Do Portals go down if maintenance is occurring on the environment that the Portals are published in?Will Portals go down if the environment goes down in general?
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No, in both cases. The Portal runs on separate infrastructure that doesn't have downtime when the environment does.
This does mean that any calls from the Portal to the environment will fail if the environment is down. I'd encourage you to put error handling around your integration calls to catch the case where they return nothing.
This also means that you can make your own maintenance pages for planned downtime. You'd make a "maintenance page" interface rule that shows whatever static message you want, then republish your Portal with that interface before the maintenance window.
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