Way to avoid Callouts during maintenance or site switching activity (during read only mode)

Salesforce maintain each instance in 2 different geographical locations and perform site switching after every 6 months. One location act as "active site" with all transaction and replicate all transaction in second location called as "ready site" in near real time.

Site switching helps Salesforce to perform maintenance activity and to plan for disaster recovery. Ideally this activity takes 1 hour and Salesforce instance will be available in read only mode.

If you have callout to external system and based on response you are trying to update anything inside salesforce then during read only mode Salesforce execute all callout but update on salesforce will be blocked. This may create some inconsistency in your program flow.

To avoid this scenario, first check if system is in read only mode or not then perform write operation in salesforce.

Salesforce provide below method through which you can identify if instance is read only mode or in default mode.

System.getApplicationReadWriteMode(); 

If this method return "ApplicationReadWriteMode.READ_ONLY" enum value, then the org is in read-only mode. If it return "ApplicationReadWriteMode.DEFAULT" then org is available in read/write mode.

Below is sample code which you can use in your apex logic

// check read only status for org
ApplicationReadWriteMode currentMode = System.getApplicationReadWriteMode();
String returnValue = '';

if (currentMode == ApplicationReadWriteMode.READ_ONLY) {
// avoid callout

} else if (currentMode == ApplicationReadWriteMode.DEFAULT) {
//perform callout
// write back in salesforce based on callout response

}

How to test read only mode in sandbox

Contact salesforce to enable read only test mode option for sandbox. Once enabled, you can toggle read only mode test functionality in sandbox.

Hope this will help!!!