Tuesday, July 31, 2012

Debugging ELMAH on Azure

Recently I was deploying an application to the new Azure Websites service, and I noticed a slight problem with my app.

ELMAH worked on my machine, but not in the cloud.

Needless to say, if your exception logging isn't working, then you're probably not going to sleep well at night. So my first question, naturally, was "How in the world do I see what is going on inside ELMAH?".

One thing that many folks don't realize until they need to do something like this is that logging libraries tend to "eat" many of their internal exceptions since they are usually the layer that is added to take care of exactly that. I say they "eat" their exceptions not in a strict sense, but more so because there really is no where you can do a "TRY CATCH" to see what is happening.

Tracing to the Rescue

Instead of throwing exceptions both ELMAH and Log4net output their exceptions to the System.Diagnostics.Trace object. This is sweet when debugging locally, as any application traces will automatically write to the Output window in Visual Studio. But as the title of this post reads, that doesn't help me at all.

I needed to figure out how to output the traces of my app running in Azure to some storage that I could then view. I did have Log4net working, and it was writing to an SQL Azure database, so I decided to go with that option.

First, I had to learn a few things about ASP.NET and Tracing. ASP.NET has its own tracing mechanism that is somewhat separate from the Diagnostics Tracing (which you can read about here). I could try to write the Diagnostic Trace messages out to the ASP.NET Page Trace mechanism, but in MVC (which I'm using), it doesn't really work. So, honestly my best bet is to write to a database (as writing to a file in the cloud is not very reliable) using a TraceListener class setup to redirect Trace messages to Log4net's log.

My TraceListener looked like this:

public class Log4netTraceListener : System.Diagnostics.TraceListener
    private static readonly log4net.ILog _log;

    public Log4netTraceListener()
        _log = log4net.LogManager.GetLogger(typeof(Log4netTraceListener));

    public override void Write(string message)
        if (_log != null)

    public override void WriteLine(string message)
        if (_log != null)

Now, I first tried adding my Log4netTraceListener in the web.config. However, this Listener is initialized and added to the "Trace.Listeners" collection before Log4net has initialized in my web application. So instead, I have to add it programmatically during the application startup. I did the following in my Global.asax.cs:

    System.Diagnostics.Trace.Listeners.Add(new Log4netTraceListener());

After all of that, I finally started seeing the following in my Log4net log:
System.Data.SqlClient.SqlException (0x80131904): Tables without a clustered index are not supported in this version of SQL Server. Please create a clustered index and try again.
Whoa, that is not something I expected to see. Apparently the "ELMAH_Error" table has a non-clustered primary key and no clustered index on the table (not really sure why). So we have two options:

  1. Add another column to the table and setup a clustered index on that column.
  2. The "Sequence" column is already an "int IDENTITY(1,1)" column - so just setup a clustered index on that column.
I chose option 2. And here's the change to apply after executing the default ELMAH SQL Script:

    CREATE CLUSTERED INDEX [IC_Elmah_Sequence] on dbo.[ELMAH_Error] (Sequence)

Wednesday, July 4, 2012

Getting Started with LocalDB

With the launch of SQL Server 2012, a new product called "LocalDB" was launched. This new toy is a great fit for needs that fall somewhere in the space between SQL Express and SQL Compact.

While you can read the full list of features for LocalDB, I'll highlight a few below:

  • Easy, and simple MSI to install. I've had very little problems with this, and it is light-years better than trying to setup SQL Express
  • Ability to control user access to the database
  • Supports virtually all features of SQL Express (Stored Procedures, Triggers, ... even some basic Replication)
  • Not an extra service on your machine to manage

Initially, there was some confusion as to whether LocalDB was high-powered file-based database or just a dumbed-down version of SQL Express. I'm not going to debate the semantics of what it is exactly, but I will tell you that you get your standard ".mdf" and ".ldf" files with LocalDB. However, the MSI installs some central libraries and an engine that power the LocalDB instances. The engine does have to have some knowledge as to the existence of any instances of LocalDB in order for those to work. So if you were looking for something where you can just drop a file into a directory and it will just work... not quite, but close.


LocalDB has the notion of "Automatic" and "Named" instances. Per version of LocalDB installed on your machine, there is only one "Automatic" instance per user. You can think of this as the default "SQLEXPRESS" instance that is installed when you first download and install SQL Express on a machine, only it is unique per user on the machine. So if you create an application that creates and uses the "Automatic" instance of LocalDB and this application is installed on a machine with 3 different user accounts, you will have 3 different ".mdf" and ".ldf" files after each user logs in and uses the application the first time.  This "Automatic" instance is:

  • "Public" – meaning any application process under the user's account can access the instance when logged into a machine
  • Accessed via the connection string "(localdb)\{major version}" – which for right now means you'll use "(localdb)\v11.0"


  • Great for development purposes when you need to just throw together a DB and ship something

Named Instances

The "Named" instance of LocalDB refers to private instances of a database that you can setup. These are useful if you want an application to have a separate, private database per user on a machine (a "Named" instance will run in a separate process under the current user on a machine).

Sharing Instances

So what if you want to allow multiple users on a machine to have access to a single LocalDB database instance? Well you can also "share" a "Named" instance of LocalDB. You can control who the instance is shared with, and you can enable/disable sharing at any time.

When connecting with a shared instance, your connection string will need to include an extra ".\" in the server portion. Going with the example above, one would use: "(localdb)\.\MySharedInstance". This tells the engine that this is a shared instance.

SqlLocalDb Utility

When you install LocalDB on a machine, it also installs the SqlLocalDb command line utility. It's pretty simple to use, and allows you to setup batch files to control the creation of your LocalDB's. One of the first things I do is create a "SetupDb.cmd" file in my source code folder of an app I'm building. This allows other devs to easily replicate setting up a development database on their machine in the same way.


Where Does Everything Go?

One of my first questions was, where are the actual database files?

The default is: "C:\Users\{USER}\AppData\Local\Microsoft\Microsoft SQL Server Local DB\Instances"

If I've created the "Northwind" Named instance as above, and I go to the default folder, I will see:


Inside each of these folders are your typical "master.mdf" , "temp.mdf", log files, etc... If you want to create your database's ".mdf" and ".ldf" files in a separate directory, you can easily do that by specifying the location when running a T-SQL statement to "CREATE DATABASE ...". Specifying where your data lives is not really any different than what you've done with SQL Server in the past.

If you want to point your application to specific '.mdf" or ".ldf" files at runtime, you can specify that in your connection string using the "AttachDbFileName" attribute, you can also read more here.