First, the init/2 callback is called. This callback is common to all handlers. To use REST for the current request, this function must return a cowboy_rest tuple.

Cowboy will then switch to the REST protocol and start executing the state machine.

After reaching the end of the flowchart, the terminate/3 callback will be called if it is defined.

The REST component has code for handling the following HTTP methods: HEAD, GET, POST, PATCH, PUT, DELETE and OPTIONS.

Other methods can be accepted, however they have no specific callback defined for them at this time.

All callbacks are optional. Some may become mandatory depending on what other defined callbacks return. The various flowcharts in the next chapter should be a useful to determine which callbacks you need.

All callbacks take two arguments, the Req object and the State, and return a three-element tuple of the form {Value, Req, State}.

Nearly all callbacks can also return {stop, Req, State} to stop execution of the request, and {{switch_handler, Module}, Req, State} or {{switch_handler, Module, Opts}, Req, State} to switch to a different handler type. The exceptions are expires generate_etag, last_modified and variances.

The following table summarizes the callbacks and their default values. If the callback isn’t defined, then the default value will be used. Please look at the flowcharts to find out the result of each return value.

In the following table, "skip" means the callback is entirely skipped if it is undefined, moving directly to the next step. Similarly, "none" means there is no default value for this callback.

As you can see, Cowboy tries to move on with the request whenever possible by using well thought out default values.

In addition to these, there can be any number of user-defined callbacks that are specified through content_types_accepted/2 and content_types_provided/2. They can take any name, however it is recommended to use a separate prefix for the callbacks of each function. For example, from_html and to_html indicate in the first case that we’re accepting a resource given as HTML, and in the second case that we send one as HTML.

Cowboy will set informative values to the Req object at various points of the execution. You can retrieve them by matching the Req object directly. The values are defined in the following table:

They can be used to send a proper body with the response to a request that used a method other than HEAD or GET.

Cowboy will set response headers automatically over the execution of the REST code. They are listed in the following table.