title: AWS SDK Ruby DynamoDB session_token invalid
body: <p>I get: The security token included in the request is invalid</p>
  
  <p>Where do I get a session token? The current documentation is confusing. On the one hand it says:</p>
  
  <p><em>If you make a request using AWS::DynamoDB with long-term credentials a request is made to Amazon STS for temporary session credentials. These will be cached in the process and re-used.</em></p>
  
  <p>and next it says:</p>
  
  <p><em>Amazon DynamoDB requires that all requests are made with short-term credentials (e.g. requires a session token).</em></p>
  
  <p>so exactly where does the session_token come from if I don't provide it? And if the call provides it, why would it be invalid? </p>
  
  <pre><code>requires 'aws-sdk'
  
  cred = {:access_key_id =&gt; 'xxxx', 
  :secret_access_key =&gt; 'yyyy'}
  
  #:session_token =&gt; ''}
  
  ddb = AWS::DynamoDB.new(cred)
  items = {...}
  ddb.batch_write do |batch|
    batch.put('my_mappings', items)
  end
  </code></pre>