HTTP Header Guidelines

Deprecated X-Foo Naming Scheme

In RFC 6648 the recommendation to prefix application-specific headers with X- was retracted. It is mentioned in RFC 2616 as a permanently-reserved prefix for implementors, but is deprecated due to the complexities of migrating prefixed headers to standardized ones. This has resulted in some standards reserving X-prefixed names in addition to their non-prefixed headers. (see X-Archived-At/Archived-At) In the more recent RFC 7231 designers of new protocols are discouraged from using X-prefixed headers and to keep new headers short where possible.

Guidance

This does not mean it is recommended to replace existing uses of X-, or in using X- in private/local/development contexts. New APIs (or new API features) should make their best effort to not use header names that conflict with other applications. To do this, use “OpenStack” and the service name in the header. An example might be “OpenStack-Compute-FooBar”, which is unlikely to be standardized already or conflict with existing headers.

RFC 6648 intentionally does not disallow using X- as a prefix, but does remove the experimental/unstandardized semantics from the prefix. For existing projects, it is acceptable to create new headers prefixed with X since it is likely that the rest of the headers already standardized in the API begin with X-.

Examples

Some good header names that are clear, unlikely to conflict, and could become standardized might be OpenStack-Identity-Status Some headers that are at risk for conflicts might look like:

Account-ID
Host-Name
Storage-Policy

In these cases, adding OpenStack- as a prefix resolves the ambiguity, as in:

OpenStack-Identity-Account-ID
OpenStack-Networking-Host-Name
OpenStack-Object-Storage-Policy

Avoid Proliferating Headers

It can be tempting to use the names of headers as a way of passing specific information between the client and the server. Where possible this should be avoided in favor of using a more generic header name and placing the specifics in the value. For example compare the following two headers:

OpenStack-API-Version: compute 2.1
OpenStack-Nova-API-Version: 2.1

Note

The first header is the recommended form. The second header is in the form of a microversion header currently in use. It effectively demonstrates the problem. Also note that whereas the second header uses a service name, the first header uses the more correct service type.

At first glance these header name and value pairs convey the same information, with the second option being a bit easier to parse on the server side. However consider the following problems when using the second form:

  • A new header is needed every time there is a new service.
  • It violates the principle that in key-value based data structures the key should be an accessor only, that is: It should be opaque and generic.
  • If CORS [1] middleware is being used, it needs to be configured to allow a multitude of headers instead of a generic one.
  • Generic library code (in either the client or the server) that is supposed to deal with this class of header has to construct or parse strings on both sides of the name-value pair.
[1]https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS

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