# Attribute Value Sources Device attributes can be populated using external sources using Attribute Value Sources. These provide mechanisms to automatically update some device attributes according to some rules. All the concepts used below are detailed in the [Core Concepts](user_core_concepts.html#tags-attributes-and-groups) page, this guide is oriented towards operational details to use Attribute Value Sources. ## Populating attributes using Astarte values To populate an attribute using a value coming from an Astarte interface, an Attribute Value Source of type `astarte-value` must be added. This will populate an attribute whose value will be eventually consistent with the value of the target Astarte interface and path. When creating an `astarte-value` Attribute Value Source, the following information must be provided: - Interface: the target interface to be used - Major version: the target major version of the interface - Path: the target path containing the value that will be used as attribute value After its creation, the Attribute Value Source will install an Astarte trigger, so when the target interface value changes, the change will be (eventually) reflected in the Attribute value. The Attribute key will be `astarte-value:`. The value will also be initialized asynchronously for all devices by querying AppEngine API in a background task.