When do physical partitions get created? - azure

Question

I have a partitioned collection that uses a 5-digit membership code as its partition key. There could be thousands of partition keys in the collection.
I upserted about 32K documents in it. Using the Partition Stats sample:
Summary:
partitions: 1
documentsCount: 32,190
documentsSize: 0.045 GB
But there is only a single physical partition! If I use the portal metrics, i see a similar thing:
Doesn't this mean that all my queries are going against a single physical partition? When does Cosmos add more physical partitions?
The reason I ask is because I am seeing really poor performance that seriously deteriorates when I load test. For example this simple count method starts off fast in a light test and then takes seconds when the system is under pressure (ignore the handler stuff):
private async Task<int> RetrieveDigitalMembershipRefreshItemsCount(string code, string correlationId)
{
var error = "";
double cost = 0;
var handler = HandlersFactory.GetProfilerHandler(_loggerService, _settingService);
handler.Start(LOG_TAG, "RetrieveDigitalMembershipRefreshItemsCount", correlationId);
try
{
if (this._docDbClient == null)
throw new Exception("No singleton DocDb client!");
// Check to see if there is a URL
if (string.IsNullOrEmpty(_docDbDigitalMembershipsCollectionName))
throw new Exception("No Digital Memberships collection defined!");
FeedOptions queryOptions = new FeedOptions { MaxItemCount = 1, PartitionKey = new Microsoft.Azure.Documents.PartitionKey(code.ToUpper()) };
return await _docDbClient.CreateDocumentQuery<DigitalMembershipDeviceRegistrationItem>(
UriFactory.CreateDocumentCollectionUri(_docDbDatabaseName, _docDbDigitalMembershipsCollectionName), queryOptions)
.Where(e => e.CollectionType == DigitalMembershipCollectionTypes.RefreshItem && e.Code.ToUpper() == code.ToUpper())
.CountAsync();
}
catch (Exception ex)
{
error = ex.Message;
throw new Exception(error);
}
finally
{
handler.Stop(error, cost, new
{
Code = code
});
}
}
Here is the log for this method as the test progresses ordered by the highest duration. Initially it takes only a few milliseconds:
I tried most of the performance tips i.e. direct mode, same region, singleton. Any help would be really appreciated.
Thanks.

Answer 1

Partition Key is used for logical partition, distributing data across physical partitions. Physical partition management is managed by Azure Cosmos DB.
Auto-split of physical partition can be achieved, as long as the initial container was created with at least 1000 RU/s of throughput and a partition key is specified. The split mainly distributes logical partitions in one physical partition to different physical partitions. And the process is transparent to us.
Two scenarios for physical parition:
Provision throughput higher than settings value, Azure Cosmos DB splits one or more of your physical partitions to support the higher throughput.
Physical partition p reaches its storage limit, Azure Cosmos DB seamlessly splits p into two new physical partitions. If p only has one logical partition inside, split won't occur.
So if those conditions are not met, you only have one single physical partition. But the queries are going against specified logical partition using partition key.
For more details, please refer to Azure Cosmos DB partition.