Using SetConfigurationSettingPublisher()

The CloudStorageAccount class in the Windows Azure Storage Client library encapsulates a StorageCredential instance that can be used to authenticate against the Windows Azure Storage Service. It also exposes a FromConfigurationSetting() factory method that creates a CloudStorageAccount instance from a configuration setting.

This method has caused much confusion since, without additional configuration, it throws an InvalidOperationException with a message of "SetConfigurationSettingPublisher needs to be called before FromConfigurationSetting() can be used." Consequently, before using FromConfigurationSetting(), it is necessary to invoke SetConfigurationSettingPublisher() once. The intent of this method is that it can be used to specify alternate ways of retrieving the data connection string that FromConfigurationSetting() uses to initialize the CloudStorageAccount instance. This setting is process-wide, so is typically done in the OnStart() method of the RoleEntryPoint class for the role.

The following is a simple implementation for SetConfigurationSettingPublisher():

CloudStorageAccount.SetConfigurationSettingPublisher((configName, configSetter) =>
{
   configSetter(RoleEnvironment.GetConfigurationSettingValue(configName));
});

There are several levels of indirection here, but the central feature is the use of a method that takes a String parameter specifying the name of the configuration setting and returns the value of that setting. In the example here, the method used is RoleEnvironment.GetConfigurationSettingValue(). The configuration-setting publisher can be set to retrieve configuration settings from any location including app.config or web.config.

The use of SetConfigurationSettingPublisher() is no longer encouraged. Instead, it is better to use CloudStorageAccount.Parse(), which takes a data connection string in canonical form and creates a CloudStorageAccount instance from it. We see how to do this in the Connecting to the Windows Azure Storage Service recipe.

In this recipe, we will learn how to set and use a configuration-setting publisher to retrieve the data connection string from a configuration file.

How to do it...

We are going to add an implementation for SetConfigurationSettingPublisher() to a worker role. We do this as follows:

1. Create a new cloud project.

2. Add a worker role to the project.

3. Add the following to the WorkerRole section of the ServiceDefinition.csdef file:

   <ConfigurationSettings>
      <Setting name="DataConnectionString" />
   </ConfigurationSettings>

4. Add the following to the ConfigurationSettings section of the ServiceConfiguration.cscfg file:

   <Setting name="DataConnectionString" value="DefaultEndpointsProtocol=https;AccountName={ACCOUNT_NAME};AccountKey={ACCOUNT_KEY}"/>

5. Replace WorkerRole.Run() with the following:

   public override void Run()
   {
      UseFromConfigurationSetting("{CONTAINER_NAME}");
   
      while (true)
      {
         Thread.Sleep(10000);
         Trace.WriteLine("Working", "Information");
      }
   }

6. Replace WorkerRole.OnStart() with the following:

   public override bool OnStart()
   {
      ServicePointManager.DefaultConnectionLimit = 12;
   
      CloudStorageAccount.SetConfigurationSettingPublisher((configName, configSetter) =>
      {
         configSetter(RoleEnvironment.GetConfigurationSettingValue(configName));
      });
   
      return base.OnStart();
   }

7. Add the following method, implicitly using the configuration setting publisher, to the WorkerRole class:

   private void UseFromConfigurationSetting(String containerName)
   {
      CloudStorageAccount cloudStorageAccount =CloudStorageAccount.FromConfigurationSetting("DataConnectionString");
   
      CloudBlobClient cloudBlobClient =cloudStorageAccount.CreateCloudBlobClient();
      CloudBlobContainer cloudBlobContainer =cloudBlobClient.GetContainerReference(containerName);
   
      cloudBlobContainer.Create();
   }

How it works...

In steps 1 and 2, we set up the project.

In steps 3 and 4, we define and provide a value for the DataConnectionString setting in the service definition and service configuration files. We must replace {ACCOUNT_NAME} and {ACCOUNT_KEY} with appropriate values for the account name and access key.

In step 5, we modify the Run() method to invoke a method that accesses the storage service. We must provide an appropriate value for {CONTAINER_NAME}.

In step 6, we modify the OnStart() method to set a configuration setting publisher for the role instance. We set it to retrieve configuration settings from the service configuration file.

In step 7, we invoke CloudStorageAccount.FromConfigurationSetting(), which uses the configuration setting publisher we added in step 6. We then use the CloudStorageAccount instance to create CloudBlobClient and CloudBlobContainer instances that we use to create a new container in blob storage.