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/**
* Copyright (c) 2019-present, GraphQL Foundation
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*
* @flow strict
*/
// A Function, which when given an Array of keys, returns a Promise of an Array
// of values or Errors.
export type BatchLoadFn<K, V> =
(keys: $ReadOnlyArray<K>) => Promise<$ReadOnlyArray<V | Error>>;
// Optionally turn off batching or caching or provide a cache key function or a
// custom cache instance.
export type Options<K, V, C = K> = {
batch?: boolean;
maxBatchSize?: number;
batchScheduleFn?: (callback: () => void) => void;
cache?: boolean;
cacheKeyFn?: (key: K) => C;
cacheMap?: CacheMap<C, Promise<V>> | null;
};
// If a custom cache is provided, it must be of this type (a subset of ES6 Map).
export type CacheMap<K, V> = {
get(key: K): V | void;
set(key: K, value: V): any;
delete(key: K): any;
clear(): any;
};
/**
* A `DataLoader` creates a public API for loading data from a particular
* data back-end with unique keys such as the `id` column of a SQL table or
* document name in a MongoDB database, given a batch loading function.
*
* Each `DataLoader` instance contains a unique memoized cache. Use caution when
* used in long-lived applications or those which serve many users with
* different access permissions and consider creating a new instance per
* web request.
*/
class DataLoader<K, V, C = K> {
constructor(
batchLoadFn: BatchLoadFn<K, V>,
options?: Options<K, V, C>
) {
if (typeof batchLoadFn !== 'function') {
throw new TypeError(
'DataLoader must be constructed with a function which accepts ' +
`Array<key> and returns Promise<Array<value>>, but got: ${batchLoadFn}.`
);
}
this._batchLoadFn = batchLoadFn;
this._maxBatchSize = getValidMaxBatchSize(options);
this._batchScheduleFn = getValidBatchScheduleFn(options);
this._cacheKeyFn = getValidCacheKeyFn(options);
this._cacheMap = getValidCacheMap(options);
this._batch = null;
}
// Private
_batchLoadFn: BatchLoadFn<K, V>;
_maxBatchSize: number;
_batchScheduleFn: (() => void) => void;
_cacheKeyFn: K => C;
_cacheMap: CacheMap<C, Promise<V>> | null;
_batch: Batch<K, V> | null;
/**
* Loads a key, returning a `Promise` for the value represented by that key.
*/
load(key: K): Promise<V> {
if (key === null || key === undefined) {
throw new TypeError(
'The loader.load() function must be called with a value, ' +
`but got: ${String(key)}.`
);
}
var batch = getCurrentBatch(this);
var cacheMap = this._cacheMap;
var cacheKey = this._cacheKeyFn(key);
// If caching and there is a cache-hit, return cached Promise.
if (cacheMap) {
var cachedPromise = cacheMap.get(cacheKey);
if (cachedPromise) {
var cacheHits = batch.cacheHits || (batch.cacheHits = []);
return new Promise(resolve => {
cacheHits.push(() => {
resolve(cachedPromise);
});
});
}
}
// 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) => {
batch.callbacks.push({ resolve, reject });
});
// If caching, cache this promise.
if (cacheMap) {
cacheMap.set(cacheKey, promise);
}
return promise;
}
/**
* Loads multiple keys, promising an array of values:
*
* var [ a, b ] = await myLoader.loadMany([ 'a', 'b' ]);
*
* This is similar to the more verbose:
*
* var [ a, b ] = await Promise.all([
* myLoader.load('a'),
* myLoader.load('b')
* ]);
*
* However it is different in the case where any load fails. Where
* Promise.all() would reject, loadMany() always resolves, however each result
* is either a value or an Error instance.
*
* var [ a, b, c ] = await myLoader.loadMany([ 'a', 'b', 'badkey' ]);
* // c instanceof Error
*
*/
loadMany(keys: $ReadOnlyArray<K>): Promise<Array<V | Error>> {
if (!isArrayLike(keys)) {
throw new TypeError(
'The loader.loadMany() function must be called with Array<key> ' +
`but got: ${(keys: any)}.`
);
}
// Support ArrayLike by using only minimal property access
const loadPromises = [];
for (let i = 0; i < keys.length; i++) {
loadPromises.push(this.load(keys[i]).catch(error => error));
}
return Promise.all(loadPromises);
}
/**
* Clears the value at `key` from the cache, if it exists. Returns itself for
* method chaining.
