title: Re-using an existing SNS Topic in a Cloudformation template
body: <p>I have a SAM cloudformation template:</p>
  
  <pre><code>Transform: AWS::Serverless-2016-10-31
  Description: Create SNS with a sub
  
  Parameters:
      NotificationEmail:
          Type: String
          Description: Email address to subscribe to SNS topic
  
  Resources:
      NotificationTopic:
          Type: AWS::SNS::Topic
          DeletionPolicy: Retain
          Properties:
              TopicName: sam-test-sns
              Subscription:
                  - Endpoint: !Ref NotificationEmail
                    Protocol: email
  
  Outputs:
      SNSTopic:
          Value: !Ref NotificationTopic
  </code></pre>
  
  <p>So I want to keep the topic <strong>sam-test-sns</strong> around since there are several subscribers already, and I don't want subscribers to tediously re-subscribe if I tear down the service and bring it back up.</p>
  
  <p>Tearing down the service with <strong>Retain</strong> keeps the topic around, so that's fine. But when I try <a href="https://github.com/kaihendry/sam-cloudtrail-ec2/blob/master/Makefile#L10" rel="nofollow noreferrer">deploy</a> the template, it fails because it already exists.</p>
  
  <p><a href="https://i.stack.imgur.com/l8tPu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/l8tPu.png" alt="SNS already exists"></a></p>
  
  <p>So what is the right approach to use an existing SNS topic?</p>