Recently I've been into multiple arguments on whether to throw an exception on false User Input.
Example: I'm trying to login though my account is not activated. As the programmer in an OO-language, I could handle this in a few ways. For this case, lets stick to these two:
Throw a custom Exception from the local Service with a representative way, extending Exception. Catching this in the class handling User Input.
Use a Validator to call the local Service to check whether this account is logged in.
My vision, like many others, an Exception represents a fault in the program. E.g. database unreachable, error in parsing data.
Vision of many others as well, the case of logging in without being activated is not a succesful scenario on any use case and will thus fail. This shouldn't not happen and is worth throwing an Exception for.
Personally, I would handle this kind of problem with a Validator, sticking to Exceptions for just the faults in the program. However though, I would like to get a constructive answer on which case is preferred. If possible, referring to any documentation. I'm using Java, though this problem is not restricted to any language (as long as it's OO I guess).
In case of a validation error, the flow of your application must be interrupted. For example, you must terminate a singing up progress if an invalid mail address supplied. Thus, the exceptions can be used for the purpose of user input validation.
As an example use, you can check JSF. It benefits from exception mechanism of Java to handle user input validations. The following links can be useful:
http://www.mkyong.com/jsf2/custom-validator-in-jsf-2-0/
http://www.ibm.com/developerworks/library/j-jsf3/
Related
My goal is to influence the error descriptions that appear in BizTalk Administration Console in the Error Information tab of suspended instance windows, after errors occur in my custom functoids. If possible, I would also like the ErrorReport.Description promoted property to display this error description on the failed message.
I've read everything I can find about custom functoid development, but I can't find much about error handling within them. In particular, whenever my functoids throw exceptions, I see the boilerplate "Exception has been thrown at the target of an invocation" message that occurs whenever exceptions occur through reflection, rather than the message on the exception itself.
I had hoped to find something within the BaseFunctoid class framework that would allow me to submit an error string, so as to traverse the reflection boundary. Is there some way to pass error information from within a custom functoid, such that the BizTalk Administration Console will display it?
If I emulate the approach taken by DatabaseLookupFunctoid and DatabaseErrorExtractFunctoid, is there some way I can fail the map with the extracted error, rather than mapping it to a field on the destination schema as is shown in its examples?
The simplest way to do this is using custom C#, writing something like this in your code:
System.Diagnostics.EventLog.WriteEntry("EVENT_LOG_SOURCE", "Error message...", System.Diagnostics.EventLogEntryType.Error);
As Johns-305 mentions, you need to make sure your event source is registered (e.g. System.Diagnostics.EventLog.CreateEventSource("EVENT_LOG_SOURCE", "Application") - but this should really be done as part of your installation steps with an EventLogInstaller or some kind of script to set up the environment). It's certainly true that error handling in BizTalk is just .NET error handling, but one thing to keep in mind is that maps are actually executing as XSLT, and the context in which their executing can have a major impact on how exceptions and errors will be handled, particularly unhandled exceptions.
Orchestrations
If you're executing a transform in an orchestration that has exception handling in it, the exception thrown will be handled and may even fall into additional logging you have in the orchestration - in other words, executing a throw from a C# functiod will work the way you'd think it would work elsewhere in C#. However, I try to avoid this since you never know if a map will at some point be used else where and because exception handling in XSLT doesn't always work the way you'd think (see below).
Send/Receive Ports
Unfortunately, if you're executing a map on a send or receive port and throw an exception within it, you will almost definitely get very unhelpful error message in the event log and a suspended instance in the group hub. There is no easy, straightforward way to simple "cancel" a transform - XSLT 1.0 doesn't have any specified way of doing that (see for example Throwing an exception from XSLT). That leaves you with outputting an error string to a particular node in the output (and/or to the EventLog), or writing lots of completely custom XSLT to try to validate input, or designing your schemas properly and using a validating component where necessary. For example, if you have a node that must match a particular regex, or should never be empty, or should never repeat more than X times, make sure you set those restrictions on the schema and then make sure you pass it through the XmlValidator or similar before attempting to map.
The simplest answer is, you just do.
Keep in mind, there is nothing special at all about error handling in BizTalk apps, it's just regular plain old .Net error handling.