*/
clear(key: K): this {
var cacheMap = this._cacheMap;
if (cacheMap) {
var cacheKey = this._cacheKeyFn(key);
cacheMap.delete(cacheKey);
}
return this;
}
/**
* Clears the entire cache. To be used when some event results in unknown
* invalidations across this particular `DataLoader`. Returns itself for
* method chaining.
*/
clearAll(): this {
var cacheMap = this._cacheMap;
if (cacheMap) {
cacheMap.clear();
}
return this;
}
/**
* Adds the provided key and value to the cache. If the key already
* exists, no change is made. Returns itself for method chaining.
*
* To prime the cache with an error at a key, provide an Error instance.
*/
prime(key: K, value: V | Error): this {
var cacheMap = this._cacheMap;
if (cacheMap) {
var cacheKey = this._cacheKeyFn(key);
// Only add the key if it does not already exist.
if (cacheMap.get(cacheKey) === undefined) {
// Cache a rejected promise if the value is an Error, in order to match
// the behavior of load(key).
var promise;
if (value instanceof Error) {
promise = Promise.reject(value);
// Since this is a case where an Error is intentionally being primed
// for a given key, we want to disable unhandled promise rejection.
promise.catch(() => {});
} else {
promise = Promise.resolve(value);
}
cacheMap.set(cacheKey, promise);
}
}
return this;
}
}
// Private: Enqueue a Job to be executed after all "PromiseJobs" Jobs.
//
// ES6 JavaScript uses the concepts Job and JobQueue to schedule work to occur
// after the current execution context has completed:
// http://www.ecma-international.org/ecma-262/6.0/#sec-jobs-and-job-queues
//
// Node.js uses the `process.nextTick` mechanism to implement the concept of a
// Job, maintaining a global FIFO JobQueue for all Jobs, which is flushed after
// the current call stack ends.
//
// When calling `then` on a Promise, it enqueues a Job on a specific
// "PromiseJobs" JobQueue which is flushed in Node as a single Job on the
// global JobQueue.
//
// DataLoader batches all loads which occur in a single frame of execution, but
// should include in the batch all loads which occur during the flushing of the
// "PromiseJobs" JobQueue after that same execution frame.
//
// In order to avoid the DataLoader dispatch Job occuring before "PromiseJobs",
// A Promise Job is created with the sole purpose of enqueuing a global Job,
// ensuring that it always occurs after "PromiseJobs" ends.
//
// Node.js's job queue is unique. Browsers do not have an equivalent mechanism
// for enqueuing a job to be performed after promise microtasks and before the
// next macrotask. For browser environments, a macrotask is used (via
// setImmediate or setTimeout) at a potential performance penalty.
var enqueuePostPromiseJob =
typeof process === 'object' && typeof process.nextTick === 'function' ?
function (fn) {
if (!resolvedPromise) {
resolvedPromise = Promise.resolve();
}
resolvedPromise.then(() => {
process.nextTick(fn);
});
} :
typeof setImmediate === 'function' ? function (fn) {
setImmediate(fn);
} : function (fn) {
setTimeout(fn);
};
// Private: cached resolved Promise instance
var resolvedPromise;
// 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;
}>,
cacheHits?: Array<() => 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> {
// 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 &&
existingBatch.keys.length < loader._maxBatchSize &&
(!existingBatch.cacheHits ||
existingBatch.cacheHits.length < loader._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.
loader._batchScheduleFn(() => {
dispatchBatch(loader, newBatch);
});
return newBatch;
}
function dispatchBatch<K, V>(
loader: DataLoader<K, V, any>,
batch: Batch<K, V>
) {
// Mark this batch as having been dispatched.
batch.hasDispatched = true;
// If there's nothing to load, resolve any cache hits and return early.
