RTKQuery on the server: how to prevent carry-over of redux state between requests?

[azangru]

TLDR:

Question: When fetching data with RTKQ on the server side, what is the proper way of making sure that redux state does not persist across requests?

Subquestion: If one creates an RTKQ api slice using the createSlice api (and then uses the injectEndpoints method on that slice), doesn't it make the slice into a singleton that, on the server, persists between requests, along with its state?

Another subquestion Can the resetApiState be used on the server when a new store is created? If an api slice is a singleton, wouldn't this interrupt any other possible fetches that were started during other requests and that may still be in flight?

===========

EXPANDED VERSION OF THE QUESTION

I am using redux-toolkit with RTKQ in a react app with a little bit of server rendering. The app is completely custom-made and does not use any off-the-shelf framework, but perhaps resembles the early Next.js with its pages router. The pages that render on the server export a serverFetch function that fetches data with the help of RTKQ.

I have recently realized that I am seeing a contamination of redux state between routes. To my utter astonishment, that contamination only appears in some routes and not in others (I can't explain this; I would have thought that if I am leaking state between requests, this would be obvious on any request to any route). This state contamination looks like this in the stringified state sent for rehydration:

A fragment of serialized state sent to the client for rehydration

{
"restApi": {
  "queries": {
    "genomeSummaryByGenomeSlug(\"grch38\")": {
      "status": "fulfilled",
      "endpointName": "genomeSummaryByGenomeSlug",
      "requestId": "tOb9Kl-OiDuefN4IojSix",
      "originalArgs": "grch38",
      "startedTimeStamp": 1760628699336,
      "data": {
        "genome_id": "a7335667-93e7-11ec-a39d-005056b38ce3",
        "genome_tag": "grch38",
        "common_name": "Human",
        "scientific_name": "Homo sapiens",
        "species_taxonomy_id": "9606",
        "type": null,
        "is_reference": true,
        "assembly": {
          "accession_id": "GCA_000001405.29",
          "name": "GRCh38.p14"
        },
        "release": {
          "name": "2025-02",
          "type": "integrated"
        },
        "latest_genome": {
          "genome_id": "2b5fb047-5992-4dfb-b2fa-1fb4e18d1abb",
          "genome_tag": null,
          "common_name": "Human",
          "scientific_name": "Homo sapiens",
          "species_taxonomy_id": "9606",
          "type": null,
          "is_reference": true,
          "assembly": {
            "accession_id": "GCA_000001405.29",
            "name": "GRCh38.p14",
            "url": "https://identifiers.org/insdc.gca/GCA_000001405.29"
          },
          "release": {
            "name": "2025-04-28",
            "type": "partial",
            "is_current": false
          }
        }
      },
      "fulfilledTimeStamp": 1760628699339
    }
  },
  "mutations": {},
  "provided": {
    "tags": {},
    "keys": {
      "genomeSummaryByGenomeSlug(\"grch38\")": []
    }
  },
  "subscriptions": {
    "getHelpArticle({\"pathname\":\"/help/articles/gene-annotation\"})": {
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}

One obvious lesson that I learnt from this was that I forgot to unsubscribe from a particular query. But it also made it painfully obvious to me that I do not fully understand how redux works on the server.

In particular, here is how I am creating a redux store:

import restApiSlice from 'src/shared/state/api-slices/restSlice';

import createRootReducer from 'src/root/rootReducer';

const middleware = [graphqlApiSlice.middleware, restApiSlice.middleware];

export const getServerSideReduxStore = () => {
  return configureStore({
    reducer: createRootReducer(),
    middleware: (getDefaultMiddleware) =>
      getDefaultMiddleware().concat(middleware)
  });
};

The restApiSlice, in turn, is created and exported like this:

// from the restSlice file

export default createApi({
  reducerPath: 'restApi',
  baseQuery: staggeredBaseQueryWithBailout,
  endpoints: () => ({}) // will inject endpoints in other files
});

The above snippet suggests to me that restApiSlice becomes a singleton that is created only once during the first module evaluation, and is not recreated at every request to the server. It must be a singleton, because it would be imported in other files that would inject their endpoints into it, like so:

// another api slice
import restApiSlice from 'src/shared/state/api-slices/restSlice';

// ...

const helpApiSlice = restApiSlice.injectEndpoints({
  endpoints: (builder) => ({
    getHelpArticle: builder.query<HelpArticleResponse, HelpArticleQueryParams>({
      query: (params) => ({
        url: `${config.docsBaseUrl}/article?url=${encodeURIComponent(
          params.pathname
        )}`
      })
    })
  })
});

Eventually, this getServerSideReduxStore function is called in an Express route per every request; so every request creates a brand new store:

const viewRouter = async (req: Request, res: Response) => {
  const reduxStore = getServerSideReduxStore();

  const matchedPageConfig = routesConfig.find((route) =>
    matchPath(route.path, req.path)
  ) as RouteConfig;

  let statusCode = 200;
  let didError = false;

  if (matchedPageConfig.serverFetch) {
    try {
      const fetchResult = await matchedPageConfig.serverFetch({
          path: req.path,
          store: reduxStore
        })
    }
  }

  // ...

But, if restApiSlice is a singleton, then creating a fresh copy of the store every request won't help, if it uses the same restApiSlice every time. And, if restApiSlice holds a state and if it persists through the whole lifetime of a server, how to prevent it from carrying over its state between requests?

[phryneas]

restApiSlice migh be a singleton, but it only holds logic, not data or state. There should not be any cross-contamination - all state is bound to the individual store instances.
So tbh., I'm a bit at a loss. Could you create a reproduction?