So, what you do is catch the error in you code, write the details to the Windows Event Log (be sure to create a custom Event Source) and...that's it. That is all I do. I don't worry about what appear in BT Admin specifically.strong text
Short and simple:
How to catch pgPL/SQL exceptions with QtSQL?
I haven't found any example or info at the docs.
More details:
I'm developing the front end for a system with Qt 5.3. The database perform some validation at some of the inputs and raise custom exceptions when something is invalid. What I want is to be able to handle these exceptions at he front end code, showing appropriate messages to the user.
EDIT:
I have tried to use the QSqlError class. Although it gave me the exception, it gave me too much data which will need to be parsed to be useful. Is there a way to get the exception raised without parsing the messages?
I am trying to develop a standard when I code applications.
I was curious as to what other developers did when it comes to sql errors or general program errors. Do you output the error to the screen, write to a log file, or something else?
It really depends on the severity of the error.
Is it a show stopper?
Can the software automatically retry and get away with no message?
Can it be ignored?
You can log every exception, or just certain ones, or none. I have a custom Exception class which logs every exception created (of that type).
I have an unhandled exception handler which emails me when there is one.
I'd only send a message to the user when it will change the way the application works from the user's point of view.
Your question is a bit subjective and you would get opinion-based answers if the entire community bothered to answer.
If the error is relevant and important to the user (e.g.: invalid username/password) display it to the user using a message box.
If the error is relevant to the developer, or can be used in the debugging process, use a log or a console output.
The trick is to identify which and how the errors should be displayed to the user. You don't want to bombard the user with exceptions and complicated errors on which the user has no idea on how to act upon.
I have a class called Hotmail that contains various method such as login, logout etc.
To illustrate the confusion I'm having I have a login method that logs the user into Hotmail via my software. The login method returns a HttpWebResponse object. But, within the login method any number of things could happen such as wrong credentials being entered or a timeout.
I'm in some confusion about how, and where to handle such errors.
In the case of the wrong credentials being entered, or a timeout, it would be pointless, or sometimes not possible to return a HttpWebResponse object. What would be the best way to handle such errors?
Should I create custom Exceptions so the code that's calling the method can check for such errors and handle them?
What's the conventional way to handle these sorts of errors as I'm sure this is a common point of confusion?
Assuming that you have something like Hotmail>>login(user, password) I would definitely use exceptions. How fine grained to be with exceptions its up to you (and your domain model) and it can be hard to achieve a balance.
For this case I would definitely have exceptions for the most important events (like WrongCredentialsException) but I wouldn't have an exception class for every 4XX and 5XX response errors. However, according to your domain and personal tastes you could have a ClientException and ServerException, with an instance variable stating the error number instead of just having a ConnectionException.
HTH
I'm having a discussion with a colleague about when to throw faults and when not to throw faults in a WCF service.
One opinion is, that we only throw faults when the service operation could not do its work due to some error; and something may be in an invalid state because of it. So, some examples:
ValidateMember(string name, string password, string country)
-> would throw a fault if the mandatory parameters are not passed, because the validation itself could not be executed;
-> would throw fault if some internal error occured, like database was down
-> would return a status contract in all other cases, that specifies the result of the validation (MemberValidated, WrongPassword, MemberNotKnown,...)
GetMember(int memberId)
-> would only throw fault if something is down, in all other cases it would return the member or null if not found
The other opinion is that we should also throw faults when GetMember does not find the member, or in the case of ValidateMember the password is wrong.
What do you think?
My take on this...
There are three causes of failure:
The service code threw an exception, e.g. database error, logic error in your code. This is your fault.
The client code failed to use your service properly according to your documentation, e.g. it didn't set a required flag value, it failed to pass in an ID. This is the client software developer's fault.
The end user typed in something silly on screen, e.g. missing date of birth, negative salary. This is the end user's fault.
It's up to you how you choose to map actual fault contracts to each cause of failure. For example, we do this:
For causes 1 and 2, all the client code needs to know is that the service failed. We define a very simple "fatal error" fault contract that contains only a unique error ID. The full details of the error are logged on the server.
For cause 3, the end user needs to know exactly what he/she did wrong. We define a "validation errors" fault contract containing a collection of friendly error messages for the client code to display on screen.