if (batch.keys.length === 0) {
resolveCacheHits(batch);
return;
}
// 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, 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)}.`
));
}
// Await the resolution of the call to batchLoadFn.
batchPromise.then(values => {
// Assert the expected resolution from batchLoadFn.
if (!isArrayLike(values)) {
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: ${String(values)}.`
);
}
if (values.length !== batch.keys.length) {
throw new TypeError(
'DataLoader must be constructed with a function which accepts ' +
'Array<key> and returns Promise<Array<value>>, but the function did ' +
'not return a Promise of an Array of the same length as the Array ' +
'of keys.' +
`\n\nKeys:\n${String(batch.keys)}` +
`\n\nValues:\n${String(values)}`
);
}
// Resolve all cache hits in the same micro-task as freshly loaded values.
resolveCacheHits(batch);
// 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) {
batch.callbacks[i].reject(value);
} else {
batch.callbacks[i].resolve(value);
}
}
}).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>,
batch: Batch<K, V>,
error: Error
) {
// Cache hits are resolved, even though the batch failed.
resolveCacheHits(batch);
for (var i = 0; i < batch.keys.length; i++) {
loader.clear(batch.keys[i]);
batch.callbacks[i].reject(error);
}
}
// Private: Resolves the Promises for any cache hits in this batch.
function resolveCacheHits(batch: Batch<any, any>) {
if (batch.cacheHits) {
for (var i = 0; i < batch.cacheHits.length; i++) {
batch.cacheHits[i]();
}
}
}
// Private: given the DataLoader's options, produce a valid max batch size.
function getValidMaxBatchSize(options: ?Options<any, any, any>): number {
var shouldBatch = !options || options.batch !== false;
if (!shouldBatch) {
return 1;
}
var maxBatchSize = options && options.maxBatchSize;
if (maxBatchSize === undefined) {
return Infinity;
}
if (typeof maxBatchSize !== 'number' || maxBatchSize < 1) {
throw new TypeError(
`maxBatchSize must be a positive number: ${(maxBatchSize: any)}`
);
}
return maxBatchSize;
}
// Private
function getValidBatchScheduleFn(
options: ?Options<any, any, any>
): (() => void) => void {
var batchScheduleFn = options && options.batchScheduleFn;
if (batchScheduleFn === undefined) {
return enqueuePostPromiseJob;
}
if (typeof batchScheduleFn !== 'function') {
throw new TypeError(
`batchScheduleFn must be a function: ${(batchScheduleFn: any)}`
);
}
return batchScheduleFn;
}
// Private: given the DataLoader's options, produce a cache key function.
function getValidCacheKeyFn<K, C>(options: ?Options<K, any, C>): (K => C) {
var cacheKeyFn = options && options.cacheKeyFn;
if (cacheKeyFn === undefined) {
return (key => key: any);
}
if (typeof cacheKeyFn !== 'function') {
throw new TypeError(`cacheKeyFn must be a function: ${(cacheKeyFn: any)}`);
}
return cacheKeyFn;
}
// Private: given the DataLoader's options, produce a CacheMap to be used.
function getValidCacheMap<K, V, C>(
options: ?Options<K, V, C>
): CacheMap<C, Promise<V>> | null {
var shouldCache = !options || options.cache !== false;
if (!shouldCache) {
return null;
}
var cacheMap = options && options.cacheMap;
if (cacheMap === undefined) {
return new Map();
}
if (cacheMap !== null) {
var cacheFunctions = [ 'get', 'set', 'delete', 'clear' ];
var missingFunctions = cacheFunctions
.filter(fnName => cacheMap && typeof cacheMap[fnName] !== 'function');
if (missingFunctions.length !== 0) {
throw new TypeError(
'Custom cacheMap missing methods: ' + missingFunctions.join(', ')
);
}
}
return cacheMap;
}
// Private
function isArrayLike(x: mixed): boolean {
return (
typeof x === 'object' &&
x !== null &&
typeof x.length === 'number' &&
(x.length === 0 ||
(x.length > 0 && Object.prototype.hasOwnProperty.call(x, x.length - 1)))
);
}
module.exports = DataLoader;