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https://mrotaru.wordpress.com/2019/05/10/users-go-offline-but-you-can-still-keep-them-updated-use-migratorydata-presence-api-to-implement-your-fallback-from-realtime-messaging-to-notifications-push-notifications-email-sms-and/
Users go offline, but you can still keep them updated – Use MigratoryData Presence API to Implement your Fallback from Realtime Messaging to Notifications: Push Notifications, Email, SMS, and more