Made some progress

app/services/process_interaction_service.rb

@@ -0,0 +1,38 @@
+class ProcessInteractionService
+  def call(envelope, target_account)
+    body = salmon.unpack(envelope)
+    xml  = Nokogiri::XML(body)
+
+    return if xml.at_xpath('//author/name').nil? || xml.at_xpath('//author/uri').nil?
+
+    username = xml.at_xpath('//author/name').content
+    url      = xml.at_xpath('//author/uri').content
+    domain   = Addressable::URI.parse(url).host
+    account  = Account.find_by(username: username, domain: domain)
+
+    if account.nil?
+      account = follow_remote_account_service.("acct:#{username}@#{domain}")
+    end
+
+    if salmon.verify(envelope, account.keypair)
+      verb = xml.at_path('//activity:verb').content
+
+      case verb
+      when 'http://activitystrea.ms/schema/1.0/follow', 'follow'
+        account.follow!(target_account)
+      when 'http://activitystrea.ms/schema/1.0/unfollow', 'unfollow'
+        account.unfollow!(target_account)
+      end
+    end
+  end
+
+  private
+
+  def salmon
+    OStatus2::Salmon.new
+  end
+
+  def follow_remote_account_service
+    FollowRemoteAccountService.new
+  end
+end