Subscribe   storage, it could be data table in data base or even XML file.

https://msdn.microsoft.com/EN-US/library/office/jj163976.aspx

Set   trigger on business data table which will fire when data change

https://msdn.microsoft.com/EN-US/library/office/jj163976.aspx

Create   OData Service base on business data, implement subscribe/ unsubscribe in   service

https://msdn.microsoft.com/EN-US/library/office/jj163976.aspx

http://blogs.msdn.com/b/microsoft_press/archive/2013/08/26/from-the-mvps-sharepoint-2013-business-data-connectivity-odata-models-with-notification-subscriptions.aspx

Create   SharePoint add-in to consume OData Service

https://msdn.microsoft.com/en-us/library/office/jj163967.aspx

Convert   app-scoped to tenant-scoped, upload to SharePoint CA with permission,

Create   external list etc.

https://msdn.microsoft.com/en-us/library/office/dn130133.aspx

Implement   subscribe/ unsubscribe functionality in SharePoint side

http://blogs.msdn.com/b/microsoft_press/archive/2013/08/26/from-the-mvps-sharepoint-2013-business-data-connectivity-odata-models-with-notification-subscriptions.aspx

Implement   event receiver on external list in SharePoint

https://msdn.microsoft.com/en-us/library/jj164024.aspx

Implement   function for sending notification to SharePoint

http://blogs.msdn.com/b/microsoft_press/archive/2013/08/26/from-the-mvps-sharepoint-2013-business-data-connectivity-odata-models-with-notification-subscriptions.aspx