In the Azure Cosmos DB, the cost of executing operations is predictable and measured in Request Units (RU). Each operation has a fixed cost in terms of RUs, whether it is a read request, a write operation, or a complex SQL query. The cost associated with these operations is determined by the complexity of the operation, the processing required, the amount of data transferred, and its indexing cost.
Understanding Request Units (RUs)
Azure Cosmos DB employs the concept of Request Units (RUs) to abstract and simplify capacity planning. An RU represents a single billable unit for Azure Cosmos DB operations. The cost of all database operations, such as insert, read, replace, delete, and query operations, are expressed in RUs.
Different operations consume different amounts of RUs. For example, a read operation on a 1-KB document typically consumes 1 RU. A write operation on the same document may consume around 5 RUs, while a simple query could consume 2.3 RUs, and a complex query might consume 50 RUs.
Calculating Request Units (RUs)
The Azure Cosmos DB capacity calculator can be used to estimate the number of RUs required by your app. This tool accepts several parameters such as Number of Reads/second, Number of Writes/second, and Data Read/Written per operation, and then instantly provides an estimated RUs/second.
For instance, if an application performs 10 document reads per second, 5 document writes per second, and 5 queries per second on a 1 KB document, the capacity calculator will determine that the application will require approximately 85 RUs per second.
Factors affecting RU Cost:
- Item Size: Larger items consume more RUs. If an item’s content is small, fewer RUs are consumed to read or write this item.
- Item Indexing: Each time an item is created or updated, change feed is updated and indexes are maintained. This requires consuming additional RUs.
- Query Complexity: RUs are charged for each query execution, and more complex queries require more RUs. Factors such as the number of returned items, the size of the returned data, and whether or not the query uses an index can all impact the RU cost.
Monitoring and Diagnostic for RUs
Azure Cosmos DB provides built-in monitoring and diagnostic tools that allow developers to track how many RUs their operations are consuming.
In the Azure portal, you can navigate to the Metrics section of your Azure Cosmos DB account. From here, you can view your total RU consumption over time, average RU consumption, and a breakdown of RUs consumed by operation type.
In addition to the Azure portal, the .NET SDK also returns a `Response` object for each operation which includes a `RequestCharge` property. This property indicates the number of RUs consumed by that operation.
Example:
CosmosItemResponse itemResponse = await this.container.ReadItemAsync(id, partitionKey);
Console.WriteLine($"Read item request charge: {itemResponse.RequestCharge}");
Conclusion
Understanding Request Units in Azure Cosmos DB and how they correlate with different operations can enable great efficiency both in terms of application performance and cost optimization. Developers should keep in mind item size, indexing and query complexity when taking into account the RU considerations of their Azure Cosmos DB. They should leverage the built-in tools provided by Azure to monitor and control RU usage, and thus ensure that their applications perform adequately while staying within budget. These principles are essential to successfully performing the DP-420 Designing and Implementing Native Applications Using Microsoft Azure Cosmos DB exam.
Practice Test
True or False: The Request Unit (RU) charge of a query in Azure Cosmos DB is the same for all queries, regardless of complexity.
- True
- False
Answer: False
Explanation: The RU charge for a query depends on various factors such as the query’s complexity, the number of returned items, and the total payload size of the query results.
Multiple Select: Which of the following operations consume Request Units (RUs) in Azure Cosmos DB?
- Execute stored procedures
- SQL query operations
- Write operations
- Read operations
Answer: All of the above
Explanation: All these operations such as executing stored procedures, SQL query operations, write and read operations consume Request Units (RUs) in Azure Cosmos DB.
True or False: Azure Cosmos DB guarantees low latency at the 99th percentile for all requests.
- True
- False
Answer: True
Explanation: Azure Cosmos DB guarantees single-digit millisecond latency at the 99th percentile for all requests.
Multiple Select: What factors can affect the cost of a point read operation in Azure Cosmos DB?
- Size of the item being read
- Number of items being read
- Number of properties in the item
- Consistency level
Answer: All of the above
Explanation: All these factors, including the size of the item being read, the number of items being read, the number of properties in the item, and the consistency level, can affect the RUs consumed by a point read operation in Azure Cosmos DB.
Single Select: What is the minimum RU/s that can be provisioned for a database in Azure Cosmos DB?
- 100 RU/s
- 400 RU/s
- 1000 RU/s
- 5000 RU/s
Answer: 400 RU/s
Explanation: The minimum throughput you can provision for an Azure Cosmos database is 400 Request Units per second (RU/s).
True or False: When estimating the Request Unit (RU) cost for a query, the Azure Cosmos DB SQL API provides a built-in SQL query execution estimator.
