Schemafirst

Schema-first is a development approach in which the data contract of a service is defined before implementing the operational code. The contract—commonly expressed as a schema, such as a GraphQL schema or an OpenAPI specification—defines the available data types, operations, inputs, outputs, and constraints. This approach creates a single source of truth that guides both client and server development and supports contract-driven governance across teams.

In GraphQL, the schema-first workflow starts with designing the GraphQL schema in the schema language, declaring types, queries, mutations, and subscriptions. Backend developers then implement resolvers that fulfill the contract while frontend teams build clients against the published schema. In REST/OpenAPI contexts, teams define the OpenAPI document first, including endpoints, parameter schemas, and response formats; server stubs and client SDKs can be generated from the spec.

Advantages include a clear, language-agnostic contract, improved collaboration between frontend and backend teams, easier testing with mocks, and better API governance and versioning. It also supports client-driven development, documentation accuracy, and automatic validation and client generation.

Challenges include schema drift when implementations diverge from the contract, potential over-design or under-optimized schemas, and the upfront investment in schema design. Maintaining backward compatibility and handling schema evolution require governance. It is often most effective for public APIs, microservices, or systems with multiple consumer teams, whereas small internal projects with rapid experimentation may favor code-first approaches.