Create patient resource using a deterministic id

src/lib/Microsoft.Health.Extensions.Fhir.R4/Service/ResourceManagementService.cs

@@ -4,6 +4,8 @@
 // -------------------------------------------------------------------------------------------------
 
 using System;
+using System.Security.Cryptography;
+using System.Text;
 using System.Threading.Tasks;
 using EnsureThat;
 using Hl7.Fhir.Model;
@@ -69,13 +71,36 @@ protected async Task<TResource> CreateResourceByIdentityAsync<TResource>(Model.I
 
             propertySetter?.Invoke(resource, identifier);
 
-            return await FhirService.CreateResourceAsync(resource).ConfigureAwait(false);
+            // Generate id programatically and issue an update to FHIR service
+            // The resource will be created if the resource with the given id does not already exist.
+            resource.Id = ComputeSha256(identifier);
+            return await FhirService.UpdateResourceAsync<TResource>(resource).ConfigureAwait(false);
         }
 
         private static Model.Identifier BuildIdentifier(string value, string system)
         {
             var identifier = new Model.Identifier { Value = value, System = string.IsNullOrWhiteSpace(system) ? null : system };
             return identifier;
         }
+
+        private static string ComputeSha256(Model.Identifier identifier)
+        {
+            EnsureArg.IsNotNullOrWhiteSpace(identifier.Value, nameof(identifier.Value));
+
+            string plainTextSystemAndId = $"{identifier.System}_{identifier.Value}";
+
+            using (SHA256 hashAlgorithm = SHA256.Create())
+            {
+                byte[] bytes = hashAlgorithm.ComputeHash(Encoding.UTF8.GetBytes(plainTextSystemAndId));
+
+                StringBuilder sb = new StringBuilder();
+                for (int i = 0; i < bytes.Length; i++)
+                {
+                    sb.Append(bytes[i].ToString("x2"));
+                }
+
+                return sb.ToString();
+            }
+        }
     }
 }