checkswhere

Checkswhere is a software library and rule-engine designed to perform conditional data validation in data processing pipelines. It allows developers to express validation rules that should apply only to data items meeting specific conditions, using a where-like predicate language.

At runtime, each data item passes through a set of checks that are activated by its context. A rule is defined as a pair of a predicate and one or more checks; if the predicate evaluates to true for a given item, its associated checks are executed. This design enables separating business conditions from validation logic and supports dynamic rule sets that can be modified without touching core code.

Core components include a lightweight engine that schedules and evaluates rules, a library of common checks (for example, non-null, type, range, pattern matching, and cross-field validations), and a context binder that attaches data items to a working environment and captures results. Results can be reported as passes or failures with error messages and metadata.

Typical use cases include data intake validation in ETL pipelines, API payload validation before processing, and data quality monitoring where checks should run only for records meeting certain risk criteria.

Origin and reception: Checkswhere originated as an open-source project in the early 2020s and has been adopted by data engineering teams seeking modular validation with minimal boilerplate. Critics note that complex rule sets can become difficult to trace and that performance requires careful indexing of predicates in large datasets.

See also: data validation, rule engine, assertion library, ETL, data quality.