General principles for good URI design

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General principles for good URI design:

Don't use query parameters to alter state
Don't use mixed-case paths if you can help it; lowercase is best
Don't use implementation-specific extensions in your URIs (.php, .py, .pl, etc.)
Don't fall into RPC with your URIs
Do limit your URI space as much as possible
Do keep path segments short
Do prefer either /resource or /resource/; create 301 redirects from the one you don't use
Do use query parameters for sub-selection of a resource; i.e. pagination, search queries
Do move stuff out of the URI that should be in an HTTP header or a body

General principles for HTTP method choice:

Don't ever use GET to alter state
Don't use PUT unless you are updating an entire resource
Don't use PUT unless you can also legitimately do a GET on the same URI
Don't use POST to retrieve information that is long-lived or that might be reasonable to cache
Don't perform an operation that is not idempotent with PUT
Do use GET for as much as possible
Do use POST in preference to PUT when in doubt
Do use POST whenever you have to do something that feels RPC-like
Do use PUT for classes of resources that are larger or hierarchical
Do use DELETE in preference to POST to remove resources
Do use GET for things like calculations, unless your input is large, in which case use POST

General principles of web service design with HTTP:

Don't put metadata in the body of a response that should be in a header
Don't put metadata in a separate resource unless including it would create significant overhead
Do use the appropriate status code
201 Created after creating a resource; resource must exist at the time the response is sent
202 Accepted after performing an operation successfully or creating a resource asynchronously
400 Bad Request when someone does an operation on data that's clearly bogus; for your application this could be a validation error; generally reserve 500 for uncaught exceptions
403 Forbidden when someone accesses your API in a way that might be malicious or if they aren't authorized
405 Method Not Allowed when someone uses POST when they should have used PUT, etc
413 Request Entity Too Large when someone attempts to send you an unacceptably large file
418 I'm a teapot when attempting to brew coffee with a teapot
Do use caching headers whenever you can
ETag headers are good when you can easily reduce a resource to a hash value
Last-Modified should indicate to you that keeping around a timestamp of when resources are updated is a good idea
Cache-Control and Expires should be given sensible values
Do everything you can to honor caching headers in a request (If-None-Modified, If-Modified-Since)
Do use redirects when they make sense, but these should be rare for a web service