Dynamic Type Expressions Enable Runtime Schema Resolution in Drupal Config

Drupal’s configuration schema supports dynamic type expressions that resolve schema definitions at runtime using contextual data. In a technical analysis, Matt Glaman explains how expressions such as [%parent.type] allow schema types to be constructed dynamically based on values in the surrounding configuration.

The mechanism relies on tokens including %parent, %key, and %type, which are resolved during schema processing. These tokens enable patterns such as deriving schema types from sibling values, mapping sequence items using their keys, and resolving types from within an element’s own data. The same resolution logic is also used in constraint definitions, allowing validation rules to reference contextual values without duplicating schema definitions.

The analysis also notes implementation changes in Drupal 11, where resolution logic moved to an internal TypeResolver class while preserving the existing YAML syntax. It also highlights limitations such as silent failures when expressions cannot be resolved. The post focuses on explaining the mechanics and usage patterns and does not provide performance benchmarks or comparative evaluation.