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How to configure and run startup tasks for an Azure Cloud Service (classic)

  • Artikkeli

Important

Cloud Services (classic) is now deprecated for all customers as of September 1st, 2024. Any existing running deployments will be stopped and shut down by Microsoft and the data will be permanantly lost starting October 2024. New deployments should use the new Azure Resource Manager based deployment model Azure Cloud Services (extended support).

You can use startup tasks to perform operations before a role starts. Operations that you might want to perform include installing a component, registering Component Object Model (COM) components, setting registry keys, or starting a long running process.

Note

Startup tasks are not applicable to Virtual Machines, only to Cloud Service Web and Worker roles.

How startup tasks work

Startup tasks are actions taken before your roles begin. The ServiceDefinition.csdef file defines startup tasks by using the Task element within the Startup element. Frequently startup tasks are batch files, but they can also be console applications, or batch files that start PowerShell scripts.

Environment variables pass information into a startup task, and local storage can be used to pass information out of a startup task. For example, an environment variable can specify the path to a program you want to install, and files can be written to local storage. From there, your roles can read the files.

Your startup task can log information and errors to the directory specified by the TEMP environment variable. During the startup task, the TEMP environment variable resolves to the C:\Resources\temp\[guid].[rolename]\RoleTemp directory when running on the cloud.

Startup tasks can also be executed several times between reboots. For example, the startup task runs each time the role recycles, and role recycles may not always include a reboot. Startup tasks should be written in a way that allows them to run several times without problems.

Startup tasks must end with an errorlevel (or exit code) of zero for the startup process to complete. If a startup task ends with a nonzero errorlevel, the role fails to start.

Role startup order

The following lists the role startup procedure in Azure:

  1. The instance is marked as Starting and doesn't receive traffic.

  2. All startup tasks are executed according to their taskType attribute.

    • The simple tasks are executed synchronously, one at a time.

    • The background and foreground tasks are started asynchronously, parallel to the startup task.

      Warning

      IIS may not be fully configured during the startup task stage in the startup process, so role-specific data may not be available. Startup tasks that require role-specific data should use Microsoft.WindowsAzure.ServiceRuntime.RoleEntryPoint.OnStart.

  3. The role host process is started and the site is created in Internet Information Services (IIS).

  4. The Microsoft.WindowsAzure.ServiceRuntime.RoleEntryPoint.OnStart method is called.

  5. The instance is marked as Ready and traffic is routed to the instance.

  6. The Microsoft.WindowsAzure.ServiceRuntime.RoleEntryPoint.Run method is called.

Example of a startup task

Startup tasks are defined in the ServiceDefinition.csdef file, in the Task element. The commandLine attribute specifies the name and parameters of the startup batch file or console command, the executionContext attribute specifies the privilege level of the startup task, and the taskType attribute specifies how the task executes.

In this example, an environment variable, MyVersionNumber, is created for the startup task and set to the value "1.0.0.0".

ServiceDefinition.csdef:

<Startup>
    <Task commandLine="Startup.cmd" executionContext="limited" taskType="simple" >
        <Environment>
            <Variable name="MyVersionNumber" value="1.0.0.0" />
        </Environment>
    </Task>
</Startup>

In the following example, the Startup.cmd batch file writes the line "The current version is 1.0.0.0" to the StartupLog.txt file in the directory specified by the TEMP environment variable. The EXIT /B 0 line ensures that the startup task ends with an errorlevel of zero.

ECHO The current version is %MyVersionNumber% >> "%TEMP%\StartupLog.txt" 2>&1
EXIT /B 0

Note

In Visual Studio, the Copy to Output Directory property for your startup batch file should be set to Copy Always to be sure that your startup batch file is properly deployed to your project on Azure (approot\bin for Web roles, and approot for worker roles).

Description of task attributes

The following describes the attributes of the Task element in the ServiceDefinition.csdef file:

commandLine - Specifies the command line for the startup task:

  • The command, with optional command line parameters, which begins the startup task.
  • Frequently this attribute is the filename of a .cmd or .bat batch file.
  • The task is relative to the AppRoot\Bin folder for the deployment. Environment variables aren't expanded in determining the path and file of the task. If environment expansion is required, you can create a small .cmd script that calls your startup task.
  • Can be a console application or a batch file that starts a PowerShell script.

executionContext - Specifies the privilege level for the startup task. The privilege level can be limited or elevated:

  • limited
    The startup task runs with the same privileges as the role. When the executionContext attribute for the Runtime element is also limited, then user privileges are used.
  • elevated
    The startup task runs with administrator privileges. These privileges allow startup tasks to install programs, make IIS configuration changes, perform registry changes, and other administrator level tasks, without increasing the privilege level of the role itself.

Note

The privilege level of a startup task does not need to be the same as the role itself.

taskType - Specifies the way a startup task is executed.

  • simple
    Tasks are executed synchronously, one at a time, in the order specified in the ServiceDefinition.csdef file. When one simple startup task ends with an errorlevel of zero, the next simple startup task is executed. If there are no more simple startup tasks to execute, then the role itself starts.

    Note

    If the simple task ends with a non-zero errorlevel, the instance will be blocked. Subsequent simple startup tasks, and the role itself, will not start.

    To ensure that your batch file ends with an errorlevel of zero, execute the command EXIT /B 0 at the end of your batch file process.

  • background
    Tasks are executed asynchronously, in parallel with the startup of the role.

  • foreground
    Tasks are executed asynchronously, in parallel with the startup of the role. The key difference between a foreground and a background task is that a foreground task prevents the role from recycling or shutting down until the task ends. The background tasks don't have this restriction.

Environment variables

Environment variables are a way to pass information to a startup task. For example, you can put the path to a blob that contains a program to install, or port numbers that your role uses, or settings to control features of your startup task.

There are two kinds of environment variables for startup tasks; static environment variables and environment variables based on members of the RoleEnvironment class. Both are in the Environment section of the ServiceDefinition.csdef file, and both use the Variable element and name attribute.

Static environment variables use the value attribute of the Variable element. The preceding example creates the environment variable MyVersionNumber which has a static value of "1.0.0.0". Another example would be to create a StagingOrProduction environment variable, which you can manually set to values of "staging" or "production" to perform different startup actions based on the value of the StagingOrProduction environment variable.

Environment variables based on members of the RoleEnvironment class don't use the value attribute of the Variable element. Instead, the RoleInstanceValue child element, with the appropriate XPath attribute value, are used to create an environment variable based on a specific member of the RoleEnvironment class. Values for the XPath attribute to access various RoleEnvironment values can be found here.

For example, to create an environment variable that is "true" when the instance is running in the compute emulator, and "false" when running in the cloud, use the following Variable and RoleInstanceValue elements:

<Startup>
    <Task commandLine="Startup.cmd" executionContext="limited" taskType="simple">
        <Environment>

            <!-- Create the environment variable that informs the startup task whether it is running
                in the Compute Emulator or in the cloud. "%ComputeEmulatorRunning%"=="true" when
                running in the Compute Emulator, "%ComputeEmulatorRunning%"=="false" when running
                in the cloud. -->

            <Variable name="ComputeEmulatorRunning">
                <RoleInstanceValue xpath="/RoleEnvironment/Deployment/@emulated" />
            </Variable>

        </Environment>
    </Task>
</Startup>

Next steps

Learn how to perform some common startup tasks with your Cloud Service.

Package your Cloud Service.