APIファースト開発とは？

API-first development is one approach to making software. It means making the parts that let different programs talk to each other, the APIs (application programming interfaces), first. (before creating the rest). In API platform Postman’s 2025 State of the API survey, 25% of respondents’ organizations operated as fully API-first.

APIファースト開発は、ビジネスと開発者の両方にとって重要です。開発プロセスの初期段階からAPIに焦点を当てることが重要な理由は次のとおりです。

• 開発の効率化: Think of APIs as building blocks. Because they are designed for repeated use across different systems, they help engineering teams save time and optimize operational resources.
• 柔軟性の向上: APIファーストアーキテクチャにはモジュール性があります。
• コラボレーションの促進: API定義は、開発者間の契約として機能します。
• イノベーションの促進： APIs create partnerships and integration. Think of, Twilio, who offers APIs for making calls and sending messages as well as running video calls, all in the cloud.

An API-first approach has four key advantages and are stated in their simplest terms below:

市場投入までの時間短縮： Just get your product or feature into the hands of customers in less time.

スケールの拡大： Get Growing! and get your business the ability to handle more customers, work or data without ever breaking or slowing down.

保守性の向上： Please do make software faster and less expensive and also, easier to update and keep running smoothly would be great.

開発者エクスペリエンス： How easy and pleasant is it for a developer to use a tool platform, or understand your API documentation.

The OpenAPI Specification (OAS), previously called the Swagger Specification, is a common standard used to describe RESTful APIs. This includes a machine-readable way to define API endpoints, request and response formats, authentication methods, and more.

OAS in an API-first approach has the following advantages:

• 標準化されたドキュメント： OAS supports consistent API documentation.
• 自動コード生成： Tools generate client and server code from an OAS document.
• 簡素化されたテスト： OAS is useful to create test cases for your APIs.

In a design-first—or contract-first— the OpenAPI specification is written prior to code development, establishing a structural framework and enabling automated boilerplate generation (OpenAPI Initiative).

マイクロサービス is when several small self-contained pieces come together to provide the features of an application, similar to an assemblage of a puzzle.

This means they have to communicate with each other so that the process works well. And this is where APIs come in.

The API-first approach focuses on the design of cross-system communication before looking at core development. API-first and microservices tend to work together, since they are both connected to how application components integrate.

Here is a clean, easy-to-read table organizing those features: