Refactor batching logic (#220)

Author: leebyron

README.md

@@ -321,7 +321,7 @@ Create a new `DataLoader` given a batch loading function and options.
 
   | Option Key | Type | Default | Description |
   | ---------- | ---- | ------- | ----------- |
-  | *batch*  | Boolean | `true` | Set to `false` to disable batching, invoking `batchLoadFn` with a single load key.
+  | *batch*  | Boolean | `true` | Set to `false` to disable batching, invoking `batchLoadFn` with a single load key. This is equivalent to setting `maxBatchSize` to `1`.
   | *maxBatchSize* | Number | `Infinity` | Limits the number of items that get passed in to the `batchLoadFn`.
   | *cache* | Boolean | `true` | Set to `false` to disable memoization caching, creating a new Promise and new key in the `batchLoadFn` for every load of the same key.
   | *cacheKeyFn* | Function | `key => key` | Produces cache key for a given load key. Useful when objects are keys and two objects should be considered equivalent.

src/__tests__/dataloader.test.js

@@ -120,6 +120,24 @@ describe('Primary API', () => {
     expect(loadCalls).toEqual([ [ 1 ] ]);
   });
 
+  it('coalesces identical requests across sized batches', async () => {
+    const [ identityLoader, loadCalls ] = idLoader<number>({ maxBatchSize: 2 });
+
+    const promise1a = identityLoader.load(1);
+    const promise2 = identityLoader.load(2);
+    const promise1b = identityLoader.load(1);
+    const promise3 = identityLoader.load(3);
+
+    const [ value1a, value2, value1b, value3 ] =
+      await Promise.all([ promise1a, promise2, promise1b, promise3 ]);
+    expect(value1a).toBe(1);
+    expect(value2).toBe(2);
+    expect(value1b).toBe(1);
+    expect(value3).toBe(3);
+
+    expect(loadCalls).toEqual([ [ 1, 2 ], [ 3 ] ]);
+  });
+
   it('caches repeated requests', async () => {
     const [ identityLoader, loadCalls ] = idLoader<string>();
 

src/index.js

@@ -54,14 +54,14 @@ class DataLoader<K, V, C = K> {
     this._batchLoadFn = batchLoadFn;
     this._options = options;
     this._promiseCache = getValidCacheMap(options);
-    this._queue = [];
+    this._batch = null;
   }
 
   // Private
   _batchLoadFn: BatchLoadFn<K, V>;
   _options: ?Options<K, V, C>;
   _promiseCache: ?CacheMap<C, Promise<V>>;
-  _queue: LoaderQueue<K, V>;
+  _batch: Batch<K, V> | null;
 
   /**
    * Loads a key, returning a `Promise` for the value represented by that key.
@@ -76,7 +76,7 @@ class DataLoader<K, V, C = K> {
 
     // Determine options
     var options = this._options;
-    var shouldBatch = !options || options.batch !== false;
+    var batch = getCurrentBatch(this);
     var cache = this._promiseCache;
     var cacheKey = getCacheKey(options, key);
 
@@ -88,23 +88,11 @@ class DataLoader<K, V, C = K> {
       }
     }
 
-    // Otherwise, produce a new Promise for this value.
+    // Otherwise, produce a new Promise for this key, and enqueue it to be
+    // dispatched along with the current batch.
+    batch.keys.push(key);
     var promise = new Promise((resolve, reject) => {
-      // Enqueue this Promise to be dispatched.
-      this._queue.push({ key, resolve, reject });
-
-      // Determine if a dispatch of this queue should be scheduled.
-      // A single dispatch should be scheduled per queue at the time when the
-      // queue changes from "empty" to "full".
-      if (this._queue.length === 1) {
-        if (shouldBatch) {
-          // If batching, schedule a task to dispatch the queue.
-          enqueuePostPromiseJob(() => dispatchQueue(this));
-        } else {
-          // Otherwise dispatch the (queue of one) immediately.
-          dispatchQueue(this);
-        }
-      }
+      batch.callbacks.push({ resolve, reject });
     });
 
     // If caching, cache this promise.
@@ -246,43 +234,61 @@ var enqueuePostPromiseJob =
 // Private: cached resolved Promise instance
 var resolvedPromise;
 
