The bloody arena – Elmah and custom error pages.

 

Let’s start with a great quote from TR Roosevelt – a man who knew the value of a little drama:

It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat.

  • T.R. Roosevelt

Ties in well with Plato’s “Be kind, for everyone you meet is fighting a great battle”, no?

Elmah – the courageous “little” error logging handler that could

Kind of a jump but let’s skip from the bloody arena to the even bloodier arena of exception handling. Scott H has been very vocal in how unfair it is on the amount of attention ELMAH hasn’t gotten. And the excellent CodingHorror site (GOD bless you sirs!) notes the compelling arguments behind exception driven development – where teams use exception logs as a defacto to-do list, checking them almost hourly:

  • 5-10% of application code is devoted to error handling. That’s significant – it’s not so “little” in terms of impact – and by its nature often isn’t reliably tested. So, the golden rule is –when a problem occurs in your application, always check first that the error was handled appropriately. If it wasn’t, always fix the handling code first. Always fix error handling before fixing errors. It’s like the airline rule of putting an oxygen mask on yourself before putting one on your child.
  • Broad-based trend analysis of error reporting data shows that 80% of customer issues can be solved by fixing 20% of the top-reported bugs. Even addressing 1% of the top bugs would address 50% of the customer issues. The same analysis results are generally true on a company-by-company basis too.
  • If I fix a bug that no actual user will ever encounter, what have I actually fixed? While I love TDD, as a bug-fixing mechanism its too much like premature optimization… I’d rather fix bugs that are problems in practice rather than theory. (Hmmmmm.) Bug-driven development has the inherent shortcoming of being hard to test/reproduce and crisis-driven. Like the guy with the leaky roof – when its raining, you can’t fix it because its raining out, and when it isn’t raining, there’s no leak!
  • Exception logs are possibly the most powerful form of feedback your customers can give you. It’s feedback based on shipping software that you don’t have to ask or cajole users to give you. Nor do you have to interpret your users’ weird, semi-coherent ramblings about what the problems are. The actual problems, with stack traces and dumps, are collected for you, automatically and silently. Exception logs are the ultimate in customer feedback.

 

Note that Scott’s blog, normally oh-so-reliable, only shows part of the steps you’ll need to get Elmah up and running for your app. Microsoft (in a rare exception) actually has a good post on start-to-finish implementation of Elmah.

Basically we’ll be following these steps:

  1. Download ELMAH and add Elmah.dll to your web app.
  2. Register ELMAH’s HTTP Modules and Handler in web.config
  3. Specify config options
  4. Create error log source infrastructure

 

Let’s get started. Go into NuGet and select ELMAH:

 

 

Let’s check our Web.config. What changes do we see?

A new HTTP handler has been configured in your application for consulting the

error log and its feeds. It is reachable at elmah.axd under your application

root. If, for example, your application is deployed at http://www.example.com,

the URL for ELMAH would be http://www.example.com/elmah.axd. You can, of

course, change this path in your application’s configuration file.

 

If I was to look in packages.config I see the following two lines (and a new “Elmah” node under References):

<package
id=elmah
version=1.2.2
targetFramework=net45 />

<package
id=elmah.corelibrary
version=1.2.2
targetFramework=net45 />

 

So far so good. Let’s get this actually hooked up to our app. I pulled the latest source code from the code site for Google and opened up SqlServer.sql, and ran the footer of the script on my local database instance. This created one table and three sprocs – nice and neat:

  • Elmah_Error (table)
  • Elmah_GetErrorXml (sproc)
  • Elmah_GetErrorsXml (sproc)
  • Elmah_LogError (sproc)

 

Now I add the following to web.config:

 

<configuration>

<configSections>

<!– For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 –>

<section
name=entityFramework
type=System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
requirePermission=false />

 

 

<sectionGroup
name=elmah>

<section
name=security
requirePermission=false
type=Elmah.SecuritySectionHandler, Elmah/>

<section
name=errorLog
requirePermission=false
type=Elmah.ErrorLogSectionHandler, Elmah />

<section
name=errorMail
requirePermission=false
type=Elmah.ErrorMailSectionHandler, Elmah />

<section
name=errorFilter
requirePermission=false
type=Elmah.ErrorFilterSectionHandler, Elmah/>

</sectionGroup>

 

</configSections>

 

 

To the location path where I set up folder-level rolebased security, I added this node:

 

<!– AND lock down our exception logging page –>

<location
path=elmah.axd>

<system.web>

<authorization>

<allow
roles=Admin />

<deny
users=* />

</authorization>

</system.web>

</location>

 

 

 

… and right above system.web I added this node

 

<!– This must be on same level as <system.web> nod. NOTE – allowRemoteAccess means its visible remotely, a potential security risk

dev points to our database instance –>

<elmah>

<security
allowRemoteAccess=1 />

<errorLog
type=Elmah.SqlErrorLog, Elmah
connectionStringName=dev />

<errorMail
from=support.perceptive@microsoft.com


to=v-davhar@XXX.com


subject=Perceptive Site Runtime Error


async=true />

 

