When persisting a JSON document to Azure Cosmos DB, you used to have to ask, “Should I use a Create or a Replace operation?” With added support for atomic Upsert operations in Azure Cosmos DB, the database makes this decision for you. This saves you the additional request unit charges, and because the operation is atomic, it removes the possibility of a race condition. In addition to making Upsert requests through the client, you can use the JavaScript server-side SDK for this functionality when you are building stored procedures and triggers. To make use of the Upsert feature, you need to download the latest SDK, based on the platform you use:
For more information, please visit the Azure blog: Azure Cosmos DB: To create, or not to create, that is the question.