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Windows Azure

Azure Monitoring and Diagnostics : Azure Diagnostics­ under the hood, Enabling diagnostic logging

10/11/2012 6:15:42 PM
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Azure Diagnostics­ under the hood

When we consider working with Azure diagnostics, we need to decide what to collect and how to store the collected data. The following table summarizes the information available to us:

Data Collected by default Role(s) Storage Storage location name
Windows Azure logs Yes Web, Worker Table WadLogsTable
IIS logs Yes Web Blob wad-iis-logfiles
Windows diagnostic logs Yes Web, Worker Table WadLogsTable
Failed request logs No Web Blob wad-iis-failedreqlogfiles
Windows event logs No Web, Worker Table WadWindowsEventLogsTable
Performance counters No Web, Worker Table WadPerformanceCountersTable
Crash dumps No Web, Worker Blob wad-crash-dumps
Custom error logs No Web, Worker Blob user defined storage

On a traditional Windows system, IIS logs, crash dumps and failed request logs would be stored in files. These three logs are referred to as Directory logs in Azure's jargon, and end up in blobs. Except for custom logs, the rest of the logs are all persisted in tables.

During data collection, information is buffered in a blob, inside a container named wad-control-container. Diagnostic data are not accessible until they are transferred from wad-control-container to the proper blob or table storage. The diagnostic buffers can be manipulated via descendants of the DiagnosticDataBufferConfiguration class.

There is also a much more sophisticated framework for logging and tracing, based in part on the Event Tracing for Windows (ETW) framework we may already be familiar with. Samples are available for downloading at http://code.msdn.microsoft.com/WADiagnostics. Microsoft also provides a number of additional samples in the Azure SDK and other sample downloads.

Diagnostic ETW data are available through classes in the System.Diagnostics namespace. Windows Azure Diagnostics extends this namespace with the Microsoft.WindowsAzure.Diagnostics namespace. References for the Microsoft.WindowsAzure.Diagnostics can be found at http://msdn.microsoft.com/en-us/library/microsoft.windowsazure.diagnostics.aspx and http://msdn.microsoft.com/en-us/library/microsoft.windowsazure.diagnostics.management.aspx. As a general outline, when we implement logging, the general process flow looks like the following diagram:

In the architecture depicted in this diagram, our application pipes diagnostic data through TraceSources, which are intermediate objects (of type System.Diagnostics.TraceSource) used to route data to various endpoints. A SourceSwitch, where we see values such as "verbose" or "critical", determines which messages should be passed to the DiagnosticMonitorTraceListener. On a normal system, verbose data could be routed through a TraceSource with a log file endpoint, while critical errors might be routed through a TraceSource that sends a text message to a sysadmin.

TraceListeners subscribe to TraceSources, and route the data to the desired endpoint. The default TraceListener in Azure is the DiagnosticMonitor, which is a special class used to configure the collection of diagnostic data. A diagnostic monitor can be configured to listen to a number of TraceSources. In Azure, diagnostic data are all routed to either Table or Blob Storage.

The three logs indicated as being part of the default Diagnostic Monitor collection Azure, IIS 7, and Windows Diagnostic are enabled by default when we use one of the Azure templates. Because our sample is using a default template, there is nothing much that we have to do. If we were to use our own template, we'd have to make sure we included the initialization. The other logs can be collected, but additional coding is required to activate them.

Although we may activate log collection, that does not make the log data available to us immediately upon collection. We also need to transfer the log data to a store we can access, such as Table Storage. This must be done whether we're using the default Diagnostic Monitor or we've activated an additional log source.

The Hello Fabric sample in the Azure SDK is a useful reference for experimenting with diagnostic logging. It's a very simple application that can be run in the development fabric on a development machine.

Enabling diagnostic logging

When we create an Azure application using one of the default templates, collection of Windows Azure, IIS, and Windows Diagnostic logs is enabled by default. We can see the setup in the template files.

The DiagnosticMonitorTraceListener configuration for our WCF and web role projects is found in the web.config file:

<system.diagnostics>
diagnostic loggingenabling<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>

There are a number of additional configuration options in the<system.diagnostics> element; complete documentation can be found at http://msdn.microsoft.com/en-us/library/1txedc80.aspx. Specifically, the<filter> element (http://msdn.microsoft.com/en-us/library/ms229326.aspx) is used to set the SourceSwitch filtering values.

The DiagnosticMonitorTraceListener is started in our projects' OnStart method, found in the webrole.vb file:

Public Overrides Function OnStart() As Boolean
DiagnosticMonitor.Start("DiagnosticsConnectionString")
...
End Function

In our workerrole.vb file, we have the following entry in the Run method that logs the start of the role.

