Handling two upserts on the same object in cosmos db - azure

Question

Suppose I have a document with a 'count' field and I want to increase the count field everytime a function is called, something like:
container.items.upsert({id: "test", count:currentCount+1})
Where 'currentCount' was the lasts value received from the document with id 'test'.
If an async call is made to increment the count during another count (from the time between currentCount is retrieved and the upsert happens) the second async call will have the wrong data (i.e. the currentCount from before the first calls incrementing of the currentCount).
How would I go about preventing such a scenario?

Answer 1

How would I go about preventing such a scenario?
To prevent such scenarios, you should use Optimistic Concurrency. Essentially make use of document's ETag property to include in your upsert requests. When you fetch the document, you get it's ETag back. You need to include the same value in your upsert request. If the document has not changed on the server (i.e. the ETag value is the same), then the update operation will succeed otherwise it will fail.
From this blog post, here's the code sample:
using System;
using System.Linq;
using System.Net;
using System.Threading.Tasks;
using Microsoft.Azure.Documents;
using Microsoft.Azure.Documents.Client;
using Shouldly;
using Xunit;
namespace Demo
{
public class OptimtimisticConcurrencyTests
{
private readonly DocumentClient _client;
private const string EndpointUrl = "https://localhost:8081";
private const string AuthorizationKey = "C2y6yDjf5/R+ob0N8A7Cgv30VRDJIWEHLM+4QDU5DE2nQ9nDuVTqobD4b8mGGyPMbIZnqyMsEcaGQy67XIw/Jw==";
private const string DatabaseId = "ConcurrencyDemo";
private const string CollectionId = "Customers";
public OptimtimisticConcurrencyTests()
{
_client = new DocumentClient(new Uri(EndpointUrl), AuthorizationKey);
}
[Fact]
public async Task Should_Throw_With_PreconditionFailed()
{
// Setup our Database and add a new Customer
var dbSetup = new DatabaseSetup(_client);
await dbSetup.Init(DatabaseId, CollectionId);
var addCustomer = new Customer(Guid.NewGuid().ToString(), "Demo");
await dbSetup.AddCustomer(addCustomer);
// Fetch out the Document (Customer)
var document = (from f in dbSetup.Client.CreateDocumentQuery(dbSetup.Collection.SelfLink)
where f.Id == addCustomer.Id
select f).AsEnumerable().FirstOrDefault();
// Cast the Document to our Customer & make a data change
var editCustomer = (Customer) (dynamic) document;
editCustomer.Name = "Changed";
// Using Access Conditions gives us the ability to use the ETag from our fetched document for optimistic concurrency.
var ac = new AccessCondition {Condition = document.ETag, Type = AccessConditionType.IfMatch};
// Replace our document, which will succeed with the correct ETag
await dbSetup.Client.ReplaceDocumentAsync(document.SelfLink, editCustomer,
new RequestOptions {AccessCondition = ac});
// Replace again, which will fail since our (same) ETag is now invalid
var ex = await dbSetup.Client.ReplaceDocumentAsync(document.SelfLink, editCustomer,
new RequestOptions {AccessCondition = ac}).ShouldThrowAsync<DocumentClientException>();
ex.StatusCode.ShouldBe(HttpStatusCode.PreconditionFailed);
}
}
}