From the apex docs System.ApplicationReadWriteMode is set to READONLY mode during major platform upgrades. Does this mean that any public sites or webservice/REST methods will continue to work so long as they do not perform any DML operations?
Specifically I want to know that they will be able to query existing records and make httprequests to external services.
In the help there is something called 5 minute upgrade which says that you will have readonly access but only to standard objects which seems to contradict this.
At what point during platform upgrades and other maintenance windows is saleforce in READONLY mode and at what point is even that down?