Plan for transactions when choosing a partition key

What is a Partition Key?

A partition key in Cosmos DB is a property within the data which Cosmos DB uses to distribute data and workload across multiple partitions. How data, documents, or items are distributed across logical partitions is defined by the partition key.

Choosing an inappropriate partition key could result in hot partitions, where one partition receives more load than others, thus creating an imbalance. To achieve load balancing, a partition key with a good distribution of data is crucial.

Planning for Transactions

When planning for transactions, it’s essential to understand that in Cosmos DB, all operations within a transaction must be done on items that are part of the same logical partition. This means, if you are planning to execute multiple operations within a transaction, they MUST all have the same partition key value.

For instance, in an e-commerce application, if Transaction A involves adding a product to the cart and updating the user’s total spent amount, all these activities should be tied to the same partition key to execute them in a single transaction.

Examples

Consider a simple example of a student database in a school.

{
"StudentID": "123",
"Name": "John Doe",
"Age": 14,
"Class": "7A",
"Subjects": ["Maths", "Science", "History"]
}

Here, if we use ‘StudentID’ as the partition key, each partition would contain data relevant to a single student. Any transactions pertaining to the student can be performed optimally as all required data resides within the same partition.

However, if we choose ‘Class’ as the partition key, all students belonging to the same class would be grouped in the same partition and transactions concerning an entire class or group of students can be performed optimally.

Conclusion

It’s crucial to remember that selecting the right partition key is paramount to the design and generation of a scalable and performant Cosmos DB implementation. The choice of a partition key becomes even more important when you are planning for transactions as all the operations within a transaction need to target the same logical partition. Therefore, understanding the nature of the transactions in your application will guide you in choosing a suitable partition key.

In the DP-420 Exam perspective, recognizing the impacts of partition key selection on transaction performance is significant. It can essentially drive the overall performance and scalability of your Cosmos DB application. By aligning your partition key with transactional requirements, you can achieve a more robust and performance-oriented Cosmos DB Architecture.

Practice Test

True/False: The partition key in Azure Cosmos DB should always be chosen based on the data that will be accessed the most frequently.

• True
• False

Answer: False

Explanation: While data accessibility is an important factor, the partition key should also evenly distribute the workload and storage across all partitions.

Single Select: Which of the following factors should you consider while choosing a partition key in Cosmos DB?

• a) Data volume
• b) Request units per second
• c) Data distribution
• d) All of the above

Answer: d) All of the above

Explanation: A well-chosen partition key is a fundamental aspect of Azure Cosmos DB’s ability to scale and perform well. All these factors are important to consider while deciding on a partition key.

Multiple Select: What are the consequences of a poorly chosen partition key in Cosmos DB?

• a) Reduced throughput
• b) Higher cost
• c) Hot partitions
• d) None of the above

Answer: a) Reduced throughput, b) Higher cost, c) Hot partitions

Explanation: A poorly chosen partition key can create “hot” partitions that receive a higher workload than others, reducing throughput, increasing cost due to inefficient resource use.

True/False: The partition key should be designed to avoid ‘hot partitions’.

• True
• False

Answer: True

Explanation: A ‘hot partition’ is one that is receiving a higher workload compared to others, leading to inefficient use of resources. The partition key should be chosen to evenly distribute requests.

Single Select: A good partition key is:

• a) High cardinality
• b) High write and read traffic
• c) Evenly distributes data and traffic across logical partitions
• d) All of the above

Answer: d) All of the above

Explanation: A good partition key is high cardinality, high write and read traffic, and ensures data and traffic are evenly distributed across all logical partitions.

True/False: It’s not important to plan for transactions while choosing the partition key.

• True
• False

Answer: False

Explanation: Planning for transactions is important because transactions in Cosmos DB are scoped to a single partition. Setting the partition key correctly can ensure all data in a single transaction is co-located in the same partition.