- True
- False
Answer: False
Explanation: While Azure Cosmos DB provides a lot of scalability and performance features, it does not offer a built-in SQL query execution cost estimator.
Multiple Select: What can help in reducing the RU charge of a query in Azure Cosmos DB?
- Proper indexing
- Partitioning
- Denormalize data
- Use of stored procedures
Answer: All of the above
Explanation: All these practices can help in reducing the RU charge of a query by improving the efficiency of operations in Azure Cosmos DB.
True or False: The larger the size of the returned query result set from a query in Azure Cosmos DB, the lower the RU charge.
- True
- False
Answer: False
Explanation: The larger the size of the returned query result, the higher the RU charge because it requires more processing power and resources to handle larger result sets.
Single Select: What is the unit of measure for throughput in Azure Cosmos DB?
- Gigabytes
- RUs
- Terabytes
- IOPS
Answer: RUs
Explanation: Throughput in Azure Cosmos DB is measured in Request Units, or RUs.
True or False: Changes made to the indexing policy of a container in Azure Cosmos DB do not consume RUs.
- True
- False
Answer: False
Explanation: Changes to the indexing policy of a container would consume RUs as it involves updating the indexes which is a write operation.
Multiple Select: In Azure Cosmos DB, which operations are considered point operations?
- Insert or upsert of a single item
- DELETE operation of an item
- Query with a filter condition
- Read operation of a single item
Answer: Insert or upsert of a single item, DELETE operation of an item, Read operation of a single item
Explanation: Point operations include basic CRUD operations like read, insert, replace, upsert, and delete of single items based on their identity. A Query with a filter condition is not considered a point operation, as it does not necessarily operate on a single item.
Interview Questions
What is the standard Request Unit (RU) charge for a read operation in Azure Cosmos DB?
The standard charge for a read operation is 1 Request Unit (RU) per 1 KB of data read.
How is the Request Unit (RU) cost calculated for a write operation in Azure Cosmos DB?
The RU cost of a write operation is determined by the amount of data being written and the indexed paths in the indexing policy, with a minimum of 5 RUs for point write operations.
What is a ‘point read operation’ in Azure Cosmos DB?
A ‘Point read operation’ is when you use the ReadItemAsync method to read a single item based on its unique id.
How can you estimate the number of Request Units required for a query in Azure Cosmos DB?
By using the ‘Query Stats’ option in Azure portal’s Data Explorer or by using the SDK’s GetItemQueryIterator method, you can get an estimate of the RU cost of a query.
What does the ‘MaxItemCount’ option do in Azure Cosmos DB?
The ‘MaxItemCount’ option, when set in query requests, specifies the maximum number of items to be returned in the enumeration operation. It helps to control the consumption of RUs.
Would increasing throughput (RUs) on an Azure Cosmos DB container improve the performance of a point read operation?
No, since point read operations typically consume very few RUs, increasing throughput would not necessarily improve the performance of these operations.
How is the RU charge for a ‘point operation’ or a ‘query’ determined in Azure Cosmos DB?
The RU charge for a point operation or a query is determined by the computational and I/O resources required to fulfill that operation or query.
Can the RU charge for a read operation exceed 1 RU if the item size is less than 1 KB in Azure Cosmos DB?
No, even if the item size is less than 1KB, the RU charge for a read operation would exactly be 1 RU.
Does a ‘query’ that retrieves multiple records in Azure Cosmos DB cost differently than multiple ‘point read operations’?
Yes, a single query that retrieves multiple records could potentially be more cost-efficient in terms of RUs than multiple point read operations depending on index utilization and query complexity.
How can you reduce the RU charge for a query operation in Azure Cosmos DB?
You can reduce the RU charge by fine-tuning your query, adjusting the indexing policy, or restructuring your data model to better suit your querying needs.
Does write operation costs more RUs than read operation in Azure Cosmos DB?
Yes, write operations generally cost more RUs than read operations due to the overhead of indexing and replication.
How much RU does a point write cost in Cosmos DB?
A point write operation costs at least 5 RUs and might cost more depending on the size of the item and the indexed paths in the indexing policy.
Do all operations in Azure Cosmos DB consume RUs?
Yes, all read, write, delete, and query operations consume RUs in Azure Cosmos DB.
What operation types can you perform in Azure Cosmos DB and how do they affect the cost in RUs?
In Azure Cosmos DB, you can perform read, write, delete, and query operations. Each type of operation requires different levels of computational and I/O resources, and therefore the cost in RUs varies.
What factors should be considered while estimating the RU charges of a query in Azure Cosmos DB?
You should consider factors such as query complexity, item size, indexing overhead, and the number of items returned to accurately estimate the RU charges of a query.
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