Comparing SOA web service interfacing style of SOAP vs. REST, the former tend to be centered around operations that are usually use-case specific and specialized. In contrast, REST is centered around business (data) entities exposed as resources that are identified via URIs and can be manipulated via standardized CRUD-like methods using different representations, and hypermedia. RESTful APIs tend to be less use-case specific and come with less rigid client / server coupling and are more suitable for an ecosystem of (core) services providing a platform of APIs to build diverse new business services. We apply the RESTful web service principles to all kind of application (micro-) service components, independently from whether they provide functionality via the internet or intranet.
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We prefer REST-based APIs with JSON payloads
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We prefer systems to be truly RESTful [1]
An important principle for API design and usage is Postel’s Law, aka The Robustness Principle (see also RFC 1122):
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Be liberal in what you accept, be conservative in what you send
Readings: Some interesting reads on the RESTful API design style and service architecture:
As mentioned above, Zalando is transforming from an online shop into an expansive fashion platform comprising a rich set of products following a Software as a Platform (SaaP) model for our business partners. As a company we want to deliver products to our (internal and external) customers which can be consumed like a service.
Platform products provide their functionality via (public) APIs; hence, the design of our APIs should be based on the API as a Product principle:
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Treat your API as product and act like a product owner
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Put yourself into the place of your customers; be an advocate for their needs
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Emphasize simplicity, comprehensibility, and usability of APIs to make them irresistible for client engineers
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Actively improve and maintain API consistency over the long term
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Make use of customer feedback and provide service level support
Embracing 'API as a Product' facilitates a service ecosystem, which can be evolved more easily and used to experiment quickly with new business ideas by recombining core capabilities. It makes the difference between agile, innovative product service business built on a platform of APIs and ordinary enterprise integration business where APIs are provided as "appendix" of existing products to support system integration and optimised for local server-side realization.
Understand the concrete use cases of your customers and carefully check the trade-offs of your API design variants with a product mindset. Avoid short-term implementation optimizations at the expense of unnecessary client side obligations, and have a high attention on API quality and client developer experience.
API as a Product is closely related to our API First principle (see next chapter) which is more focused on how we engineer high quality APIs.
API First is one of our engineering and architecture principles. In a nutshell API First requires two aspects:
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define APIs first, before coding its implementation, using a standard specification language
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get early review feedback from peers and client developers
By defining APIs outside the code, we want to facilitate early review feedback and also a development discipline that focus service interface design on…
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profound understanding of the domain and required functionality
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generalized business entities / resources, i.e. avoidance of use case specific APIs
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clear separation of WHAT vs. HOW concerns, i.e. abstraction from implementation aspects — APIs should be stable even if we replace complete service implementation including its underlying technology stack
Moreover, API definitions with standardized specification format also facilitate…
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single source of truth for the API specification; it is a crucial part of a contract between service provider and client users
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infrastructure tooling for API discovery, API GUIs, API documents, automated quality checks
Elements of API First are also this API Guidelines and a standardized API review process as to get early review feedback from peers and client developers. Peer review is important for us to get high quality APIs, to enable architectural and design alignment and to supported development of client applications decoupled from service provider engineering life cycle.
It is important to learn, that API First is not in conflict with the agile development principles that we love. Service applications should evolve incrementally — and so its APIs. Of course, our API specification will and should evolve iteratively in different cycles; however, each starting with draft status and early team and peer review feedback. API may change and profit from implementation concerns and automated testing feedback. API evolution during development life cycle may include breaking changes for not yet productive features and as long as we have aligned the changes with the clients. Hence, API First does not mean that you must have 100% domain and requirement understanding and can never produce code before you have defined the complete API and get it confirmed by peer review.
On the other hand, API First obviously is in conflict with the bad practice of publishing API definition and asking for peer review after the service integration or even the service productive operation has started. It is crucial to request and get early feedback — as early as possible, but not before the API changes are comprehensive with focus to the next evolution step and have a certain quality (including API Guideline compliance), already confirmed via team internal reviews.