Envoy utilizes metadata to transport arbitrary untyped or typed data from the control plane to Envoy. Metadata configurations can be applied to Listeners, clusters, routes, virtual hosts, endpoints, and other elements.
Unlike other configurations, Envoy does not explicitly define the purpose of metadata configurations, which can be used for stats, logging, or filter/extension behavior. Users can define the purpose of metadata configurations for their specific use cases. Metadata configurations offer a flexible way to transport user-defined data from the control plane to Envoy without modifying Envoy’s core API or implementation.
For instance, users can add extra attributes to routes, such as the route owner or upstream service maintainer, to metadata. They can then enable Envoy to log these attributes to the access log or report them to StatsD, among other possibilities. Moreover, users can write a filter/extension to read these attributes and execute any specific logic.