Public Overrides Sub Run()
Trace.WriteLine("JupiterMotorsWorkerRole entry point called.", "Information")
...
End Sub


					  

If we weren't using a template, we'd need to add this code manually to the proper files to enable collection of diagnostic data.

Enabling the additional logging types is easy, but not all the logs are implemented in the same way. For instance, the Failed Request Logs are enabled by editing the web.config file, whereas the Windows Event Logs are enabled with some code in the Role.OnStart method. The MSDN documentation for enabling other sources of diagnostic information can be found at http://msdn.microsoft.com/en-us/library/ee843890.aspx.

One of the more interesting and useful sources of diagnostic information is performance counters. We've all probably used performance counters in the past when debugging all sorts of issues, and Azure provides us with the same capabilities. The downside to performance counters is that we configure them in the Role.OnStart() method, so we must redeploy our application if we want to change them. One possibility would be to preconfigure a number of performance counters, wrapped in if...then blocks that check for values in the config files. This way, we can turn collection of performance counters on and off by editing the proper config file. The following is the configuration for the % Processor Time counter. Note that there is a very specific format used for the CounterSpecifier. For additional information on this naming format, we can review the documentation at http://msdn.microsoft.com/en-us/library/aa373193%28VS.85%29.aspx.

Public Overrides Function OnStart() As Boolean
Dim diagConfig As DiagnosticMonitorConfiguration = DiagnosticMonitor.GetDefaultInitialConfiguration()
Dim procTimeConfig As PerformanceCounterConfiguration = New PerformanceCounterConfiguration()
procTimeConfig.CounterSpecifier = "\Processor(*)\% Processor Time"
procTimeConfig.SampleRate = System.TimeSpan.FromSeconds(1.0)
diagConfig.PerformanceCounters.DataSources.Add(procTimeConfig)
DiagnosticMonitor.Start("DiagnosticsConnectionString")
Return MyBase.OnStart()
End Function


					  
Diagnostic configuration is not global configuration applies only to the role where we have added the code. We need to be sure to configure data collection for each and every role we want to be able to debug (which is pretty much every role). If we're interested in the same information, it's just copy-and-paste code, but we still need to remember to do it.

Changing the location of the logging configuration

In an ASP.NET application, the usual place to configure SourceSwitches and other configuration details is in the web.config file. This way, if we need to change the level of diagnostic information collected, we can simply edit the web.config and commence debugging. By default, the diagnostic configuration information for an Azure application is stored in the web.config file. However, Azure applications are not deployed in the same way as ASP.NET applications. Azure applications are compiled into a single file and deployed as a single file. We cannot simply edit a web.config (or an app.config in the case of a worker role) once an Azure application is deployed. If we need to edit the web.config or app.config, we would need to edit the files locally and redeploy the entire application post-edit.

With Azure applications, the recommendation is to duplicate configuration settings into both the web.config and the .cscfg file. The .cscfg file can be changed while the application is running, but the web.config cannot be changed in the application. This setup requires a little extra effort at the beginning, but the effort can be worth it when needed. The idea behind duplicating the configuration is that an application can be deployed on Azure, and then later redeployed on premises without any additional modification. If this application is sure to not be deployed on IIS, the web.config edit can be skipped.

The easiest way to add the required settings to the .cscfg file is to open the Roles subfolder, right-click the role we want to modify, and choose Properties.

In the Properties panel, select the Settings tab, click the Add Setting button, and add a String setting. This setting will act as our SourceSwitch and help us determine the level of information we want to capture.

Adding a setting via the Properties panel makes an entry in both the .cscfg and the .csdef files. The .csdef file must contain a definition for every setting in the .cscfg file, and cannot be edited in a live application. Therefore, we cannot add additional settings to a live application, but we can change their values.

The final step is to add a few lines of code to help our application determine if it's running on IIS or Azure. We simply need to check for RoleEnvironment.IsAvailable, which is available only in Azure.

Dim _debugLevel As String
If RoleEnvironment.IsAvailable Then
' gets the value from .cscfg file
_debugLevel = RoleEnvironment.GetConfigurationSettingValue( "DebugLevel")
Else
' gets the value from web.config file
_debugLevel = WebConfigurationManager.AppSettings("DebugLevel")
End If

The template projects include configuration settings for DiagnosticsConnectionString and DataConnectionString. By default, these connections point to a local SQL Server Express instance on our development machines, and obviously the connection strings need to be updated when we deploy our application. Again, if we weren't using a default template, we'd need to add these configuration settings too.

There are two useful resources about working with diagnostic logging and configuration files in Azure one is an MSDN article titled Take Control of Logging and Tracing in Windows Azure (http://msdn.microsoft.com/en-us/magazine/ff714589.aspx), and a code project article titled Windows Azure Development Deep Dive: Working With Configuration (http://www.codeproject.com/KB/azure/azure-configuration.aspx), on which part of the MSDN article is based.


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