Runtime Reconfiguration Pattern

Comments

On a change to configuration, a change will re-load settings.

Two types of checks on changes; changed and changing.

Code

the following code from the WebRole.cs class in the Runtime Reconfiguration solution of the examples you can download for this guide shows how you can use the RoleEnvironment.Changing event to cancel the update for all settings except the ones that can be applied at runtime without requiring a restart.

This example allows a change to the settings named “CustomSetting” to be applied at runtime without restarting the application (the component that uses this setting will be able to read the new value and change its behavior accordingly at runtime). Any other change to the configuration will automatically cause the web or worker role to restart.

private void RoleEnvironment_Changing(object sender,
                               RoleEnvironmentChangingEventArgs e)
{
  var changedSettings = e.Changes.OfType()
                                 .Select(c => c.ConfigurationSettingName).ToList();
  Trace.TraceInformation("Changing notification. Settings being changed: "
                         + string.Join(", ", changedSettings));

  if (changedSettings
    .Any(settingName => !string.Equals(settingName, CustomSettingName,
                               StringComparison.Ordinal)))
  {
    Trace.TraceInformation("Cancelling dynamic configuration change (restarting).");

    // Setting this to true will restart the role gracefully. If Cancel is not 
    // set to true, and the change is not handled by the application, the 
    // application will not use the new value until it is restarted (either 
    // manually or for some other reason).
    e.Cancel = true; 
  }
  Else
  {
    Trace.TraceInformation("Handling configuration change without restarting. ");
  }
}

Updates that are not cancelled in the RoleEnvironment.Changing event handler can then be detected and applied to the application components after the new configuration has been accepted by the Azure framework.

For example, the following code in the Global.asax file of the example solution handles the RoleEnvironment.Changed event.

It examines each configuration setting and, when it finds the setting named “CustomSetting”, calls a function (shown earlier) that applies the new setting to the appropriate component in the application.

private void RoleEnvironment_Changed(object sender, 
                               RoleEnvironmentChangedEventArgs e)
{
  Trace.TraceInformation("Updating instance with new configuration settings.");

  foreach (var settingChange in
           e.Changes.OfType())
  {
    if (string.Equals(settingChange.ConfigurationSettingName, 
                      CustomSettingName, 
                      StringComparison.Ordinal))
    {
      // Execute a function to update the configuration of the component.
      ConfigureFromSetting(CustomSettingName );
    }
  }
}