Updating the salesforce field values using patch by constructing the JSON payload for the data which needs updating. How to specify update the records only if the field is blank or null? please help. thanks.

Pulling in all the contacts from salesforce. But, how to get only the modiifed objects either using etags / if modified since as I dont want to import millions of contacts again if not modified ?

Connecting the Salesforce Account through Oauth via connected apps. Is it possible to create push topic for various organisation salesforce account through one developer account in workbench. Any inputs on this as am new to salesforce streaming. Thanks 

How to import all the cotacts from salesforce accounts through REST efficiently and push to MYSQL without time out issues.

As per documentation,
' If you enter multiple callback URLs, at run time Salesforce matches the callback URL value specified by the application with one of the values in Callback URL. It must match one of the values to pass validation.'
https://help.salesforce.com/HTViewHelpDoc?id=connected_app_create.htm

How can the value be specified in the field? Is it separated by comma or spaces? Thanks.

Updating the salesforce field values using patch by constructing the JSON payload for the data which needs updating. How to specify update the records only if the field is blank or null? please help. thanks.

Pulling in all the contacts from salesforce. But, how to get only the modiifed objects either using etags / if modified since as I dont want to import millions of contacts again if not modified ?

Connecting the Salesforce Account through Oauth via connected apps. Is it possible to create push topic for various organisation salesforce account through one developer account in workbench. Any inputs on this as am new to salesforce streaming. Thanks 

As per documentation,
' If you enter multiple callback URLs, at run time Salesforce matches the callback URL value specified by the application with one of the values in Callback URL. It must match one of the values to pass validation.'
https://help.salesforce.com/HTViewHelpDoc?id=connected_app_create.htm

How can the value be specified in the field? Is it separated by comma or spaces? Thanks.