useGETForQueries is accepted but doesn't work with BatchHttpLink

[lhguerra]

Issue Description

I'm setting my link like this:

const httpLink = new BatchHttpLink({
  uri: apiUri,
  batchMax: 5,
  batchInterval: 20,
  useGETForQueries: true,
  fetchOptions: {
    agent: new Agent({ keepAlive: true }),
  },
})

I got no warning from TS or at runtime, so I just assumed my batched link would keep doing get requests as previously configured, but it only does post. I don't get why only do batched queries with post, it's not like there is a limitation, I simulated the same behavior with get (serializing the queries array so it can be used in a get request string) and my graphql server gave me the same response.

Link to Reproduction

.

Reproduction Steps

• Create the Apollo Link with BatchHttpLink constructor
• Set useGETForQueries to true
• Make any request to the graphql api, it will be sent always via post method

[jerelmiller]

Hey @lhguerra 👋

Digging through the codebase, I found this comment:

apollo-client/src/link/batch-http/batchHttpLink.ts

Lines 112 to 117 in a1dac63

	// There's no spec for using GET with batches.
	if (options.method === 'GET') {
	return fromError<FetchResult[]>(
	new Error('apollo-link-batch-http does not support GET requests'),
	);
	}

It looks like this code was originally pulled over from the apollo-link-batch-http package. Looking through the Git history, I found this commit: apollographql/apollo-link@2084620

While your server might understand batching via GET requests, my best guess is not every server might have support for it, hence why we disable it. If you're able to help me do a bit of sleuthing to determine if popular GraphQL server implementations allow HTTP batching via GET, that would go a long ways to help us determine whether to add formal support for it in the library.

At the very least, we should fix the TypeScript issue. Allowing this option via TypeScript seems like an oversight. It looks like we naively pull in all HttpOptions from HttpLink, which is why TypeScript isn't yelling at you.

[lhguerra]

If you're able to help me do a bit of sleuthing to determine if popular GraphQL server implementations allow HTTP batching via GET, that would go a long ways to help us determine whether to add formal support for it in the library.

I don't think that's the case, as this automatic interpretation of arrays in the get request is a feature of the request lib of the framework i'm using (RoR), not part of the graphql server implementation (I actually discovered it by accident when writting tests for the graphql controller). So my use case might be too specific to support, and a type error should do.

[jerelmiller]

Something I'll mention is that if you absolutely need GET support for batch requests, there is nothing stopping you from providing a custom fetch function that switches the request from a POST to a GET. It might be a little bit hacky, but this should work as well:

const link = new BatchHttpLink({
  fetch: (uri, options) => {
    // NOTE: You'll probably need to serialize `uri` differently here
    return window.fetch(uri, {
      ...options,
      method: 'GET',
    })
  }
});

At the very least, this allows you to use GET without official support from the library via useGETForQueries. Something to consider!

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