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<a class="" href="../executing-queries/">Executing Queries</a>
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<a class="current" href="./">Fetching Data</a>
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<ul>
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<li class="toctree-l3"><a href="#overview">Overview</a></li>
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<li class="toctree-l3"><a href="#default-field-resolver">Default Field Resolver</a></li>
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<li class="toctree-l3"><a href="#default-field-resolver-per-type">Default Field Resolver per Type</a></li>
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<li class="toctree-l3"><a href="#solving-n1-problem">Solving N+1 Problem</a></li>
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<li class="toctree-l3"><a href="#async-php">Async PHP</a></li>
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<a class="" href="../complementary-tools/">Complementary Tools</a>
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<div class="section">
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<h1 id="overview">Overview</h1>
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<p>GraphQL is data-storage agnostic. You can use any underlying data storage engine, including SQL or NoSQL database,
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plain files or in-memory data structures.</p>
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<p>In order to convert GraphQL query to PHP array <strong>graphql-php</strong> traverses query fields (using depth-first algorithm) and
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runs special <code>resolve</code> function on each field. This <code>resolve</code> function is provided by you as a part of
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<a href="../type-system/object-types/#field-configuration-options">field definition</a>.</p>
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<p>Result returned by <code>resolve</code> function is directly included in response (for scalars and enums)
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or passed down to nested fields (for objects).</p>
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<p>Let's walk through an example. Consider following GraphQL query:</p>
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<pre><code class="graphql">{
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lastStory {
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title
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author {
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name
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}
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}
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}
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</code></pre>
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<p>We need Schema that can fulfill it. On the very top level Schema contains Query type:</p>
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<pre><code class="php">$queryType = new ObjectType([
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'name' => 'Query',
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'fields' => [
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'lastStory' => [
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'type' => $blogStoryType,
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'resolve' => function() {
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return [
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'id' => 1,
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'title' => 'Example blog post',
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'authorId' => 1
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];
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}
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]
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]
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]);
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</code></pre>
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<p>As we see field <code>lastStory</code> has <code>resolve</code> function that is responsible for fetching data.</p>
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<p>In our example we simply return array value, but in real-world application you would query
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your database/cache/search index and return result.</p>
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<p>Since <code>lastStory</code> is of complex type <code>BlogStory</code> this result is passed down to fields of this type:</p>
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<pre><code class="php">$blogStoryType = new ObjectType([
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'name' => 'BlogStory',
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'fields' => [
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'author' => [
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'type' => $userType,
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'resolve' => function($blogStory) {
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$users = [
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1 => [
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'id' => 1,
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'name' => 'Smith'
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],
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2 => [
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'id' => 2,
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'name' => 'Anderson'
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]
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];
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return $users[$blogStory['authorId']];
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}
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],
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'title' => [
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'type' => Type::string()
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]
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]
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]);
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</code></pre>
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<p>Here <code>$blogStory</code> is the array returned by <code>lastStory</code> field above. </p>
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<p>Again: in real-world applications you would fetch user data from datastore by <code>authorId</code> and return it.
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Also note that you don't have to return arrays. You can return any value, <strong>graphql-php</strong> will pass it untouched
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to nested resolvers.</p>
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<p>But then the question appears - field <code>title</code> has no <code>resolve</code> option. How is it resolved?</p>
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<p>The answer is: there is default resolver for all fields. When you define your own <code>resolve</code> function
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for a field you simply override this default resolver.</p>
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<h1 id="default-field-resolver">Default Field Resolver</h1>
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<p><strong>graphql-php</strong> provides following default field resolver:</p>
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<pre><code class="php">function defaultFieldResolver($source, $args, $context, ResolveInfo $info)
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{
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$fieldName = $info->fieldName;
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$property = null;
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if (is_array($source) || $source instanceof \ArrayAccess) {
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if (isset($source[$fieldName])) {
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$property = $source[$fieldName];
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}
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} else if (is_object($source)) {
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if (isset($source->{$fieldName})) {
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$property = $source->{$fieldName};
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}
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}
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return $property instanceof \Closure ? $property($source, $args, $context) : $property;
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}
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</code></pre>
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<p>As you see it returns value by key (for arrays) or property (for objects). If value is not set - it returns <code>null</code>.</p>
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<p>To override default resolver - use:</p>
