Unable to properly handle errors - scala

Question

I have found the reason why my application isn't behaving the way I would like it to but I don't know how to resolve the issue. To summarise, my application has a custom error handler which gets invoked if there is an error. The error handler sends json messages. But in one application startup error scenario (Future fails), I want to Redirect the user to homepage instead of sending a json message.but it doesn't happen because the custom error handler sends json message before I can send a Redirect from Future's recover.
One feature of the application is signup verification. The user clicks on a url which has a token. When the url is clicked, verifyUser Action is called. It does some checks (using database queries using Futures) and depending on the success or failure, it sends Redirect with signup=success or signup=error attribute (not failure here is decided based on whether something exists in the database or not). However, if the Future fails (I queried a wrong field which is not part of database's schema), I want to Redirect again but it doesn't work as the custom error handler gets invoked before recover. How could I make my application Redirect?
val result:Future[Result] = for{tokenOption:Option[UserToken] <- userTokenRepo.findOne(UserTokenKey(UUID.fromString(token))) //generator 1 - get token from database
userOption:Option[User] <- if (tokenOption.isDefined) userRepo.findOne(tokenOption.get.userKeys) else Future.successful(None) //generator2. found token, look for corresponding user to which the token belongs
modifiedUser:Option[User] <- if (userOption.isDefined) confirmSignupforUser(userOption.get) else Future.successful(None) //generator 3. found user and token. Update profile
deletedToken:Option[UserTokenKey] <- if(modifiedUser.isDefined) userTokenRepo.delete(UserTokenKey(UUID.fromString(token))) else Future.successful(None)
}
yield { //check if we have user and token and modified user here. If any is missing, return error else success
println("db query results tokenOption: "+tokenOption+", userOption: "+userOption+" : modifiedUserOption: "+modifiedUser+", deletedToken: "+deletedToken)
if(tokenOption.isDefined && userOption.isDefined && modifiedUser.isDefined && deletedToken.isDefined)
Redirect("http://localhost:9000/home"+";signup=success")//TODOM - pick from config
else
/*TODOM - when redirecting with error, can provide additional info why sign up failed*/
if(tokenOption.isEmpty)
Redirect("http://localhost:9000/home"+";signup=error")//TODOM - pick from config
else if(userOption.isEmpty)
Redirect("http://localhost:9000/home"+";signup=error")//TODOM - pick from config
else if(modifiedUser.isEmpty)
Redirect("http://localhost:9000/home"+";signup=error")//TODOM - pick from config
else //this shouldn't happen. Unexpected
Redirect("http://localhost:9000/home"+";signup=error")//TODOM - pick from config
}
result.recover { case x => {
println("Future failed in validateUserSession. Recovering. Returning Internal Server Error" + x)
//before this Redirect, the custom error handler sends json response
Redirect("http://localhost:9000/home"+";signup=error")//TODOM - pick from config
}
}
Custom error handler
class CustomHttpErrorHandler extends HttpErrorHandler {
def onClientError(request: RequestHeader, statusCode: Int, message: String) = {
println("client error: request "+request+", statusCode: "+statusCode+", message:"+message)
Future.successful(
Status(statusCode)(Json.toJson(JsonResultError(message)))
)
}
def onServerError(request: RequestHeader, exception: Throwable) = {
println("server error: request: "+request+", exception: "+exception.getMessage)
Future.successful(
InternalServerError(Json.toJson(JsonResultError(exception.getMessage)))
)
}
}
I am able to verify the isse as I see two debugs (one from custom error handler and other from recover)
server error: request: GET /ws/users/signup/312c9eaf-f27b-43c7-8dac-445a628c3be8, exception: bucket_id is not a column defined in this metadata
Future failed in validateUserSession. Recovering. Returning Internal Server Errorjava.lang.IllegalArgumentException: bucket_id is not a column defined in this metadata
I could try to check based on the exception received in custom error handler but I think it is too generic and might not be a good design approach.

Answer 1

It is possible that the error is happening before the Future is created so the code is throwing an exception that is handled by the default handler rather than being caught by the Future.
Specifically, the expression
userTokenRepo.findOne(UserTokenKey(UUID.fromString(token)))
is evaluated in the current thread, so any exception in this code will not be caught and will invoke the default error handler.
The solution is to compute this in a Try and process the error immediately if there is one.
This is what it might look like:
for {
tokenKey <- Future.fromTry(Try(UserTokenKey(UUID.fromString(token))))
tokenOption <- userTokenRepo.findOne(tokenKey)
userOption <- tokenOption.fold(Future.successful)(userRepo.findOne(_.userKeys)) //generator2. found token, look for corresponding user to which the token belongs
modifiedUser <- userOption.fold(Future.successful)(confirmSignupforUser) //generator 3. found user and token. Update profile
...
Any exception in this code will result in a failed Future which your recover code will handle.