-// Private: given the current state of a Loader instance, perform a batch load
-// from its current queue.
-function dispatchQueue<K, V>(loader: DataLoader<K, V, any>) {
-  // Take the current loader queue, replacing it with an empty queue.
-  var queue = loader._queue;
-  loader._queue = [];
-
-  // If a maxBatchSize was provided and the queue is longer, then segment the
-  // queue into multiple batches, otherwise treat the queue as a single batch.
-  var maxBatchSize = loader._options && loader._options.maxBatchSize;
-  if (maxBatchSize && maxBatchSize > 0 && maxBatchSize < queue.length) {
-    for (var i = 0; i < queue.length / maxBatchSize; i++) {
-      dispatchQueueBatch(
-        loader,
-        queue.slice(i * maxBatchSize, (i + 1) * maxBatchSize)
-      );
-    }
-  } else {
-    dispatchQueueBatch(loader, queue);
+// Private: Describes a batch of requests
+type Batch<K, V> = {
+  hasDispatched: boolean,
+  keys: Array<K>,
+  callbacks: Array<{
+    resolve: (value: V) => void;
+    reject: (error: Error) => void;
+  }>
+}
+
+// Private: Either returns the current batch, or creates and schedules a
+// dispatch of a new batch for the given loader.
+function getCurrentBatch<K, V>(loader: DataLoader<K, V, any>): Batch<K, V> {
+  var options = loader._options;
+  var maxBatchSize =
+    (options && options.maxBatchSize) ||
+    (options && options.batch === false ? 1 : 0);
+
+  // If there is an existing batch which has not yet dispatched and is within
+  // the limit of the batch size, then return it.
+  var existingBatch = loader._batch;
+  if (
+    existingBatch !== null &&
+    !existingBatch.hasDispatched &&
+    (maxBatchSize === 0 || existingBatch.keys.length < maxBatchSize)
+  ) {
+    return existingBatch;
   }
+
+  // Otherwise, create a new batch for this loader.
+  var newBatch = { hasDispatched: false, keys: [], callbacks: [] };
+
+  // Store it on the loader so it may be reused.
+  loader._batch = newBatch;
+
+  // Then schedule a task to dispatch this batch of requests.
+  enqueuePostPromiseJob(() => dispatchBatch(loader, newBatch));
+
+  return newBatch;
 }
 
-function dispatchQueueBatch<K, V>(
+function dispatchBatch<K, V>(
   loader: DataLoader<K, V, any>,
-  queue: LoaderQueue<K, V>
+  batch: Batch<K, V>
 ) {
-  // Collect all keys to be loaded in this dispatch
-  var keys = queue.map(({ key }) => key);
+  // Mark this batch as having been dispatched.
+  batch.hasDispatched = true;
 
-  // Call the provided batchLoadFn for this loader with the loader queue's keys.
-  var batchLoadFn = loader._batchLoadFn;
-  // Call with the loader as the `this` context.
-  var batchPromise = batchLoadFn.call(loader, keys);
+  // Call the provided batchLoadFn for this loader with the batch's keys and
+  // with the loader as the `this` context.
+  var batchPromise = loader._batchLoadFn(batch.keys);
 
   // Assert the expected response from batchLoadFn
   if (!batchPromise || typeof batchPromise.then !== 'function') {
-    return failedDispatch(loader, queue, new TypeError(
+    return failedDispatch(loader, batch, new TypeError(
       'DataLoader must be constructed with a function which accepts ' +
       'Array<key> and returns Promise<Array<value>>, but the function did ' +
       `not return a Promise: ${String(batchPromise)}.`
@@ -300,41 +306,40 @@ function dispatchQueueBatch<K, V>(
         `not return a Promise of an Array: ${String(values)}.`
       );
     }
-    if (values.length !== keys.length) {
+    if (values.length !== batch.keys.length) {
       throw new TypeError(
         'DataLoader must be constructed with a function which accepts ' +
         'Array<key> and returns Promise<Array<value>>, but the function did ' +
         'not return a Promise of an Array of the same length as the Array ' +
         'of keys.' +
-        `\n\nKeys:\n${String(keys)}` +
+        `\n\nKeys:\n${String(batch.keys)}` +
         `\n\nValues:\n${String(values)}`
       );
     }
 
-    // Step through the values, resolving or rejecting each Promise in the
-    // loaded queue.
-    queue.forEach(({ resolve, reject }, index) => {
-      var value = values[index];
+    // Step through values, resolving or rejecting each Promise in the batch.
+    for (var i = 0; i < batch.callbacks.length; i++) {
+      var value = values[i];
       if (value instanceof Error) {
-        reject(value);
+        batch.callbacks[i].reject(value);
       } else {
-        resolve(value);
+        batch.callbacks[i].resolve(value);
       }
-    });
-  }).catch(error => failedDispatch(loader, queue, error));
+    }
+  }).catch(error => failedDispatch(loader, batch, error));
 }
 
 // Private: do not cache individual loads if the entire batch dispatch fails,
 // but still reject each request so they do not hang.
 function failedDispatch<K, V>(
   loader: DataLoader<K, V, any>,
-  queue: LoaderQueue<K, V>,
+  batch: Batch<K, V>,
   error: Error
 ) {
-  queue.forEach(({ key, reject }) => {
-    loader.clear(key);
-    reject(error);
-  });
+  for (var i = 0; i < batch.keys.length; i++) {
+    loader.clear(batch.keys[i]);
+    batch.callbacks[i].reject(error);
+  }
 }
 
 // Private: produce a cache key for a given key (and options)
@@ -369,13 +374,6 @@ function getValidCacheMap<K, V, C>(
   return cacheMap;
 }
 
-// Private
-type LoaderQueue<K, V> = Array<{
-  key: K;
-  resolve: (value: V) => void;
-  reject: (error: Error) => void;
-}>;
-
 // Private
 function isArrayLike(x: mixed): boolean {
   return (