Drupal・3 min read・Posted on July 5, 2024

# How to alter the exception message thrown from core REST API

 

 

 

 ![How to alter the exception message thrown from core REST API](/sites/default/files/remediation-media/6687bafa19a6fe02e7605326_How_to_alter_error_message_of_core_Rest_API_1.avif) 

 



 

   Table of contents  - [Introduction](#introduction)
- [Example](#example)
- [Steps to alter error messages thrown from core REST API](#steps-to-alter-error-messages-thrown-from-core-rest-api)
- [Conclusion](#conclusion)
- [Reference links](#reference-links)
 
  

 

## Introduction

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Recently, I was working with a REST API where I needed to register a user account. I decided to reuse the core user registration REST API, however, the only problem was that the error messages were not user-friendly.

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At that time, I felt the need to alter the exception messages thrown by the core REST API. I got the opportunity to explore the REST API workflow and discovered the 'Event Subscriber' approach that could be used.

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### Example

Let’s take a very generic scenario: a user tries to create an account with an email ID that already exists. When the REST API endpoint is hit at the backend, a more user-friendly exception message is thrown from the core user registration API using the 'Event Subscriber' approach through customization.

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Default core user registers Rest API thrown error message:   
 "message": "Unprocessable Entity: validation failed.\\nmail: email address nia@gmail.com is already taken.\\n"s

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Custom thrown error message for core user register REST API: By using event subscriber approach: see attached screencast for demo:  
 "message": "The email address nia@gmail.com is already taken."

  ![‍Custom  thrown error message ](/sites/default/files/c3-body-media/6730b7cc4d5c788974b89cbd.avif) ## Steps to alter error messages thrown from core REST API

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Steps to alter the error message thrown from core REST API by using the “event subscriber” approach:

  
1\. Create a new service: “custom\_rest\_validation.custom\_rest\_validation\_subscriber” in which tags key pass event\_subscriber & class key pass name of the class that needs to call when this specified event subscriber calls.

  ![Creation of new service](/sites/default/files/c3-body-media/439474bf96129f6f4324c03d.avif)   
Note: Services are defined in a file called 'custom\_rest\_validation

.services.yml', assuming that your module is named 'custom\_rest\_validation. This file should be located in the root directory of your module project.   
  
2\. Create an event subscriber class that is attached to the newly created service in the above-mentioned step.

  ![Creation of an event subscriber class](/sites/default/files/c3-body-media/0e1d850c953e06b414516fe6.avif)   
Here, getPriority() handles the exceptions we want for JSON calls before core subscribers (with priority -70/-75). Therefore, it is given a higher priority value (-69) than the core subscriber.

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  ![getPriority() handleing the exceptions ](/sites/default/files/c3-body-media/c97c8ac4a105a5ad79919e16.avif)   
Here, onException() handles errors for this subscriber and is used to throw custom exception messages. A condition is added so that the custom exception is thrown only for the user registration API and for the email field.

  ![onException() handling errors](/sites/default/files/c3-body-media/b0f6a6aa9da672a032e40811.avif) Here call the custom parseErrorMsesage($msg) conditionally, which returns a human-readable user-friendly error message “The email address example@gmail.com is already taken.” if the user enters an existing email while creating a new user account.

  ![Human-readable user-friendly error message](/sites/default/files/c3-body-media/82bdaf61f8f8bd630fd8a48d.avif) ‍

Note: The event subscriber class CustomExceptionSubscriber.php will be placed in the following folder structure: root\_folder\_module/src/EventSubscriber/.

  
We can alter the exception message of the core REST API by extending an "event subscribe" approach.

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## Conclusion

This approach allows you to utilize the core REST API implementation and integrate desired functionality to provide more human-readable, user-friendly exception messages to end users, thereby enhancing the user experience. The following actions are implemented using the REST API:

- Get data from the endpoint (GET request)
- Create data at the endpoint (POST request)
- Update data at the endpoint (PATCH request)
- Delete data from the endpoint (DELETE request)

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## Reference links

**‍** For more details about the REST API, you can follow the [GitHub demo link](https://github.com/pooja-sharma-qed42/custom_rest_validation). For example, in this demo, I have altered the error message of the core user registration API. If a user enters an existing email ID, a custom exception message will be thrown instead of the default error message from the core REST API for user registration.

Rest API:[ documentation](https://www.drupal.org/docs/drupal-apis/restful-web-services-api/restful-web-services-api-overview)

 



Written by

Pooja Sharma

Backend Engineer

 

 

 

 

 

 

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