How should I handle expected errors? eg. "username already exists"

I am struggling to understand how I should design the error handling parts of my code. I recently asked a similar question about how I should go about returning server error codes to the user, eg. 404 errors. I learnt that I should handle the error from within the current part of the application; seem's simple enough.

However, what should I do when I can't handle the error from the current link in the chain? For example, I may have a class that is used to manage authentication. One of it's methods could be createUser($username, $password). Edit: This method will return a user id or user object. Within that function, I need to determine if the username already exists. If this is true, how should I alert the calling code about this? Returning null instead of a user object is one way. But how do I then know what caused the error?

How should I handle errors in such a way that calling code can easily find out what caused the error? Is there a design pattern commonly used for this kind of situation?

Edit: I forgot to mention: I am using PHP.

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Solved: While many people argue that ex开发者_StackOverflow社区ceptions should not be used in this situation, I have come to the conclusion that it is the best solution.

Firstly, there is no simple, elegant alternative to exceptions. (I recon that it is not possible without a system built into the language... exceptions are already built in.)

Secondly, in response to the "exceptions should only be used for exceptional situations, and this is not one" argument: "When I call getFoo(), I actually expect to get a Foo. If I don't get it, it's by definition an exceptional event." (via, pkainulainen)

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There are a few common patterns:

1. Throw an exception.

2. Return NULL or FALSE and set a passed in reference to an error. E.g.

function createUser($user, $password, &$error)
{
      //...
      // You could use any type of object here.  This is just a simple example.
      if(uniqueKeyFailure)
      {
          $error = new UserAlreadyExists();
          return NULL;
      }
      //..
}

It could be called like:

$userCreateError = NULL;
$res = createUser($user, $pass, $userCreateError);
if($res === NULL)
{
  // do something with $userCreateError
}

3. Return NULL or FALSE and provide a get last error (e.g. curl_error).

I would recommend 1 or 2. The main reason people avoid exceptions for "unexceptional" errors like user input is performance. There is a fair amount of discussion on this, such as Performance of try-catch in php.

I do not recommend 3 as it is not reentrant.

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I often handle this kind of thing by sticking my validation inside some domain object (like a user).

For example:

<?PHP
$user->name = 'Joe';
$user->password = 'secret';

//try to save the user.
if (! $user->save()){
   $errors = $user->getMessages();
   // ... do something with error messages
}else{
   //the user object set it's own ID.
   $id = $user->id;
}

Now, a lot of stuff is abstracted behind the user. There might be whole universe of objects in there, but that's up to you.

The bigger issue here is: Why in the world would you have an authentication class create a user? Creation of user objects is probably outside of the scope of Authentication.

It might be sensible to have some method like authenticate($username,$password) return an object of type 'user' (representing the user that just authenticated), but even that is a little messy.

Instead consider something like:

<?PHP
$auth = new AuthService();
$u = new User();
$u->username='Joe';
$u->password='secret';

$authResult = $auth->authenticate($user);

if ($authResult->success){
  //$user has properties set as a side effect.
}else{
  //find out why it failed.
  $errors = $authResult->getErrors();
}

Of course, this requires that you define some kind of result-value class for AuthService::authenticate() to populate and return. But at least you don't end up with methods that sometimes return booleans and sometimes return objects, etc.

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You can return integers that are defined in a global header file with names such as DUPLICATE_USER or INVALID_PASSWORD or whatever is appropriate for your usage. Then the caller function can easily check these return codes against the global header declarations to determine what sort of error occurred.

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This would have to be done in following manner -

Create an error object with error codes in them, error text and the method which threw the error.

Pass the error object back.

You could throw exceptions back but exceptions are expensive

EDIT - Based on the OP's edit. In that case you would have to throw an exception object back.

Create an exception class with error code, error description, class name, method name. Throw the exception back.

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If you control the return type of the method, you can also change its signature to return something like an OperationResult object that would have an ErrorDescription property and a UserID or User object property.

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Easiest way: Return NULL if the validation passed, and the error message if it failed. Prevents the need to make Error classes or exceptions. Also the best for performance.

eg.

function numeric($input) {
    if (!is_numeric($input)) return 'Must be numeric.';
    return NULL;
}

function usernameavailable($input) {
    //query database....
    if ($result) return 'Username already taken, please choose another one.';
    return NULL;
}

You can use functions with parameters as well:

function length($input, $min, $max) {
    if (strlen($input) < $min) return "Must be longer than $min characters.";
    if (strlen($input) > $max) return "Must be shorter than $max characters.";
    return NULL;
}

Validating the each element in the entire form is then extremely easy. Just loop over all the elements, call all the validation functions for that element and collect the error strings, if any. If there are no error strings, then all the user input is valid.

I would strongly discourage the suggestions made by one of the other answers. Invalid user input is not Exceptional and does not require an exception or an error. It's simply an expected use case that has to be dealt with in the simplest possible manner.

With this method, you can build a Form class quite easily and simply instantiate elements with the arguments being the validation functions. You can also expand this to include AJAX validation sending asynchronous requests to the exact same PHP validation functions.

Example from my framework:

$Form = new AjaxForm();
$Form->add(new TextBox('Username', 'usernameavailable|length[3,12]'));
$Form->add(new SubmitButton());

With only three lines I've created a fully functional form with both client side and server side validation. Of course all the details are up to you but I still believe that invalid user input should be expected and not be Exceptional.

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You are expected to check if the username exists BEFORE calling the createUser() method. If you don't, or something happens between the time you have checked and called createUser(), you throw an exception. It's nowhere near as expensive as it is made out to be.

Does it have a cost? Yes. Is it big enough to matter? Most probably no.