You could implement something like this.
Or maybe you can have a "ConfigTable" in your database, with property, value, expiration time.
So, when the admin updates some value, the other applications in the network can pool the new value.
Regarding the pooling approach, (We don't have enough information to give you the best solution) you could try something like this.
A notification_table with ID, Event (Event is something that you know, maybe load new images, etc)
Workflow
1) The clients pool from this table every X seconds, filtering with ID > lastNotificationId (that needs to be saved in local file) 2) The client applies the new events. 3) The client updates the local file with new notificationId.