allow projects to override getDefaultStore?

[mxmul]

The provider-less mode, with it's singleton default store, is a big footgun for server-rendered apps -- potentially allowing data to leak between requests (e.g. #1603).

I understand that the guidance for server-rendered apps is to just always use a <Provider>, but this could be difficult to enforce at scale. In a large codebase, with many React applications and many developers, it's likely that someone will eventually forget to include the necessary provider somewhere.

Would you be open to an API that allows users to override the behavior of getDefaultStore?

This would allow projects to easily disable the feature outright:

import { overrideGetDefaultStore } from 'jotai';

overrideGetDefaultStore(() => {
  throw new Error("We have disabled Jotai provider-less mode. Please add a <Provider>!");
});

or they could replace the default store with one that is scoped to a single request. Implementation would be framework-specific, and outside the scope of Jotai, but a node web app could do something with AsyncLocalStorage -- roughly:

import { createStore, overrideGetDefaultStore } from 'jotai';

const asyncLocalStorage = new AsyncLocalStorage();

overrideGetDefaultStore(() => {
  return asyncLocalStore.getStore();
});

// express middleware that ensure's request-scoped default store
app.use((req, res, next) => {
  asyncLocalStorage.run(createStore(), () => {
    next();
  });
})

[dmaskasky]

This makes sense to me.

[dai-shi]

I think that could also allow us to replace the implementation with a request-scoped one as described earlier.

Assuming the idea is to use AsyncLocalStorage, the mutation hack is not possible.

I like the mutation hack for throwing errors though.

[mxmul]

It does in fact seem to work, doing something approximating:

import { getDefaultStore } from 'jotai';

const asyncLocalStorage = new AsyncLocalStorage();

const defaultStore = getDefaultStore();
defaultStore.get = (...args) => {
  const jotaiStore = asyncLocalStore.getStore();
  return jotaiStore.get(...args);
};
defaultStore.set = (...args) => {
  const jotaiStore = asyncLocalStore.getStore();
  return jotaiStore.set(...args);
};
defaultStore.sub = (...args) => {
  const jotaiStore = asyncLocalStore.getStore();
  return jotaiStore.sub(...args);
};

app.use((req, res, next) => {
  asyncLocalStorage.run(createStore(), () => {
    // everything in here has a request-scoped `defaultStore`
    next();
  });
})

What was your concern?

[dai-shi]

Oh, I didn't think about it. It definitely works. I was concerning about mutating the store for each request. lol

[dai-shi]

I think mutating the default store is more reasonable. Can we close #3227?

[mxmul]

Works for me!

[mxmul]

An alternative would be to update getDefaultStore() to always throw in server environments. This would be a breaking change however.