Month: March 2012

These are my own findings whilst developing my first application to be hosted in the cloud. The Azure features I used were Azure SQL, Federated Authentication, Web roles, Worker roles and Queues.

Handling transients from SQL Server

Beware of transient connection issues when connecting to your Azure database. These conditions occur due to normal network related errors, throttling due to excessive requests and controlled load balancing by the AppFabric framework.

Fortunately these types of errors raise specific error codes which can be trappable from your code.  Below are some errors codes to expect.

When developing code to use SQL Azure you should code for transient connection issues. To make things easy, the team from Microsoft patterns and practices have developed  the Microsoft Enterprise Library 5.0 Integration Pack for Windows Azure library.

Logging your application

Its important to add logging to any application deployed to Azure. I use system.diagnostics debug and trace statements extensively throughout my application to emit messages to blob storage for debugging my application at runtime.

To enable some basic logging you need to wire up the Azure diagnostics listener in the app.config file as shown below.

<system.diagnostics>
        <trace>
            <listeners>
                <add type="Microsoft.WindowsAzure.Diagnostics.DiagnosticMonitorTraceListener, 
                     Microsoft.WindowsAzure.Diagnostics, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
                    name="AzureDiagnostics">
                    <filter type="" />
                </add>
            </listeners>
        </trace>
    </system.diagnostics>

Add a new setting called Microsoft.WindowsAzure.Plugins.Diagnostics.ConnectionString to the role file under the Azure project and set the value to your Azure storage account.

Then add the following code in the OnStart method to persist the logs to a table called WADLogsTable in Azure storage. The data will be written to storage every minute which is configurable as shown in the code below.

public override bool OnStart()

       {

           // Set the maximum number of concurrent connections 

           ServicePointManager.DefaultConnectionLimit = 12;

           TimeSpan tsOneMinute = TimeSpan.FromMinutes(1);

           DiagnosticMonitorConfiguration dmc = DiagnosticMonitor.GetDefaultInitialConfiguration();

           // Transfer logs to storage every minute

           dmc.Logs.ScheduledTransferPeriod = tsOneMinute;

           // Transfer verbose, critical, etc. logs

           dmc.Logs.ScheduledTransferLogLevelFilter = LogLevel.Verbose;

           // Start up the diagnostic manager with the given configuration

           DiagnosticMonitor.Start("Microsoft.WindowsAzure.Plugins.Diagnostics.ConnectionString", dmc);

           // For information on handling configuration changes

           // see the MSDN topic at http://go.microsoft.com/fwlink/?LinkId=166357.

           RoleEnvironment.Changing += RoleEnvironmentChanging;

           return base.OnStart();

       }

       private void RoleEnvironmentChanging(object sender, RoleEnvironmentChangingEventArgs e)

       {

           // If a configuration setting is changing 

           if (e.Changes.Any(change => change is RoleEnvironmentConfigurationSettingChange))

           {

               // Set e.Cancel to true to restart this role instance 

               e.Cancel = true;

           }

       }

Now in your code use the Trace or Debug statements to log any information you require as shown below.

 Trace.WriteLine("Entry point called");

Compile Messages

Always review the warnings emitted after compiling your solution. In one of my projects I was referencing an assembly that was not part of the default core installation of the .Net framework. Luckily I noticed the warning message in the output window.

Ensure you set the Copy Local to true if you need to include a referenced DLL that is required to be packaged with the solution before deploying to Azure.

Viewing blob and table storage within VS2010

You can view both Blob and Table storage contents within Visual Studio 2010 after installing the Windows Azure Tools for VS2010.

From the menu bar choose View and then Server Explorer. This will open the server explorer window where you can add a new storage account as shown below.

Once you have added the account details, you will be able to view the contents of your stores.

Message Queues

One important thing to remember about Azure queues after you get the message, you must exclusively delete it from the queue after your process finishes. If not the message will reappear on the queue after a configurable period.

The reason why your message reappears is your instance may get shut down half way through processing your data by the Azure AppFabric and then you are at risk of losing data before completing the process. Hence you should delete the message from the queue at the end of your process and not soon after getting the message off the queue.

Also remember the order of delivery is not guaranteed.

You must always build logic in your code to take into account that your process may get shutdown at any time.

Windows Live Federated Credentials

The Windows Live authentication only provides a “token string” as one of the SAML claims. Note that is token is unique to your application. If you need to get access to the users email address, you will have to capture that information within your application and persist it yourself.