Add or replace entity in Azure Table Storage

Question

I'm working with Windows Azure Table Storage and have a simple requirement: add a new row, overwriting any existing row with that PartitionKey/RowKey. However, saving the changes always throws an exception, even if I pass in the ReplaceOnUpdate option:

tableServiceContext.AddObject(TableName, entity);
tableServiceContext.SaveChangesWithRetries(SaveChangesOptions.ReplaceOnUpdate);

If the entity already exists it throws:

System.Data.Services.Client.DataServiceRequestException: An error occurred while processing this request. ---> System.Data.Services.Client.DataServiceClientException: <?xml version="1.0" encoding="utf-8" standalone="yes"?>
<error xmlns="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
  <code>EntityAlreadyExists</code>
  <message xml:lang="en-AU">The specified entity already exists.</message>
</error>

Do I really have to manually query for the existing row first and call DeleteObject on it? That seems very slow. Surely there is a better way?

Answer 1

As you've found, you can't just add another item that has the same row key and partition key, so you will need to run a query to check to see if the item already exists. In situations like this I find it helpful to look at the Azure REST API documentation to see what is available to the storage client library. You'll see that there are separate methods for inserting and updating. The ReplaceOnUpdate only has an effect when you're updating, not inserting.

While you could delete the existing item and then add the new one, you could just update the existing one (saving you one round trip to storage). Your code might look something like this:

var existsQuery = from e
                    in tableServiceContext.CreateQuery<MyEntity>(TableName)
                    where
                    e.PartitionKey == objectToUpsert.PartitionKey
                    && e.RowKey == objectToUpsert.RowKey
                    select e;

MyEntity existingObject = existsQuery.FirstOrDefault();

if (existingObject == null)
{
    tableServiceContext.AddObject(TableName, objectToUpsert);
}
else
{
    existingObject.Property1 = objectToUpsert.Property1;
    existingObject.Property2 = objectToUpsert.Property2;

    tableServiceContext.UpdateObject(existingObject);
}

tableServiceContext.SaveChangesWithRetries(SaveChangesOptions.ReplaceOnUpdate);

EDIT: While correct at the time of writing, with the September 2011 update Microsoft have updated the Azure table API to include two upsert commands, Insert or Replace Entity and Insert or Merge Entity

Answer 2

In order to operate on an existing object NOT managed by the TableContext with either Delete or SaveChanges with ReplaceOnUpdate options, you need to call AttachTo and attach the object to the TableContext, instead of calling AddObject which instructs TableContext to attempt to insert it.

http://msdn.microsoft.com/en-us/library/system.data.services.client.dataservicecontext.attachto.aspx

Answer 3

in my case it was not allowed to remove it first, thus I do it like this, this will result in one transaction to server which will first remove existing object and than add new one, removing need to copy property values

       var existing = from e in _ServiceContext.AgentTable
                       where e.PartitionKey == item.PartitionKey
                             && e.RowKey == item.RowKey
                       select e;

        _ServiceContext.IgnoreResourceNotFoundException = true;
        var existingObject = existing.FirstOrDefault();

        if (existingObject != null)
        {
            _ServiceContext.DeleteObject(existingObject);
        }

        _ServiceContext.AddObject(AgentConfigTableServiceContext.AgetnConfigTableName, item);

        _ServiceContext.SaveChangesWithRetries();
        _ServiceContext.IgnoreResourceNotFoundException = false;

Answer 4

Insert/Merge or Update was added to the API in September 2011. Here is an example using the Storage API 2.0 which is easier to understand then the way it is done in the 1.7 api and earlier.

public void InsertOrReplace(ITableEntity entity)
    {
        retryPolicy.ExecuteAction(
            () =>
            {
                try
                {
                    TableOperation operation = TableOperation.InsertOrReplace(entity);
                    cloudTable.Execute(operation);
                }
                catch (StorageException e)
                {
                    string message = "InsertOrReplace entity failed.";

                    if (e.RequestInformation.HttpStatusCode == 404)
                    {
                        message += " Make sure the table is created.";
                    }

                    // do something with message
                }
            });
    }

Answer 5

The Storage API does not allow more than one operation per entity (delete+insert) in a group transaction:

An entity can appear only once in the transaction, and only one operation may be performed against it.

see MSDN: Performing Entity Group Transactions

So in fact you need to read first and decide on insert or update.