Designing a resource handler

This chapter aims to provide you with a list of questions you must answer in order to write a good resource handler. It is meant to be usable as a step by step guide.

The service

Can the service become unavailable, and when it does, can we detect it? For example database connectivity problems may be detected early. We may also have planned outages of all or parts of the system. Implement the service_available callback.

What HTTP methods does the service implement? Do we need more than the standard OPTIONS, HEAD, GET, PUT, POST, PATCH and DELETE? Are we not using one of those at all? Implement the known_methods callback.

Type of resource handler

Am I writing a handler for a collection of resources, or for a single resource?

The semantics for each of these are quite different. You should not mix collection and single resource in the same handler.

Collection handler

Skip this section if you are not doing a collection.

Is the collection hardcoded or dynamic? For example if you use the route /users for the collection of users then the collection is hardcoded; if you use /forums/:category for the collection of threads then it isn't. When the collection is hardcoded you can safely assume the resource always exists.

What methods should I implement?

OPTIONS is used to get some information about the collection. It is recommended to allow it even if you do not implement it, as Cowboy has a default implementation built-in.

HEAD and GET are used to retrieve the collection. If you allow GET, also allow HEAD as there's no extra work required to make it work.

POST is used to create a new resource inside the collection. Creating a resource by using POST on the collection is useful when resources may be created before knowing their URI, usually because parts of it are generated dynamically. A common case is some kind of auto incremented integer identifier.

The next methods are more rarely allowed.

PUT is used to create a new collection (when the collection isn't hardcoded), or replace the entire collection.

DELETE is used to delete the entire collection.

PATCH is used to modify the collection using instructions given in the request body. A PATCH operation is atomic. The PATCH operation may be used for such things as reordering; adding, modifying or deleting parts of the collection.

Single resource handler

Skip this section if you are doing a collection.

What methods should I implement?

OPTIONS is used to get some information about the resource. It is recommended to allow it even if you do not implement it, as Cowboy has a default implementation built-in.

HEAD and GET are used to retrieve the resource. If you allow GET, also allow HEAD as there's no extra work required to make it work.

POST is used to update the resource.

PUT is used to create a new resource (when it doesn't already exist) or replace the resource.

DELETE is used to delete the resource.

PATCH is used to modify the resource using instructions given in the request body. A PATCH operation is atomic. The PATCH operation may be used for adding, removing or modifying specific values in the resource.

The resource

Following the above discussion, implement the allowed_methods callback.

Does the resource always exist? If it may not, implement the resource_exists callback.

Do I need to authenticate the client before they can access the resource? What authentication mechanisms should I provide? This may include form-based, token-based (in the URL or a cookie), HTTP basic, HTTP digest, SSL certificate or any other form of authentication. Implement the is_authorized callback.

Do I need fine-grained access control? How do I determine that they are authorized access? Handle that in your is_authorized callback.

Can access to a resource be forbidden regardless of access being authorized? A simple example of that is censorship of a resource. Implement the forbidden callback.

Is there any constraints on the length of the resource URI? For example the URI may be used as a key in storage and may have a limit in length. Implement uri_too_long.

Representations

What media types do I provide? If text based, what charsets are provided? What languages do I provide?

Implement the mandatory content_types_provided. Prefix the callbacks with to_ for clarity. For example to_html or to_text.

Implement the languages_provided or charsets_provided callbacks if applicable.

Is there any other header that may make the representation of the resource vary? Implement the variances callback.

Depending on your choices for caching content, you may want to implement one or more of the generate_etag, last_modified and expires callbacks.

Do I want the user or user agent to actively choose a representation available? Send a list of available representations in the response body and implement the multiple_choices callback.

Redirections

Do I need to keep track of what resources were deleted? For example you may have a mechanism where moving a resource leaves a redirect link to its new location. Implement the previously_existed callback.

Was the resource moved, and is the move temporary? If it is explicitly temporary, for example due to maintenance, implement the moved_temporarily callback. Otherwise, implement the moved_permanently callback.

The request

Do we need to perform extra checks to make sure the request is valid? Cowboy will do many checks when receiving the request already, do we need more? Note that this only applies to the request-line and headers of the request, and not the body. Implement malformed_request.

May there be a request body? Will I know its size? What's the maximum size of the request body I'm willing to accept? Implement valid_entity_length.

Finally, take a look at the sections corresponding to the methods you are implementing.

OPTIONS method

Cowboy by default will send back a list of allowed methods. Do I need to add more information to the response? Implement the options method.

GET and HEAD methods

If you implement the methods GET and/or HEAD, you must implement one ProvideResource callback for each content-type returned by the content_types_provided callback.

PUT, POST and PATCH methods

If you implement the methods PUT, POST and/or PATCH, you must implement the content_types_accepted callback, and one AcceptResource callback for each content-type it returns. Prefix the AcceptResource callback names with from_ for clarity. For example from_html or from_json.

Do we want to allow the POST method to create individual resources directly through their URI (like PUT)? Implement the allow_missing_post callback. It is recommended to explicitly use PUT in these cases instead.

May there be conflicts when using PUT to create or replace a resource? Do we want to make sure that two updates around the same time are not cancelling one another? Implement the is_conflict callback.

DELETE methods

If you implement the method DELETE, you must implement the delete_resource callback.

When delete_resource returns, is the resource completely removed from the server, including from any caching service? If not, and/or if the deletion is asynchronous and we have no way of knowing it has been completed yet, implement the delete_completed callback.

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