I have used REST in the past for creating/updating single record and used Bulk API to create records in bulk too. But there seems to be new way to creating multiple records using REST. Here is the article - https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_composite_sobject_tree_flat.htm
Can someone post few help articles on how to get started? I have not used curl in the past, is there way to test this using PostMaster or WorkBench?
Edit
Let me make it simple. Suppose I have below REST resource in Salesforce, how can I call it to pass multiple contact records?
@RestResource(urlMapping='/insertcontact/*')
global with sharing class Mycontacts{
@HttpPost global static List doPost(List lstcnts){ insert lstcnts; return lstcnts; }
}