OpenAPI Is The Contract For Your Microservice

I’ve talked about how generating an OpenAPI (fka Swagger) definition from code is still the dominate way that microservice owners are producing this artifact. This is a by-product of developers seeing it as just another JSON artifact in the pipeline, and from it being primarily used to create API documentation, often times using Swagger UI–which is also why it is still called Swagger, and not OpenAPI. I’m continuing my campaign to help the projects I’m consulting on be more successful with their overall microservices strategy by helping them better understand how they can work in concert by focus in on OpenAPI, and realizing that it is the central contract for their service.

Each Service Beings With An OpenAPI Contract There is no reason that microservices should start with writing code. It is expensive, rigid, and time consuming. The contract that a service provides to clients can be hammered out using OpenAPI, and made available to consumers as a machine readable artifact (JSON or YAML), as well as visualized using documentation like Swagger UI, Redocs, and other open source tooling. This means that teams need to put down their IDE’s, and begin either handwriting their OpenAPI definitions, or being using an open source editor like Swagger Editor, Apicurio, API GUI, or even within the Postman development environment. The entire surface area of a service can be defined using OpenAPI, and then provided using mocked version of the service, with documentation for usage by UI and other application developers. All before code has to be written, making microservices development much more agile, flexible, iterative, and cost effective.

Mocking Of Each Microservice To Hammer Out Contract Each OpenAPI can be used to generate a mock representation of the service using Postman,, or other OpenAPI-driven mocking solution. There are a number of services, and tooling available that takes an OpenAPI, an generates a mock API, as well as the resulting data. Each service should have the ability to be deployed locally as a mock service by any stakeholder, published and shared with other team members as a mock service, and shared as a demonstration of what the service does, or will do. Mock representations of services will minimize builds, the writing of code, and refactoring to accommodate rapid changes during the API development process. Code shouldn’t be generated or crafted until the surface area of an API has been worked out, and reflects the contract that each service will provide.

OpenAPI Documentation Always AVailable In Repository Each microservice should be self-contained, and always documented. Swagger UI, Redoc, and other API documentation generated from OpenAPI has changed how we deliver API documentation. OpenAPI generated documentation should be available by default within each service’s repository, linked from the README, and readily available for running using static website solutions like Github Pages, or available running locally through the localhost. API documentation isn’t just for the microservices owner / steward to use, it is meant for other stakeholders, and potential consumers. API documentation for a service should be always on, always available, and not something that needs to be generated, built, or deployed. API documentation is a default tool that should be present for EVERY microservice, and treated as a first class citizen as part of its evolution.

Bringing An API To Life Using It’s OpenAPI Contract Once an OpenAPI contract has been been defined, designed, and iterated upon by service owner / steward, as well as a handful of potential consumers and clients, it is ready for development. A finished (enough) OpenAPI can be used to generate server side code using a popular language framework, build out as part of an API gateway solution, or common proxy services and tooling. In some cases the resulting build will be a finished API ready for use, but most of the time it will take some further connecting, refinement, and polishing before it is a production ready API. Regardless, there is no reason for an API to be developed, generated, or built, until the OpenAPI contract is ready, providing the required business value each microservice is being designed to deliver. Writing code, when an API will change is an inefficient use of time, in a virtualized API design lifecycle.

OpenAPI-Driven Monitoring, Testing, and Performance A read-to-go OpenAPI contract can be used to generate API tests, monitors, and deliver performance tests to ensure that services are meeting their business service level agreements. The details of the OpenAPI contract become the assertions of each test, which can be executed against an API on a regular basis to measure not just the overall availability of an API, but whether or not it is actually meeting specific, granular business use cases articulated within the OpenAPI contract. Every detail of the OpenAPI becomes the contract for ensuring each microservice is doing what has been promised, and something that can be articulated and shared with humans via documentation, as well as programmatically by other systems, services, and tooling employed to monitor and test accordingly to a wider strategy.

Empowering Security To Be Directed By The OpenAPI Contract An OpenAPI provides the entire details of the surface area of an API. In addition to being used to generate tests, monitors, and performance checks, it can be used to inform security scanning, fuzzing, and other vital security practices. There are a growing number of services and tooling emerging that allow for building models, policies, and executing security audits based upon OpenAPI contracts. Taking the paths, parameters, definitions, security, and authentication and using them as actionable details for ensuring security across not just an individual service, but potentially hundreds, or thousands of services being developed across many different teams. OpenAPI quickly is becoming not just the technical and business contract, but also the political contract for how you do business on web in a secure way.

OpenAPI Provides API Discovery By Default An OpenAPI describes the entire surface area for the request and response of each API, providing 100% coverage for all interfaces a services will possess. While this OpenAPI definition will be generated mocks, code, documentation, testing, monitoring, security, and serving other stops along the lifecycle, it provides much needed discovery across groups, and by consumers. Anytime a new application is being developed, teams can search across the team Github, Gitlab, Bitbucket, or Team Foundation Server (TFS), and see what services already exist before they begin planning any new services. Service catalogs, directories, search engines, and other discovery mechanisms can use OpenAPIs across services to index, and make them available to other systems, applications, and most importantly to other humans who are looking for services that will help them solve problems.

OpenAPI Deliver The Integration Contract For Client OpenAPI definitions can be imported in Postman, Stoplight, and other API design, development, and client tooling, allowing for quick setup of environment, and collaborating with integration across teams. OpenAPIs are also used to generate SDKs, and deploy them using existing continuous integration (CI) pipelines by companies like APIMATIC. OpenAPIs deliver the client contract we need to just learn about an API, get to work developing a new web or mobile application, or manage updates and version changes as part of our existing CI pipelines. OpenAPIs deliver the integration contract needed for all levels of clients, helping teams go from discovery to integration with as little friction as possible. Without this contract in place, on-boarding with one service is time consuming, and doing it across tens, or hundreds of services becomes impossible.

OpenAPI Delivers Governance At Scale Across Teams Delivering consistent APIs within a single team takes discipline. Delivering consistent APIs across many teams takes governance. OpenAPI provides the building blocks to ensure APIs are defined, designed, mocked, deployed, documented, tested, monitored, perform, secured, discovered, and integrated with consistently. The OpenAPI contract is an artifact that governs every stop along the lifecycle, and then at scale becomes the measure for how well each service is delivering at scale across not just tens, but hundreds, or thousands of services, spread across many groups. Without the OpenAPI contract API governance is non-existent, and at best extremely cumbersome. The OpenAPI contract is not just top down governance telling what they should be doing, it is the bottom up contract for service owners / stewards who are delivering the quality services on the ground inform governance, and leading efforts across many teams.

I can’t articulate the importance of the OpenAPI contract to each microservice, as well as the overall organizational and project microservice strategy. I know that many folks will dismiss the role that OpenAPI plays, but look at the list of members who govern the specification. Consider that Amazon, Google, and Azure ALL have baked OpenAPI into their microservice delivery services and tooling. OpenAPI isn’t a WSDL. An OpenAPI contract is how you will articulate what your microservice will do from inception to deprecation. Make it a priority, don’t treat it as just an output from your legacy way of producing code. Roll up your sleeves, and spend time editing it by hand, and loading it into 3rd party services to see the contract for your microservice in different ways, through different lenses. Eventually you will begin to see it is much more than just another JSON artifact laying around in your repository.