How to manage centralized values in a sharded environment - azure

Question

I have an ASP.NET app being developed for Windows Azure. It's been deemed necessary that we use sharding for the DB to improve write times since the app is very write heavy but the data is easily isolated. However, I need to keep track of a few central variables across all instances, and I'm not sure the best place to store that info. What are my options?
Requirements:
Must be durable, can survive instance reboots
Must be synchronized. It's incredibly important to avoid conflicting updates or at least throw an exception in such cases, rather than overwriting values or failing silently.
Must be reasonably fast (2000+ read/writes per second
I thought about writing a separate component to run on a worker role that simply reads/writes the values in memory and flushes them to disk every so often, but I figure there's got to be something already written for that purpose that I can appropriate in Windows Azure.
I think what I'm looking for is a system like Apache ZooKeeper, but I dont' want to have to deal with installing the JRE during the worker role startup and all that jazz.
Edit: Based on the suggestion below, I'm trying to use Azure Table Storage using the following code:
var context = table.ServiceClient.GetTableServiceContext();
var item = context.CreateQuery<OfferDataItemTableEntity>(table.Name)
.Where(x => x.PartitionKey == Name).FirstOrDefault();
if (item == null)
{
item = new OfferDataItemTableEntity(Name);
context.AddObject(table.Name, item);
}
if (item.Allocated < Quantity)
{
allocated = ++item.Allocated;
context.UpdateObject(item);
context.SaveChanges();
return true;
}
However, the context.UpdateObject(item) call fails with The context is not currently tracking the entity. Doesn't querying the context for the item initially add it to the context tracking mechanism?

Answer 1

Have you looked into SQL Azure Federations? It seems like exactly what you're looking for:
sharding for SQL Azure.
Here are a few links to read:
http://msdn.microsoft.com/en-us/library/windowsazure/hh597452.aspx
http://convective.wordpress.com/2012/03/05/introduction-to-sql-azure-federations/
http://searchcloudapplications.techtarget.com/tip/Tips-for-deploying-SQL-Azure-Federations

Answer 2

What you need is Table Storage since it matches all your requirements:
Durable: Yes, Table Storage is part of a Storage Account, which isn't related to a specific Cloud Service or instance.
Synchronized: Yes, Table Storage is part of a Storage Account, which isn't related to a specific Cloud Service or instance.
It's incredibly important to avoid conflicting updates: Yes, this is possible with the use of ETags
Reasonably fast? Very fast, up to 20,000 entities/messages/blobs per second
Update:
Here is some sample code that uses the new storage SDK (2.0):
var storageAccount = CloudStorageAccount.DevelopmentStorageAccount;
var table = storageAccount.CreateCloudTableClient()
.GetTableReference("Records");
table.CreateIfNotExists();
// Add item.
table.Execute(TableOperation.Insert(new MyEntity() { PartitionKey = "", RowKey ="123456", Customer = "Sandrino" }));
var user1record = table.Execute(TableOperation.Retrieve<MyEntity>("", "123456")).Result as MyEntity;
var user2record = table.Execute(TableOperation.Retrieve<MyEntity>("", "123456")).Result as MyEntity;
user1record.Customer = "Steve";
table.Execute(TableOperation.Replace(user1record));
user2record.Customer = "John";
table.Execute(TableOperation.Replace(user2record));
First it adds the item 123456.
Then I'm simulating 2 users getting that same record (imagine they both opened a page displaying the record).
User 1 is fast and updates the item. This works.
User 2 still had the window open. This means he's working on an old version of the item. He updates the old item and tries to save it. This causes the following exception (this is possible because the SDK matches the ETag):
The remote server returned an error: (412) Precondition Failed.

Answer 3

I ended up with a hybrid cache / table storage solution. All instances track the variable via Azure caching, while the first instance spins up a timer that saves the value to table storage once per second. On startup, the cache variable is initialized with the value saved to table storage, if available.