Consider calling ConfigureAwait on the awaited task

Consider calling ConfigureAwait on the awaited task

Member CalculateIdempotencyKey does not access instance data and can be marked as static (Shared in VisualBasic)

In externally visible method 'string IdempotencyStore.CalculateIdempotencyKey(OrderIdComponents orderId, string orderJson)', validate parameter 'orderId' is non-null before using it. If appropriate, throw an ArgumentNullException when the argument is null or add a Code Contract precondition asserting non-null argument.

Add the following constructor to SilentRollbackException: public SilentRollbackException(string message, Exception innerException).

Add the following constructor to SilentRollbackException: public SilentRollbackException(string message).

Exceptions should be public

Avoid empty interfaces

This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.