The batch activity endpoints allow you to add or update up to 20,000 contacts by making either 1 or 2 API calls.
This process imports all contacts in your local application or database to Constant Contact, regardless of whether they are:
You do not need to filter or compare contact data in Constant Contact with your local data. The only property that will change for contacts whose data was unchanged locally is that the modified_date property, which reflects the time of the import activity.
Do this as follows:
When executed, all contacts are imported into Constant Contact:
This batch import process updates only new contacts or existing contacts whose information has changed using only 2 API calls, one to export the contacts from Constant Contact, and one to import them back in.