How to debug a Node.js application in Azure Web Sites
Azure provide built-in diagnostics to assist with debugging Node.js applications hosted in Azure Web Sites. In this article, you will learn how to enable logging of stdout and stderr, display error information in the browser, and how to download and view log files.
Diagnostics for Node.js applications hosted on Azure is provided by IISNode. While this article discusses the most common settings for gathering diagnostics information, it does not provide a complete reference for working with IISNode. For more information on working with IISNode, see the IISNode Readme on GitHub.
By default, Azure Web Sites only capture diagnostic information about deployments, such as when you deploy a web site using Git. This information is useful if you are having problems during deployment, such as a failure when installing a module referenced in package.json, or if you are using a custom deployment script.
To enable the logging of stdout and stderr streams, you must create an IISNode.yml file at the root of your Node.js application and add the following:
This enables the logging of stderr and stdout from your Node.js application.
The IISNode.yml file can also be used to control whether friendly errors or developer errors are returned to the browser when a failure occurs. To enable developer errors, add the following line to the IISNode.yml file:
Once this option is enabled, IISNode will return the last 64K of information sent to stderr instead of a friendly error such as "an internal server error occurred".
While devErrorsEnabled is useful when diagnosing problems during development, enabling it in a production environment may result in development errors being sent to end users.
If the IISNode.yml file did not already exist within your application, you must restart your web site after publishing the updated application. If you are simply changing settings in an existing IISNode.yml file that has previously been published, no restart is required.
If your web site was created using the Azure Command-Line Tools or Azure PowerShell Cmdlets, a default IISNode.yml file is automatically created.
You can restart the web site by selecting the site from the Azure Management Portal, and then selecting the RESTART button:
If the Azure Command-Line Tools are installed in your development environment, you can use the following command to restart the web site:
azure site restart [sitename]
While loggingEnabled and devErrorsEnabled are the most commonly used IISNode.yml configuration options for capturing diagnostic information, IISNode.yml can be used to configure a variety of options for your hosting environment. For a full list of the configuration options, see the iisnode_schema.xml file.
Diagnostic logs can be accessed in three ways; Using the File Transfer Protocol (FTP), downloading a Zip archive, or as a live updated stream of the log (also known as a tail). Downloading the Zip archive of the log files or viewing the live stream require the Azure Command-Line Tools. These can be installed by using the following command:
npm install azure-cli -g
Once installed, the tools can be accessed using the 'azure' command. The command-line tools must first be configured to use your Azure subscription. For information on how to accomplish this task, see the How to download and import publish settings section of the How to Use The Azure Command-Line Tools article.
To access the diagnostic information through FTP, visit the [Azure portal], select your web site, and then select the DASHBOARD. In the quick links section, the FTP DIAGNOSTIC LOGS and FTPS DIAGNOSTIC LOGS links provide access to the logs using the FTP protocol.
If you have not previously configured user name and password for FTP or deployment, you can do so from the QuickStart management page by selecting Set up deployment credentials.
The FTP URL returned in the dashboard is for the LogFiles directory, which will contain the following sub-directories:
[Deployment Method] - If you use a deployment method such as Git, a directory of the same name will be created and will contain information related to deployments.
nodejs - Stdout and stderr information captured from all instances of your application (when loggingEnabled is true.)
To download a Zip archive of the diagnostic logs, use the following command from the Azure Command-Line Tools:
azure site log download [sitename]
This will download a diagnostics.zip in the current directory. This archive contains the following directory structure:
Live stream (tail)
To view a live stream of diagnostic log information, use the following command from the Azure Command-Line Tools:
azure site log tail [sitename]
This will return a stream of log events that are updated as they occur on the server. This stream will return deployment information as well as stdout and stderr information (when loggingEnabled is true.)
In this article you learned how to enable and access diagnostics information for Azure. While this information is useful in understanding problems that occur with your application, it may point to a problem with a module you are using or that the version of Node.js used by Azure Web Sites is different than the one used in your deployment environment.
For information in working with modules on Azure, see Using Node.js Modules with Azure Applications.
For information on specifying a Node.js version for your application, see Specifying a Node.js version in an Azure application.