Without web-service support, an action which collects the data for displaying a list of people might look something like this:

def index
  @people = Person.all
end

That action implicitly responds to all formats, but formats can also be whitelisted:

def index
  @people = Person.all
  respond_to :html, :js
end

Here's the same action, with web-service support baked in:

def index
  @people = Person.all

  respond_to do |format|
    format.html
    format.js
    format.xml { render xml: @people }
  end
end

What that says is, “if the client wants HTML or JS in response to this action, just respond as we would have before, but if the client wants XML, return them the list of people in XML format.” (Rails determines the desired response format from the HTTP Accept header submitted by the client.)

Supposing you have an action that adds a new person, optionally creating their company (by name) if it does not already exist, without web-services, it might look like this:

def create
  @company = Company.find_or_create_by(name: params[:company][:name])
  @person  = @company.people.create(params[:person])

  redirect_to(person_list_url)
end

Here's the same action, with web-service support baked in:

def create
  company  = params[:person].delete(:company)
  @company = Company.find_or_create_by(name: company[:name])
  @person  = @company.people.create(params[:person])

  respond_to do |format|
    format.html { redirect_to(person_list_url) }
    format.js
    format.xml  { render xml: @person.to_xml(include: @company) }
  end
end

If the client wants HTML, we just redirect them back to the person list. If they want JavaScript, then it is an Ajax request and we render the JavaScript template associated with this action. Lastly, if the client wants XML, we render the created person as XML, but with a twist: we also include the person's company in the rendered XML, so you get something like this:

<person>
  <id>...</id>
  ...
  <company>
    <id>...</id>
    <name>...</name>
    ...
  </company>
</person>

Note, however, the extra bit at the top of that action:

company  = params[:person].delete(:company)
@company = Company.find_or_create_by(name: company[:name])

This is because the incoming XML document (if a web-service request is in process) can only contain a single root-node. So, we have to rearrange things so that the request looks like this (url-encoded):

person[name]=...&person[company][name]=...&...

And, like this (xml-encoded):

<person>
  <name>...</name>
  <company>
    <name>...</name>
  </company>
</person>

In other words, we make the request so that it operates on a single entity's person. Then, in the action, we extract the company data from the request, find or create the company, and then create the new person with the remaining data.

Note that you can define your own XML parameter parser which would allow you to describe multiple entities in a single request (i.e., by wrapping them all in a single root node), but if you just go with the flow and accept Rails' defaults, life will be much easier.

If you need to use a MIME type which isn't supported by default, you can register your own handlers in config/initializers/mime_types.rb as follows.

Mime::Type.register "image/jpg", :jpg

Respond to also allows you to specify a common block for different formats by using any:

def index
  @people = Person.all

  respond_to do |format|
    format.html
    format.any(:xml, :json) { render request.format.to_sym => @people }
  end
end

In the example above, if the format is xml, it will render:

render xml: @people

Or if the format is json:

render json: @people

Formats can have different variants.

The request variant is a specialization of the request format, like :tablet, :phone, or :desktop.

We often want to render different html/json/xml templates for phones, tablets, and desktop browsers. Variants make it easy.

You can set the variant in a before_action:

request.variant = :tablet if request.user_agent =~ /iPad/

Respond to variants in the action just like you respond to formats:

respond_to do |format|
  format.html do |variant|
    variant.tablet # renders app/views/projects/show.html+tablet.erb
    variant.phone { extra_setup; render ... }
    variant.none  { special_setup } # executed only if there is no variant set
  end
end

Provide separate templates for each format and variant:

app/views/projects/show.html.erb
app/views/projects/show.html+tablet.erb
app/views/projects/show.html+phone.erb

When you're not sharing any code within the format, you can simplify defining variants using the inline syntax:

respond_to do |format|
  format.js         { render "trash" }
  format.html.phone { redirect_to progress_path }
  format.html.none  { render "trash" }
end

Variants also support common any/all block that formats have.

It works for both inline:

respond_to do |format|
  format.html.any   { render html: "any"   }
  format.html.phone { render html: "phone" }
end

and block syntax:

respond_to do |format|
  format.html do |variant|
    variant.any(:tablet, :phablet){ render html: "any" }
    variant.phone { render html: "phone" }
  end
end

You can also set an array of variants:

request.variant = [:tablet, :phone]

This will work similarly to formats and MIME types negotiation. If there is no :tablet variant declared, :phone variant will be picked:

respond_to do |format|
  format.html.none
  format.html.phone # this gets rendered
end