You have an Azure subscription with an Azure Pipelines pipeline named Pipeline1 that builds an app named App1. A Slack channel App1chat has an incoming webhook configured. Which mechanism should you use to send a notification to App1chat when a successful build of App1 completes?

A subscription in Azure DevOps allows you to configure notifications for specific events, such as a successful build completion. You can then specify an external service hook, like a Slack incoming webhook, as the target for these notifications. This directly addresses the requirement of sending a notification to App1chat upon a successful build. An alert rule is typically used for monitoring metrics and logs in Azure Monitor and triggering actions based on thresholds, not for direct build completion notifications. An action group is a collection of notification preferences that can be used by alert rules, but it doesn't initiate the notification itself based on a build event. While "a notification" is a general concept, in Azure DevOps, the specific mechanism to set this up for pipeline events is through a "subscription.