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kotlinlang

Hi everyone! I am currently working on implementing custom error handling and logging using GraphQl and Kotlin on a federated service. Does anyone have a current example on how to do this? The examples I've found online seem to use deprecated classes.

which library are you using? if you are using `graphql-kotlin` or `graphql-java` it should be the same as in non-federated service
• for error handling you probably want to specify your own `DataFetcherExceptionHandler`
• for logging (if it is something that you want to configure globally) you could use instrumentation

Hi Dariusz and thanks for the response. We are using `graphql-kotlin` I've tried implementing `DataFetcherExceptionHandler`without success.

On the application properties I've set: `exception-handlers-enabled: true`

I've also created a class to handle exceptions like this:

```@Component
class CustomGraphQlErrorHandler: DataFetcherExceptionHandler {
    private val logger = LoggerFactory.getLogger(CustomGraphQlErrorHandler::class.java)

    @ExceptionHandler
    override fun onException(handlerParameters: DataFetcherExceptionHandlerParameters?): DataFetcherExceptionHandlerResult {
        val exception= handlerParameters?.exception
        val sourceLocation= handlerParameters?.sourceLocation
        val path= handlerParameters?.path

        val error = ExceptionWhileDataFetching(path, exception, sourceLocation)

        return DataFetcherExceptionHandlerResult.newResult().error(error).build()
    }
}```


unsure what is the `exception-handlers-enabled` property as it is not something we use within the library

as for the error handler it looks correct (<https://github.com/ExpediaGroup/graphql-kotlin/blob/master/graphql-kotlin-spring-server/src/main/kotlin/com/expediagroup/graphql/spring/exception/KotlinDataFetcherExceptionHandler.kt>)

are you using `graphql-kotlin-spring-server`?

yes, we are using `graphql-kotlin-spring-server`
I'm not sure if the way I'm trying to trigger that code may be wrong though.
What I'm currently doing is trying to retrieve a non-existing field in our schema. And that does triggers a validation error, but the code in the handler doesn't seem to execute.

If I throw an Exception in the resolver then the onHandle method executes just fine, but in cases where schema validation fails, it's not executed. Particularly we want to be able to change the message contents in errors like this:

```{
  "errors": [
    {
      "message": "Validation error of type FieldUndefined: Field 'categoryNames' in type 'Category' is undefined @ 'getCategoryById/categoryNames'",
      "locations": [
        {
          "line": 4,
          "column": 5
        }
      ]
    }
  ]
}```


query validation happens before any of the exception handlers

unsure if there is a way to customize those validation messages

we are relying on `graphql-java` logic there so you can take a look at their documentation but I don’t recall any way to customize validation exception handling

I'll take a look at their documentation then. Thank you so much Dariusz!