enable batch execution

WIP:

When `batch` is set to true for a given subschemaConfig, batches all delegated root fields into a combined request passed to the executor. Moreover, batches all requests to a given subschema into the minimum number of requests, collecting queries and mutations separately, preserving operation order. Distributes properly pathed errors to the originating requests.

Adapted from Gatsby query batcher by @vladar.

Caveats:
* Uses a Dataloader under the hood, which is created anew upon each request -- relies on a unique context argument per request to make this happen!
* Passed `info` argument from first executor call to the batched executor call, making info argument unreliable.

TODO:
- Add testing!

Related:

gatsbyjs/gatsby#22347 (comment)
#1710 (comment)
#1959 (comment)
#1954

packages/delegate/package.json

@@ -21,11 +21,12 @@
     "@graphql-tools/schema": "6.1.0",
     "@graphql-tools/utils": "6.1.0",
     "@ardatan/aggregate-error": "0.0.1",
+    "dataloader": "2.0.0",
     "is-promise": "4.0.0",
     "tslib": "~2.0.1"
   },
   "publishConfig": {
     "access": "public",
     "directory": "dist"
   }
-}
+}

packages/delegate/src/delegateToSchema.ts

@@ -24,6 +24,7 @@ import { createRequestFromInfo, getDelegatingOperation } from './createRequest';
 import { Transformer } from './Transformer';
 
 import AggregateError from '@ardatan/aggregate-error';
+import { getBatchingExecutor } from './getBatchingExecutor';
 
 export function delegateToSchema(options: IDelegateToSchemaOptions | GraphQLSchema): any {
   if (isSchema(options)) {
@@ -154,9 +155,13 @@ export function delegateRequest({
   }
 
   if (targetOperation === 'query' || targetOperation === 'mutation') {
-    const executor =
+    let executor =
       subschemaConfig?.executor || createDefaultExecutor(targetSchema, subschemaConfig?.rootValue || targetRootValue);
 
+    if (subschemaConfig?.batch) {
+      executor = getBatchingExecutor(context, subschemaConfig, executor);
+    }
+
     const executionResult = executor({
       document: processedRequest.document,
       variables: processedRequest.variables,

packages/delegate/src/getBatchingExecutor.ts

@@ -0,0 +1,74 @@
+import { getOperationAST } from 'graphql';
+
+import isPromise from 'is-promise';
+
+import DataLoader from 'dataloader';
+
+import { ExecutionResult } from '@graphql-tools/utils';
+
+import { SubschemaConfig, ExecutionParams } from './types';
+import { memoize2of3 } from './memoize';
+import { mergeExecutionParams } from './mergeExecutionParams';
+import { splitResult } from './splitResult';
+
+export const getBatchingExecutor = memoize2of3(function (
+  _context: Record<string, any>,
+  subschemaConfig: SubschemaConfig,
+  executor: ({ document, context, variables, info }: ExecutionParams) => ExecutionResult | Promise<ExecutionResult>
+) {
+  const loader = new DataLoader(
+    createLoadFn(executor ?? subschemaConfig.executor),
+    subschemaConfig.batchingOptions?.dataLoaderOptions
+  );
+  return (executionParams: ExecutionParams) => loader.load(executionParams);
+});
+
+function createLoadFn(
+  executor: ({ document, context, variables, info }: ExecutionParams) => ExecutionResult | Promise<ExecutionResult>
+) {
+  return async (execs: Array<ExecutionParams>): Promise<Array<ExecutionResult>> => {
+    const execBatches: Array<Array<ExecutionParams>> = [];
+    let index = 0;
+    const exec = execs[index];
+    let currentBatch: Array<ExecutionParams> = [exec];
+    execBatches.push(currentBatch);
+    const operationType = getOperationAST(exec.document, undefined).operation;
+    while (++index < execs.length) {
+      const currentOperationType = getOperationAST(execs[index].document, undefined).operation;
+      if (operationType === currentOperationType) {
+        currentBatch.push(execs[index]);
+      } else {
+        currentBatch = [execs[index]];
+        execBatches.push(currentBatch);
+      }
+    }
+
+    let containsPromises = false;
+    const executionResults: Array<ExecutionResult | Promise<ExecutionResult>> = [];
+    execBatches.forEach(execBatch => {
+      const mergedExecutionParams = mergeExecutionParams(execBatch);
+      const executionResult = executor(mergedExecutionParams);
+
+      if (isPromise(executionResult)) {
+        containsPromises = true;
+      }
+      executionResults.push(executionResult);
+    });
+
+    if (containsPromises) {
+      return Promise.all(executionResults).then(resultBatches => {
+        let results: Array<ExecutionResult> = [];
+        resultBatches.forEach((resultBatch, index) => {
+          results = results.concat(splitResult(resultBatch, execBatches[index].length));
+        });
+        return results;
+      });
+    }
+
+    let results: Array<ExecutionResult> = [];
+    (executionResults as Array<ExecutionResult>).forEach((resultBatch, index) => {
+      results = results.concat(splitResult(resultBatch, execBatches[index].length));
+    });
+    return results;
+  };
+}

packages/delegate/src/memoize.ts

@@ -211,3 +211,43 @@ export function memoize2<T1 extends Record<string, any>, T2 extends Record<strin
 
   return memoized;
 }
+
+export function memoize2of3<
+  T1 extends Record<string, any>,
+  T2 extends Record<string, any>,
+  T3 extends any,
+  R extends any
+>(fn: (A1: T1, A2: T2, A3: T3) => R): (A1: T1, A2: T2, A3: T3) => R {
+  let cache1: WeakMap<T1, WeakMap<T2, R>>;
+
+  function memoized(a1: T1, a2: T2, a3: T3) {
+    if (!cache1) {
+      cache1 = new WeakMap();
+      const cache2: WeakMap<T2, R> = new WeakMap();
+      cache1.set(a1, cache2);
+      const newValue = fn(a1, a2, a3);
+      cache2.set(a2, newValue);
+      return newValue;
+    }
+
+    let cache2 = cache1.get(a1);
+    if (!cache2) {
+      cache2 = new WeakMap();
+      cache1.set(a1, cache2);
+      const newValue = fn(a1, a2, a3);
+      cache2.set(a2, newValue);
+      return newValue;
+    }
+
+    const cachedValue = cache2.get(a2);
+    if (cachedValue === undefined) {
+      const newValue = fn(a1, a2, a3);
+      cache2.set(a2, newValue);
+      return newValue;
+    }
+
+    return cachedValue;
+  }
+
+  return memoized;
+}