Accessing cache from onError / onErrorRetry

[joshkel]

I'd like to access the cache from an onError handler so that I can invalidate previously cached values in response to certain types of errors.

I see that onError takes the config object as its third parameter, and in practice, the config object appears to contain a cache member. However, it doesn't appear to be permitted by the TypeScript types or documented.

Is it permitted to access this member? Can/should the types be updated to reflect this? Or can/should SWR add an alternative standardized way of accessing the cache for the current request?

Thank you.

[shuding]

so that I can invalidate previously cached values in response to certain types of errors.

I think in that case, I wouldn’t directly edit the cache, because it won’t sync that state with all hooks. Maybe we should expose mutate in those callbacks.

[joshkel]

Thanks for the reply. In my ideal world, I'd actually want to access both the cache and mutate, because I'm selectively mutating based on whether previous values had succeeded. (See #998.)

Or, if there were a way to call mutate and set both data and error, that would be better. For example, if ScopedMutator were overloaded:

(key: Key, data?: Data | Promise<Data> | MutatorCallback<Data>, shouldRevalidate?: boolean): Promise<Data | undefined>;
// To facilitate overloading, assume / require that any errors are of Error type.
// Promise and callback are not implemented, since data + error should be provided together:
(key: Key, data?: Data, error?: Error, shouldRevalidate?: boolean): Promise<Data | undefined>;

Then my error handler from #998 could be

function onError(err: Error, key: string, config) {
  if (err instanceof BackendError && err.response.statusCode === StatusCodes.NOT_FOUND) {
    // If the requested entity doesn't exist, clear any cached value and return the Not Found error.
    config.mutate(key, null, err, false);
  }
}

(Alternatively, the error object could be an optional fourth parameter to mutate, to avoid overloading. Or, to keep full support for callbacks and promises, the API could be extended to offer an optional flag or symbol for providing a {data, error}.)

[miso-belica]

Hello, I have a similar need to access the cache in onError but I don't have to mutate them. My use-case is as follows. I report all errors globally in onError to a monitoring service. But there is quite a lot of NetworkError which I can ignore if data are already in the cache. Problem is I want to report cases when data are not in the cache but network error happens. Currently I can't if I don't want to rely on private cache property. Any other idea how to do it?