID_PORTS_ASYNC_UPDATE_PORTS, updatePorts);
}

function* updatePorts(action) {
    const {response, error} = yield SagaEffects.call(portsService.updatePorts, action.ports);
}

score 30 · Accepted Answer · edited Feb 10 '23 at 19:39

30

Yes. The action hits the reducer first then Sagas.

This is mentioned in the docs under the select() effect API:

It's important to note that when an action is dispatched to the store, the middleware first forwards the action to the reducers and then notifies the Sagas. This means that when you query the Store's State, you get the State after the action has been applied. However, this behavior is only guaranteed if all subsequent middlewares call next(action) synchronously. If any subsequent middleware calls next(action) asynchronously (which is unusual but possible), then the sagas will get the state from before the action is applied. Therefore it is recommended to review the source of each subsequent middleware to ensure it calls next(action) synchronously, or else ensure that redux-saga is the last middleware in the call chain.

edited Feb 10 '23 at 19:39

romellem

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answered Jul 28 '16 at 15:51

Yassine Elouafi

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Thx, Yassine. One more thing before accepting the answer. Can you provide a link to the docs if it's there? Or is your answer based on your sole opinion? – Amio.io Jul 29 '16 at 06:23
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I wrote the library so maybe this would give some guarantee ☺ – Yassine Elouafi Jul 31 '16 at 10:23
Gotta trust you, Bro. ;) Accepted – Amio.io Jul 31 '16 at 18:41
1

@YassineElouafi as per your comment was that added to the documentation? – Kamaraju Oct 10 '17 at 03:51
@Kamaraju https://redux-saga.js.org/docs/api/index.html#selectselector-args – Erik Haider Forsén Feb 28 '19 at 08:34
1

Here are the lines in code so you can see for yourself :) https://github.com/redux-saga/redux-saga/blob/4348497e1c22bb5656c1905af19e25f8e7d29c61/packages/core/src/internal/middleware.js#L27-L28 – rottweilers_anonymous Apr 14 '21 at 03:52
1

I wonder what is the reasoning behind the design decision to run saga after reducers. "Middleware" name implies something to happen before it reaches the reducers) – T.P. Nov 23 '21 at 03:00

Order of reducer and saga

1 Answers1