Single Select: Cosmos DB transactions are scoped to:

• a) Entire database
• b) Single document
• c) Single partition
• d) Multiple partitions

Answer: c) Single partition

Explanation: Transactions in Cosmo DB are scoped to a single partition. This is why planning for transactions when choosing the partition key is important.

Multiple Select: When planning for goods transactions, it’s best to:

• a) Always choose a partition key that ensures data is evenly distributed
• b) Plan to include all data in a single document
• c) Include all data in a single partition
• d) Avoid choosing a partition key based on data size

Answer: a) Always choose a partition key that ensures data is evenly distributed and c) Include all data in a single partition

Explanation: The partition key should ensure that data and workload are evenly distributed, and planning for transactions should include placing all data in a single partition.

True/False: A partition key value can be changed after it’s initially set.

• True
• False

Answer: False

Explanation: Once you set a partition key value, you can’t change it. This is why it’s important to give careful thought to what you assign as your partition key.

Single Select: If all writes are going to a single partition, it may result in:

• a) Inefficient usage of throughput
• b) Fetching of unnecessary data
• c) Application errors
• d) All of the above

Answer: a) Inefficient usage of throughput

Explanation: Concentrating writes on a single partition can lead to inefficient use of throughput and can create hot-spots that limit the overall throughput of the container.

Interview Questions

What is a partition key in Microsoft Azure Cosmos DB?

A partition key in Azure Cosmos DB is a specific attribute within a data collection that Azure uses to distribute the data across multiple logical partitions.

Why is choosing the right partition key in Azure Cosmos DB important?

Choosing the right partition key is vital because it can affect the scalability, performance, and cost of your application. It ensures that the data is evenly distributed across all logical partitions and can help manage transactions effectively.

How does a partition key affect transactions in Azure Cosmos DB?

In Azure Cosmos DB, all the operations within a transaction must happen within a single logical partition. If operations occur across different logical partitions, it will require multiple transactions, which can affect performance.

How should a partition key be chosen to optimize transactions?

The partition key should be chosen such that the transactional workload is evenly distributed across all logical partitions. It should also be based on a property that has a wide range of values and the one which is frequently used in the where clause of your queries.

What happens when a suboptimal partition key is chosen?

A suboptimal partition key can lead to data skew and hot partitions. It can limit the scalability of the application and can result in higher costs.

Which type of operations cannot span multiple logical partitions in Azure Cosmos DB?

Operations such as transactions cannot span multiple logical partitions in Azure Cosmos DB. All operations within a transaction must be on a single logical partition.

Can you change the partition key after creating it in Cosmos DB?

No, you cannot change the partition key once it has been set in Cosmos DB. Therefore, it is crucial to choose the partition key wisely during the design phase.

What is a logical partition in Cosmos DB?

A logical partition in Cosmos DB is a subdivision of data within a container. Each logical partition consists of a set of items that have the same partition key.

What is the role of the partition key in relation to Request Units (RU)?

The partition key can affect the consumption of Request Units (RU). A poorly chosen partition key may lead to skewed RU consumption across partitions, leading to increased cost and bad performance.

How does Cosmos DB ensure transactional integrity with partition keys?

Cosmos DB ensures transactional integrity by isolating all operations within a single transaction to one logical partition, identified by a partition key. This enables atomicity of transactions in Cosmos DB.

What should you keep in mind when planning for cost while choosing a partition key?

While planning for cost, it’s recommended to avoid hot partitions that burden a single partition with many requests as it could increase the cost. Therefore, a partition key should be chosen in a way that evenly distributes the data and the workload.

Can a partition key be a composite of multiple properties?

No, in Azure Cosmos DB, a partition key cannot be a composite of multiple properties. It should be a single property of items.

Can two items with the same partition key exist in separate logical partitions?

No, all items with the same partition key will exist within the same logical partition in Azure Cosmos DB.

Can the partition key be based on a numeric property?

Yes, the partition key can be any JSON property, including numerics, strings, and Booleans.

What is the advantage of selecting a partition key with a high cardinality?

A high cardinality partition key distributes the data across many partitions, enhancing scalability and reducing the chance of hotspots.