One area of API design that doesn’t get discussed much is the semantics of the payload. That is, the names we give our attributes and elements for the values being communicated. When developing single-use APIs (usually for client applications), this is unlikely to be an issue as the team(s) involved are likely to know each other and are able to interact and resolve clarity issues easily enough (although getting the semantics right makes this easier particularly in the long term). But when it comes to providing reusable endpoints, we may know the early adopters but are unlikely to interact with consumers beyond that unless there is a problem.
This makes getting the semantics right somewhat harder. How do we know if our early adopters represent the wider customer base (internally and externally)? Conversely, if we simply use our own company terminology, how do we know that it is representative of the wider user base? It isn’t unusual for organizations to develop their own variations of a term or apply assumed meaning. Even simple things, a ‘post code’ element of an address, other parts of the world use ‘zip codes’ or PINS are they the same? Perhaps if we said ‘postal code,’ we break the direct specific country associations with ‘post code.’ We can overcome these issues by providing a dictionary of meanings and lengthy explanations. Using the right term goes beyond simply understanding the data value; it will infer specific formatting and potential application behaviors. Taking our postcode/zip code example. In the UK data is published, which means it is possible to easily validate a postcode against the address line and vice versa. In fact, in the UK to get something delivered, you only need the property number and the postcode. A US 5-digit zipcode can’t do that. For that precision, the ZIP+4 needs to be used.
If we can address these issues, then life becomes easier for us in maintaining the information and for consumers in not needing to look up the details. The question is how can we be sure of using semantics that is consistent across our APIs and widely understood and, when necessary, already documented, so we don’t have to document the information again?
Read more: API payload design getting the semantics rightPublic Data Models
There is a shortcut to some of these problems. Many industries have agreed on data models for different industries. The bodies such as OASIS, OMG, and others are developed and maintained by multiple organizations. As a result, there is a commonality in the meaning achieved. So if you align with that meaning, then use that semantic. Not only can the naming of attributes become easier, but any documentation can be simplified to reference the published definitions. in most cases, these standards are publicly available as it promotes the widest adoption – one of the goals of developing such models. But there are some pitfalls to be mindful of using this approach:
- Sometimes rather than arrive at a universal definition, the models will accommodate structural variations or aliased names – as a result, they may not necessarily be helpful to you.
- The more well-known models are internationalized. If you have no intent to support international needs and not expecting to have international consumers, then the naming may not align with localized conventions.
- If you use the semantics provided, ensure your data abides by the meaning. For example, don’t use ‘shipping address’ if you’re not shipping anything.
- Don’t slavishly copy the data models provided – the model may not be intended for API use cases. At the same time, it doesn’t stop you from asking why the data in the model is there and whether your users may want such data (and whether it makes sense for you to provide that information).
Predefined APIs
Some organizations, such as TMForum have taken the public data model to the next step and provided predefined API specifications. This is ideal where you’re following industry standards and providing standardized/common services that aren’t a differentiator but need to be offered as part of doing business.
Data Catalogs
Larger, data-mature organizations will keep some form of Data Catalog. These catalogs are often held to help understand compliance needs, such as where personal data is held, how data issues can impact data accuracy and integrity, etc. It is possible that metadata may also be kept to address the semantic meaning of data or reference the definitions. Such information is used to help inform any data cleansing that may be needed. This offers a potentially good source of information for internal API use cases.
Vendor Led
If your business is delivery/service focussed so that your unique value isn’t in IT processes but perhaps something that the company manufactures or a specialist service such as consulting in a specific industry, then it is possible that the majority of your systems are SaaS or COTs based. If your business has opted to focus on a particular vendor, e.g., Oracle or SAP, for most services, then vendor-led data models are a possibility. These vendors are often involved with public data model development, so they won’t be too divergent in most situations – but awareness of differences is necessary, but as both models should be internally consistent, the differences will also be consistent. This approach will give you better alignment and reduce the chances of needing to address any divergence. The downside of this is a change of direction on strategic vendors can create additional work going forward as the alignment is disrupted. More work will be needed to map from your naming and semantics to the new core, and attempts to move away from the selected model to try to realign semantics with a new core will potentially create breaking changes for API consumers.
Don’t Forget
Regardless of the approach taken, there are some very simple but critical rules that will keep you in a good place:
- Don’t use your underlying storage data models – this is a well-documented API anti-pattern.
- Consistency of language across your APIs, regardless of whether they are internal or external, is important.
Information Sources
Regardless of approach – be careful not to lock your API semantics and data model to that of the storage layer – these can change and even create breaking changes that you shouldn’t expose to your users. Some sources to consider.
- OAGIS – covers a broad variety of business data domains. Some ERP suppliers have used this as a foundation for their application data models.
- OASIS – covers many industries
- TMForum APIs
- ARTS (formally hosted by NRF now with the OMG). The full OMG standards catalog.
- GS1 – lots here on shipping, supply chain, and product tracking
Some more reading on the subject:
Phil (aka MP3Monster)'s Blog 