Exception Hierarchy

Background

Exceptions are good - they simplify error handling and increase readability. 

Rules

• Be aware of your exception hierarchy, and how it changes across the application layers. 

• Catch exceptions early, and transform them from very specific low level exceptions to more generic, high level ones.

Implementation hints

Have base exceptions you’ll use all over the application

• ‘temporary problem’

• ‘permanent problem’

  • ‘uncorrectable failure’

  • ‘critical internal failure’

Create exceptions inheriting from these base exceptions to constrain possible exceptions being thrown from services:

• example

  • URLFetchingService with GET method 

• exception hierarchy

  • BaseTemporaryErrorException

    • URLFetchingRemoteNotAvailableException - for 5xx and connection problems

  • BasePermanentErrorException

    • BaseCriticalInternalFailureException

      • URLFetchingCriticalInternalFailureException - for any situations that could indicate that something’s very wrong (eg. can’t serialize passed DTO to JSON) - so that we as developers are informed about this

    • URLFetchingNotFoundException - for 404

    • URLFetchingBadURIException - for internal axios exceptions dealing with uri

• now an example service that enables user to fetch remote urls would look like:

try {
    return URLFetchingService.get(uri, params);
} catch (e) {
    if (e instanceof BaseTemporaryErrorException) {
        return retry();
    } else if (e instanceof URLFetchingBadURIException) {
        this.addErrors({uri: ’Wrong URI format.’});
    } else if (e instanceof URLFetchingNotFoundException) {
        this.addErrors({nonfield: ’Resource is long gone.’});
    } else if (e instanceof BasePermanentErrorException) {
        this.onFailure();

        // this will:
        //  - fire up a notification
        //  - notify us if e is BaseCriticalInternalFailureException
        ErrorService.dealWith(e); 
    } else {
        throw e;
    }
}