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<pre><code class="php">GraphQL\GraphQL::setDefaultFieldResolver($myResolverCallback);
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</code></pre>
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<h1 id="default-field-resolver-per-type">Default Field Resolver per Type</h1>
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<p>Sometimes it might be convenient to set default field resolver per type. You can do so by providing
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<a href="../type-system/object-types/#configuration-options">resolveField option in type config</a>. For example:</p>
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<pre><code class="php">$userType = new ObjectType([
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'name' => 'User',
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'fields' => [
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'name' => Type::string(),
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'email' => Type::string()
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],
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'resolveField' => function(User $user, $args, $context, ResolveInfo $info) {
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switch ($info->fieldName) {
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case 'name':
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return $user->getName();
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case 'email':
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return $user->getEmail();
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default:
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return null;
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}
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}
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]);
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</code></pre>
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<p>Keep in mind that <strong>field resolver</strong> has precedence over <strong>default field resolver per type</strong> which in turn
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has precedence over <strong>default field resolver</strong>.</p>
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<h1 id="solving-n1-problem">Solving N+1 Problem</h1>
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<p>Since: 9.0</p>
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<p>One of the most annoying problems with data fetching is so-called <a href="https://secure.phabricator.com/book/phabcontrib/article/n_plus_one/">N+1 problem</a>.</p>
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<p>Consider following GraphQL query:</p>
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<pre><code>{
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topStories(limit: 10) {
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title
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author {
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name
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email
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}
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}
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}
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</code></pre>
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<p>Naive field resolution process would require up to 10 calls to underlying data store to fetch authors for all 10 stories.</p>
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<p><strong>graphql-php</strong> provides tools to mitigate this problem: it allows you to defer actual field resolution to later stage
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when one batched query could be executed instead of 10 distinct queries.</p>
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<p>Here is an example of <code>BlogStory</code> resolver for field <code>author</code> that uses deferring:</p>
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<pre><code class="php">'resolve' => function($blogStory) {
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MyUserBuffer::add($blogStory['authorId']);
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return new GraphQL\Deferred(function () use ($blogStory) {
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MyUserBuffer::loadOnce();
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return MyUserBuffer::get($blogStory['authorId']);
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});
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}
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</code></pre>
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<p>In this example we fill up buffer with 10 author ids first. Then <strong>graphql-php</strong> continues
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resolving other non-deferred fields until there are none of them left.</p>
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<p>After that it calls <code>Closures</code> wrapped by <code>GraphQL\Deferred</code> which in turn load all buffered
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ids once (using SQL IN(?), Redis MGET or other similar tools) and return final field value.</p>
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<p>Originally this approach was advocated by Facebook in their <a href="https://github.com/facebook/dataloader">Dataloader</a>
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project.</p>
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<p>This solution enables very interesting optimizations at no cost. Consider following query:</p>
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<pre><code>{
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topStories(limit: 10) {
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author {
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email
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}
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}
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category {
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stories(limit: 10) {
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author {
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email
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}
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}
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}
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}
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</code></pre>
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<p>Even if <code>author</code> field is located on different levels of query - it can be buffered in the same buffer.
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In this example only one query will be executed for all story authors comparing to 20 queries
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in naive implementation.</p>
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<h1 id="async-php">Async PHP</h1>
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<p>Since: 9.0</p>
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<p>If your project runs in environment that supports async operations
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(like <code>HHVM</code>, <code>ReactPHP</code>, <code>Icicle.io</code>, <code>appserver.io</code> <code>PHP threads</code>, etc) you can leverage
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the power of your platform to resolve fields asynchronously.</p>
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<p>The only requirement: your platform must support the concept of Promises compatible with
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<a href="https://promisesaplus.com/">Promises A+</a> specification.</p>
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<p>To enable async support - set adapter for promises:</p>
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<pre><code>GraphQL\GraphQL::setPromiseAdapter($adapter);
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</code></pre>
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<p>Where <code>$adapter</code> is an instance of class implementing <code>GraphQL\Executor\Promise\PromiseAdapter</code> interface.</p>
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<p>Then in your <code>resolve</code> functions you should return <code>Promises</code> of your platform instead of
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<code>GraphQL\Deferred</code> instances.</p>
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<p>Platforms supported out of the box:</p>
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<ul>
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<li><a href="https://github.com/reactphp/react">ReactPHP</a> (requires <strong>react/promise</strong> as composer dependency):
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<code>GraphQL\GraphQL::setPromiseAdapter(new GraphQL\Executor\Promise\Adapter\ReactPromiseAdapter());</code></li>
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<li>HHVM: TODO</li>
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</ul>
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<p>To integrate other platform - implement <code>GraphQL\Executor\Promise\PromiseAdapter</code> interface. </p>
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