Millions of Accounts are updated every quarter from an external system. What is the optimal way to update these in Salesforce?

The Bulk API is specifically designed for asynchronously loading or deleting large sets of data, ranging from a few thousand to millions of records. Given the requirement to update "Millions of Accounts," the Bulk API is the optimal choice. It processes data in batches, operates asynchronously in the background, and is built to handle large data volumes efficiently without being constrained by the typical governor limits of synchronous APIs.

A. The Composite REST API is for executing a series of related API requests in a single call, not for processing millions of records. It has a limit on subrequests.

B. The SOAP API is a synchronous API best suited for real-time client applications that update a few records at a time, not for mass updates of millions of records.

D. An Apex REST Web Service is subject to Apex governor limits, such as CPU time and heap size, making it unsuitable and inefficient for processing millions of records.

1. Bulk API 2.0 and Bulk API Developer Guide, "Which API Should I Use?". The guide explicitly recommends Bulk API for "loading a large set of data into your Salesforce org for the first time" or for operations involving "more than 2,000 records."

2. Salesforce Architect - Record-Triggered Automation, "Choosing the Right Automation Tool". While focused on automation, the principles highlight using asynchronous, bulk-safe tools for large data volume operations, a category where the Bulk API is the primary integration tool.