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MAY Use JSON:API to Encode Structured Data

APIs could represent resources with many different structures, but picking a standard is a good thing. JSON:API is a good approach and enables useful things like including related resources in the same request. I tend to prefer a minimal JSON:API implementation to start with and add features if required.

The most important thing, however, is to pick a structure and stick to it.

MAY Use non-JSON Media Types for Binary Data or Alternative Content Representations

Other media types may be used in following cases:

  • Transferring binary data or data whose structure is not relevant. This is the case if payload structure is not interpreted and consumed by clients as is. Example of such use case is downloading images in formats JPG, PNG, GIF.

  • In addition to JSON version alternative data representations (e.g. in formats PDF, DOC, XML) may be made available through content negotiation.

MUST Use Standard Date and Time Formats

JSON Payload

Represent date and time format as RFC 3339.

HTTP headers

HTTP headers including the proprietary headers use the HTTP date format defined in RFC 7231.

MAY Use Standards for Country, Language and Currency Codes

Use the following standard formats for country, language and currency codes:

MUST Define Format for Type Number and Integer

Whenever an API defines a property of type number or integer, the precision must be defined by the format as follows to prevent clients from guessing the precision incorrectly, and thereby changing the value unintentionally:

type format specified value range
integer int32 integer between -2^31 and 2^31-1
integer int64 integer between -2^63 and 2^63-1
integer bigint arbitrarily large signed integer number
number float IEEE 754-2008/ISO 60559:2011 binary64 decimal number
number double IEEE 754-2008/ISO 60559:2011 binary128 decimal number
number decimal arbitrarily precise signed decimal number

The precision must be translated by clients and servers into the most specific language types. E.g. for the following definitions the most specific language types in Java will translate to BigDecimal for Money.amount and int or Integer for the OrderList.page_size:

components:
  schemas:
    Money:
      type: object
      properties:
        amount:
          type: number
          description: Amount expressed as a decimal number of major currency units
          format: decimal
          example: 99.95
       ...

    OrderList:
      type: object
      properties:
        page_size:
          type: integer
          description: Number of orders in list
          format: int32
          example: 42

MUST Use camelCase property names: ^[a-z][A-Z0-9]*$

Property names are restricted to ASCII strings. The first character must be a letter, or an underscore, and subsequent characters can be a letter, or a number.

MUST Pluralize array names

To indicate they contain multiple values prefer to pluralize array names. This implies that object names should in turn be singular.

MUST Not have null boolean property values

Schema based JSON properties that are by design booleans must not be presented as nulls. A boolean is essentially a closed enumeration of two values, true and false. If the content has a meaningful null value, strongly prefer to replace the boolean with enumeration of named values or statuses - for example acceptedTermsAndConditions with true or false can be replaced with termsAndConditions with values yes, no and unknown.

SHOULD Remove null value fields

OpenAPI, which is in common use, doesn’t support null field values (it does allow omitting that field completely if it is not marked as required). However that doesn’t prevent clients and servers sending and receiving those fields with null values. Also, in some cases null may be a meaningful value - for example, JSON Merge Patch RFC 7382) using null to indicate property deletion.

MUST Use empty array values instead of null

Empty array values can unambiguously be represented as the empty list, [].

SHOULD Ensure date property values conform to RFC 3339

Use the date and time formats defined by RFC 3339:

  • for "date" use strings matching date-fullyear "-" date-month "-" date-mday, for example: 2015-05-28

  • for "date-time" use strings matching full-date "T" full-time, for example 2015-05-28T14:07:17Z

Note that the OpenAPI format "date-time" corresponds to "date-time" in the RFC) and 2015-05-28 for a date (note that the OpenAPI format "date" corresponds to "full-date" in the RFC). Both are specific profiles, a subset of the international standard ISO 8601.

A zone offset may be used (both, in request and responses) — this is simply defined by the standards. However, we encourage restricting dates to UTC and without offsets. For example 2015-05-28T14:07:17Z rather than 2015-05-28T14:07:17+00:00. From experience we have learned that zone offsets are not easy to understand and often not correctly handled. Note also that zone offsets are different from local times that might be including daylight saving time. Localization of dates should be done by the services that provide user interfaces, if required.

When it comes to storage, all dates should be consistently stored in UTC without a zone offset. Localization should be done locally by the services that provide user interfaces, if required.

Sometimes it can seem data is naturally represented using numerical timestamps, but this can introduce interpretation issues with precision

  • for example whether to represent a timestamp as 1460062925, 1460062925000 or 1460062925.000. Date strings, though more verbose and requiring more effort to parse, avoid this ambiguity.

SHOULD Ensure time durations and intervals conform to ISO 8601

Schema based JSON properties that are by design durations and intervals could be strings formatted as recommended by ISO 8601 (Appendix A of RFC 3339 contains a grammar for durations).

SHOULD Use standards for Language, Country and Currency