Lookups are a concept in Druid where dimension values are replaced with new values. In Druid lookup, a “key” refers to a dimension value to match, and a “value” refers to its replacement. So if you wanted to map “us” to “United States” then the key would be “us” and the value would be “United States”.
Druid supports passing very small lookups (a few dozen to a few hundred) at query time as a “map” lookup. Other lookup types are available as extensions, including lookups from local files, remote URIs, or JDBC through lookups-cached-global. Another supported type is globally cached lookups from a Kafka topic through kafka-extraction-namespace.
In this article, I will demonstrate how to configure Druid to use lookups from Json files using the lookups-cached-global extension.