gistfile1.txt

diff --git a/server/lib/deltacloud/backend_capability.rb b/server/lib/deltacloud/backend_capability.rb
new file mode 100644
index 0000000..bec8714
--- /dev/null
+++ b/server/lib/deltacloud/backend_capability.rb
@@ -0,0 +1,21 @@
+module Deltacloud::BackendCapability
+
+  class Failure < StandardError
+    attr_reader :capability
+    def initialize(capability, msg='')
+      super(msg)
+      @capability = capability
+    end
+  end
+
+  attr_reader :capability
+  def with_capability(capability)
+    @capability = capability
+  end
+
+  def check_capability(backend)
+    if capability and !backend.respond_to?(capability)
+      raise Failure.new(capability, "#{capability} capability not supported by backend #{backend.class.name}")
+    end
+  end
+end
diff --git a/server/views/errors/backend_capability_failure.html.haml b/server/views/errors/backend_capability_failure.html.haml
new file mode 100644
index 0000000..dc830d8
--- /dev/null
+++ b/server/views/errors/backend_capability_failure.html.haml
@@ -0,0 +1,11 @@
+%h1 Backend Capability Failure
+
+%p= @error.message
+
+%dl
+  %di
+    %dt Request URL
+    %dd= request.env['REQUEST_URI']
+  %di
+    %dt Capability
+    %dd= @error.capability
diff --git a/server/views/errors/backend_capability_failure.xml.haml b/server/views/errors/backend_capability_failure.xml.haml
new file mode 100644
index 0000000..83892fb
--- /dev/null
+++ b/server/views/errors/backend_capability_failure.xml.haml
@@ -0,0 +1,4 @@
+%error{:url => "#{request.env['REQUEST_URI']}", :status => "#{response.status}"}
+  %capability #{@error.capability}
+  %message #{@error.message}
+
diff --git a/server/lib/sinatra/rabbit.rb b/server/lib/sinatra/rabbit.rb
index f411268..94c74e3 100644
--- a/server/lib/sinatra/rabbit.rb
+++ b/server/lib/sinatra/rabbit.rb
@@ -1,6 +1,7 @@
 require 'sinatra/base'
 require 'sinatra/url_for'
 require 'deltacloud/validation'
+require 'deltacloud/backend_capability'
 
 module Sinatra
 
@@ -13,6 +14,7 @@ module Sinatra
     class Operation
       attr_reader :name, :method
 
+      include ::Deltacloud::BackendCapability
       include ::Deltacloud::Validation
 
       STANDARD = {
@@ -58,6 +60,7 @@ module Sinatra
       def control(&block)
         op = self
         @control = Proc.new do
+          op.check_capability(driver)
           op.validate(params)
           instance_eval(&block)
         end
diff --git a/server/server.rb b/server/server.rb
index af8bc09..7e226a8 100644
--- a/server/server.rb
+++ b/server/server.rb
@@ -32,6 +32,9 @@ error Deltacloud::Validation::Failure do
   report_error(400, "validation_failure")
 end
 
+error Deltacloud::BackendCapability::Failure do
+  report_error(405, "backend_capability_failure")
+end
 error Deltacloud::AuthException do
   report_error(403, "auth_exception")
 end
@@ -324,6 +327,7 @@ collection :keys do
 
   operation :index do
     description "List all available credentials which could be used for instance authentication."
+    with_capability :keys
     control do
       filter_all :keys
     end
@@ -331,18 +335,16 @@ collection :keys do
 
   operation :show do
     description "Show details about given instance credential."
+    with_capability :key
     param :id,  :string,  :required
     control { show :key }
   end
 
   operation :create do
     description "Create a new instance credential if backend supports this."
+    with_capability :create_key
     param :name,  :string,  :required
     control do
-      unless driver.respond_to?(:create_key)
-        raise Deltacloud::BackendFeatureUnsupported.new('501',
-          'Creating instance credentials is not supported in backend')
-      end
       @key = driver.create_key(credentials, { :key_name => params[:name] })
       respond_to do |format|
         format.html { haml :"keys/show" }
@@ -353,12 +355,9 @@ collection :keys do
 
   operation :destroy do
     description "Destroy given instance credential if backend supports this."
+    with_capability :destroy_key
     param :id,  :string,  :required
     control do
-      unless driver.respond_to?(:destroy_key)
-        raise Deltacloud::BackendFeatureUnsupported.new('501',
-          'Creating instance credentials is not supported in backend')
-      end
       driver.destroy_key(credentials, { :key_name => params[:id]})
       redirect(keys_url)
     end