That's probably the Rack::Cache middleware that you have. The most simple way to fix it (and have a benefit of going through most layers of the application), is to add cache related header in your rack endpoint:
Cache-Control: no-cache
This will prevent Rack::Cache from caching the result of this action. BTW, the text you're returning is not a valid json, so either change the Content-Type to text/plain, or return something like {"result":"alive"}