azure - Document DB - ETag Optimistic Concurrency with Session Consistency -

if have documentdb database session consistency, guarantees there concurrent writes of same document using etags? example, , b operating in different sessions:

• read:a - etag:1
• read:b - etag:1
• write:a - etag:1 (match, etag = 2)
• write:b - etag:1 ?

the assumption write:b fails since etag 2, haven't found explicit documentation on guarantee there documentdb reads etag value on server, in, could read secondary replica etag still 1, , let write:b still go through?

this question session guarantee on documentdb discusses issue in comments, not seem come conclusion.

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web documentation

from faq https://docs.microsoft.com/en-us/azure/documentdb/documentdb-faq

from example source (implies, not state, holds true @ consistency levels?) https://github.com/azure/azure-documentdb-dotnet/blob/master/samples/code-samples/documentmanagement/program.cs:

documentdb team member here. confirming larry's reply conditional writes etags enforced consistency levels. write request must include etag (if-match header in rest, requestoptions.accesscondition in .net).