Workers as your fallback origin
If you are building your application on Cloudflare Workers, you can use a Worker as the origin for your SaaS zone (also known as your fallback origin).
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In your SaaS zone, create and set a fallback origin. Ensure the fallback origin only has an originless DNS record: - Example: service.example.com AAAA 100::
 
- Example: 
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In that same zone, navigate to Workers Routes. 
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Click Add route. 
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Decide whether you want traffic bound for your SaaS zone ( example.com) to go to that Worker:- 
If yes, set the following values: - Route: */*(routes everything — including custom hostnames — to the Worker).
- Worker: Select the Worker used for your SaaS application.
 
- Route: 
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If no, set the following values: - Route: *.<zonename>.com/*(only routes custom hostname traffic to the Worker)
- Worker: None
 
- Route: 
 
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Click Save.