Optimistic Concurrency Control with ETags

The default repository now supports etags and will pass them when IItemWithEtag is implemented correctly or the base classes EtagItem or FullItem are used. The etag check is enforced on all updates when TItem is of the correct type. It can however be bypassed by setting the ignoreEtag optional parameter in the relevant async methods. The InMemory repository also supports OCC with Etags. The OptimisticCurrencyControl sample shows these features.

Getting the new etag

When creating a new object, if storing in memory, it is important to store the result from the create call to ensure you have the correct etag for future updated.

For example your code should look something like this:

TItem currentItem = new TItem(...);
currentItem = _repository.CreateAsync(currentItem);

Sequential updates

When doing sequential updates to the same item it is important to use the result from the update method (when OptimizeBandwith is false) or refetch the updated data each time (when OptimizeBandwith is true) otherwise the etag value will not be updated. The following code shows what to do in each case:

Optimize Bandwith Off

TItem currentItem = _repository.CreateAsync(itemConfig);
currentItem = _repository.UpdateAsync(currentItem);
currentItem = _repository.UpdateAsync(currentItem);

Optimize Bandwith On

TItem currentItem = _repository.CreateAsync(itemConfig);
currentItem = _repository.GetAsync(currentItem.Id);
currentItem = _repository.GetAsync(currentItem);
currentItem = _repository.UpdateAsync(currentItem);

Catching mismatched etag errors

The following code shows how to catch the error when the etags do not match.

      currentBankAccount = await repository.UpdateAsync(currentBankAccount);
      Console.WriteLine($"Updated bank account: {currentBankAccount}.");
  catch (CosmosException exception) when (exception.StatusCode == HttpStatusCode.PreconditionFailed)
      Console.WriteLine("Failed to update balance as the etags did not match.");

Ignoring the etag

The following code shows how to ignore the etag when doing an update.

await repository.UpdateAsync(currentBankAccount, ignoreEtag: true);

Passing the etag to a patch update

The following code shows how to pass the etag when doing a update to specific properties.

await repository.UpdateAsync(currentBankAccount.Id,
  builder => builder.Replace(account => account.Balance, currentBankAccount.Balance - 250), etag: currentBankAccount.Etag);