<!– to filter

<errorFilter>

<test>

<equal binding=”HttpStatusCode” value=”404″ type=”Int32″ />

</test>

</errorFilter> –>

 

</elmah>

 

<system.web>

 

Under <system.webserver> I added the following:

<system.webServer>

<validation
validateIntegratedModeConfiguration=false />

 

<modules>

<remove
name=FormsAuthenticationModule />

<add
name=Elmah.ErrorLog
type=Elmah.ErrorLogModule, Elmah
preCondition=managedHandler />

<add
name=Elmah.ErrorLog
type=Elmah.ErrorLogModule, Elmah
preCondition=managedHandler />

<add
name=Elmah.ErrorMail
type=Elmah.ErrorMailModule
preCondition=managedHandler />

</modules>

<handlers>

<remove
name=ExtensionlessUrlHandler-Integrated-4.0 />

<remove
name=OPTIONSVerbHandler />

<remove
name=TRACEVerbHandler />

<add
name=ExtensionlessUrlHandler-Integrated-4.0
path=*.
verb=*
type=System.Web.Handlers.TransferRequestHandler
preCondition=integratedMode,runtimeVersionv4.0 />

<!– DH added the following –>

<add
name=Elmah
path=elmah.axd
verb=POST,GET,HEAD
type=Elmah.ErrorLogPageFactory, Elmah
preCondition=integratedMode />

</handlers>

</system.webServer>

Above, if this was targeting an IIS6 webserver it would be under HttpHandlers and HttpModules.

I run the project and get a very … not too informative page. Let’s try to throw an error, shall we? Append a “/test” at the end of your elmah.axd call and then refresh – and you’ll see the following:

 

Clicking on the details gives us that treasured “yellow screen of death” that provides such awesome information in capturing and reproducing issues.

Let’s generate a few more just to be on the safe side. I add a few lines to my forms, one generating a DivideByZeroException within a try/catch block, the other straight up (where a naughty programmer forgot to wrap his logic in try/catch).

I get the above screen, the famed Yellow Screen of Death – and pointing the browser to elmah.axd gives me all the generated error messages. I can view them in the new ELMAH_Error table in SQL Server, and subscribe to it via a RSS feed. Very nifty!

Here’s the Catch logic I used to call the newer form of the elmah API:


catch(Exception ex)

{


//older API


//Elmah.ErrorLog.GetDefault(HttpContext.Current).Log(new Elmah.Error(ex));


//newer API


//This applies ELMAH filtering rules to exception, is subscription based (i.e. multi logger enabled)

Elmah.ErrorSignal.FromCurrentContext().Raise(ex);

}

Adding a Custom Error Page

Note that all we’ve done is trap application exceptions in one handy location. So, how do we trap these so the user is presented with a better, more friendly experience when the unexpected occurs?

Scott Mitchell put it best – “Every web application should have a custom error page. It provides a more professional-looking alternative to the Runtime Error YSOD, it is easy to create, and configuring the application to use the custom error page takes only a few moments.” Fair enough, and I WHOLEHEARTEDLY agree. So, how to add this to my app?

As my five year old would say, “Easy-peasy!” Add a new page to your application called “ErrorForm” – and link to a master page so its nice and spiffy. The code couldn’t be simpler:

<asp:Content
ID=”Content1″
ContentPlaceHolderID=”MainContent”
runat=”server”>


<h2>Oops!!!</h2>


<br
/><br
/>


<p>Looks like we’re having problems now with the application. Our site admins have been alerted: please follow the link below to go back to Home.</p>


<ul>


<li>


<asp:HyperLink
runat=”server”
ID=”lnkHome”
NavigateUrl=”~/Default.aspx”>Return to the homepage</asp:HyperLink>


</li>

 


</ul>

</asp:Content>

 

And add the following to your web.config:

<!– Remoteonly should be our default setting for local debugging on dev, Off to show explicit error messages outside of elmah –>

<customErrors
mode=RemoteOnly
defaultRedirect=~/ErrorForm.aspx />

 

 

 

More information on error filtering – i.e. only throwing “top 20” errors like 404 etc – is found here: http://code.google.com/p/elmah/wiki/ErrorFiltering. There’s some great information on ELMAH here –

 

This doesn’t mean that TDD is a waste of time and test projects are a brake on development speed. In our project, we’re remarkably weak still in not having a test project – that’s basic, and I pity the fool that doesn’t have “time” to write unit tests! This will be addressed over the next two weeks – it’s like not taking the time to buckle a seat belt before getting in and driving. Regression errors are starting to drive us crazy, and TDD/BDD or some form of this is the standard answer. That being said – THANKS ELMAH for making my life that much easier and not having to write some custom library to capture and log errors. This kind of direct feedback from the user community is too valuable to let go to waste.

And, side note, I’m loving GIMP for image editing. Cheaper than Photoshop, and much more functional than Paint.NET or Picasa. Yay open source!!!

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