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If you have a hosted application, that when customers signup, they choose a subdomain like:

When they do this, I want to create a custom dns record that will map to a specific i.p address.

The reason is for sharding users to different clusters.

If I do this, when the customer finishes signup up, it might take a while for the url to resolve correct?

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DNS records are created and updated instantly. It might take longer if you're using a third party service for DNS, but then that's because they purposely slow it down (for instance, Linode applies DNS changes made through their web panel every 15 minutes).

If you change DNS records, then it might take a while for the various DNS servers on the internet to drop the existing record off their cache, but the actual update on the server is instant.

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