externallanguage

Externallanguage is a proposed specification language intended to describe external interfaces and data exchange between software components, services, and systems. It aims to provide a precise, language-agnostic contract that can be used for API design, integration, and testing. The language focuses on the external behavior of a system rather than internal implementation details, supporting interoperability across programming languages and runtimes.

The core constructs include entities such as Service, Operation, Message, and DataType, together with annotations for versioning, security constraints, and quality attributes. The syntax is designed to be readable by humans and machine-processable, enabling automatic validation, client and server stubs, and documentation generation. Descriptions are intended to be backward-compatible when interfaces evolve, with explicit versioning and deprecation semantics.

Externallanguage is related to existing interface definition languages and API description formats; it overlaps with OpenAPI, Protocol Buffers, and GraphQL in purpose but differentiates itself by aiming to be a single, cross-language external contract instead of primarily code generation for a single API style.

Usage scenarios include contract-first API design, automated test generation, vendor interoperability testing, and documentation. It is often discussed in academic and industry proposals as a way to standardize how external interfaces are described across heterogeneous systems.

Status and adoption: Externallanguage remains a conceptual or proposal-level concept with no widely adopted standard. Various projects and research efforts have explored its feasibility, comparing its expressiveness, tooling, and integration with existing IDLs and API ecosystems. See also API description language, interface definition language, and OpenAPI.