We borrow the Microsoft EntLib class for cause 3, and use exception shielding to handle causes 1 and 2 declaratively. It makes for very simple code.
To Clarify:
We handle the three causes like this inside the service:
An unexpected exception is thrown in the service code. We catch it at the top level (actually exception shielding catches it, but the principle is the same). Log full details, then throw a FaultException<ServiceFault> to the client containing only the error ID.
We validate the input data and deliberately throw an exception. It's normally an ArgumentException, but any appropriate type would do. Once it is thrown, it is dealt with in exactly the same way as (1) because we want to make it appear the same to the client.
We validate the input data and deliberately throw an exception. This time, it's a FaultException<ValidationFault>. We configure exception shielding to pass this one through un-wrapped, so it appears on the client as FaultException<ValidationFault> not FaultException<ServiceFault>.
End result:
No catch blocks at all inside the service (nice clean code).
Client only has to catch FaultException<ValidationFault> if it wants to display messages to the user. All other exception types including FaultException<ServiceFault> are dealt with by the client's global error handler as fatal errors, since a fatal error in the service generally means a fatal error in the client as well.
It it is a common, routine failure, then throwing a fault is a mistake. The software should be written to handle routine items, like entering the wrong password. Fault processing is for exceptional failure which are not considered part of the program's normal design.
For example, if your program was written with the idea that it always has access to a database, and the database is not accessible, that's an issue where the "fix" is well outside of the limits of your software. A fault should be thrown.
Fault processing uses different logical flows through the structure of the programming language, and by using it only when you've "left" the normal processing of the programming problem, you will make your solution leverage the feature of the programming language in a way that seems more natural.
I believe it is good practice to separate error handling and fault handling. Any error case should be dealt by your program - fault handling is reserved for exceptional conditions. As a guide to the separation of the two I found it useful when considering such cases to remember that there are only three types of error (when handling data and messages) and only one type of fault.
The error types are related to different types of validation:
Message validation - you can determine from the message contents that the data is valid or invalid.
Example: content that is intended to be a date of birth - you can tell from the data whether it is valid or not.
Context validation - you can only determine that content is invalid by reference to the message
combined with the system state.
Example: a valid date of joining a company is earlier than that persons date of birth.
Lies to the system - you can only determine that a message was in error when a later message
throws up an anomaly.
Example: Valid date of birth stored and inspection of the person's birth certificate shows this to be incorrect. Correction of lies to the system generally require action outside of the system, for instance invoking legal or disciplinary remedies.
Your system MUST deal with all classes of error - though in case three this may be limited to issuing an alert.
Faults (exceptions) by contrast only have one cause - data corruption (which includes data truncation). Example: validation parameters are not passed.
Here the appropriate mechanism is fault or exception handling - basically handing off the problem to some other part of the system that is capable of dealing with it (which is why there should be an ultimate destination for unhandled faults).
In the old days we used to have a rule that exceptions were only for exceptional and unexpected things. One of the reasons you did not want to use them too much was that they "cost" alot of computing power.
But if you use exceptions you can reduce the amount of code, no need for alot of if else statements, just let the exception bubble up.
It depends on your project. The most important thing is that there is a project standard and everyone does it the same way.
My opinion is that exceptions/fault should be thrown whenever what the method is supposed to do can't be achieved. So validation logic should never raise exception except if the validation can't be made (i.e. for technical reasons) but never just because the data are not valid (in that case it will return validation codes/messages or anything helping the caller to correct the data).
Now the GetMember case is an interesting one because it's all about semantic. The name of the method suggest that a member can be retrieved by passing an id (compare to a TryGetMember method for exemple). Of course the method should not throw the same exception if the id is nowhere to be found or if the database does not respond but a wrong id passed to this method is probably the sign that something is going wrong somewhere before that call. Except if the user can directly enter a member-id from within the interface in which case a validation should occurred before calling the method.
I hear a lot about the performance issue. I just made a simple test using C# and trow/catch 1000 exceptions. The time it took is 23ms for 1K Exeptions. That's 23ยต per exception. I think performance is no longer the first argument here except if you plan to raise more than 2000 exception per second in which case you will have a 5% performance down, which I can start considering.
My humble opinion...