[GraphQL] Add information about debugging

_data/toc/graphql.yml

@@ -30,6 +30,9 @@ pages:
         - label: Create a custom urlResolver service
           url: /graphql/develop/create-custom-url-resolver.html
 
+        - label: Debugging GraphQL queries
+          url: /graphql/develop/debugging.html
+
         - label: Exception handling
           url: /graphql/develop/exceptions.html
 

guides/v2.3/graphql/develop/debugging.md

@@ -0,0 +1,32 @@
+---
+group: graphql
+title: Debugging GraphQL queries
+contributor_name: Atwix
+contributor_link: https://www.atwix.com/
+---
+
+This topic provides recommendations on how to debug GraphQL requests.
+
+## Debugging with PHPStorm and Xdebug
+
+When [using GraphiQL]({{ page.baseurl }}/graphql/index.html#how-to-access-graphql) or any other client for testing GraphQL queries, you might need to debug the request processing.
+You can use Xdebug for debugging the PHP execution of a GraphQL query just as you would for other HTTP requests.
+To start debugging, add the `?XDEBUG_SESSION_START=PHPSTORM` parameter to the endpoint URL.
+The following example shows how to establish a connection between Xdebug and PHPStorm IDE.
+
+```
+http://<magento2-3-server>/graphql?XDEBUG_SESSION_START=PHPSTORM
+```
+
+You can also enable an Xdebug connection for a particular request by setting the corresponding header parameter.
+
+```
+Cookie: XDEBUG_SESSION=PHPSTORM
+```
+
+As a result, Xdebug within the PHP execution attempts to make a connection to an IDE. If the IDE is listening, it will give the instructions to Xdebug about breakpoints and other relevant information.
+
+## Related Topics
+
+* [GraphQL request headers]({{ page.baseurl }}/graphql/send-request.html)
+* [Exception handling]({{ page.baseurl }}/graphql/develop/exceptions.html)