This choice is vital as it can affect the scalability, throughput, and storage of your application. Let’s delve into the concept of partition keys and discuss some guiding principles and strategies to help you make the right choice.
Understanding Partition Keys
A partition key is essentially a property of the items within a container in Azure Cosmos DB. It is used to distribute these items (data) across multiple physical partitions. Each partition in turn stores items with the same partition key value. This design allows Azure Cosmos DB to evenly distribute the data and traffic load to achieve high scalability and performance.
When you are designing your Azure Cosmos DB, you define the partition key at the time of creating the container. Once the partition key is set, it cannot be changed; hence the importance of making an accurate decision from the start.
Factors to Consider when Choosing a Partition Key
The right partition key can depend on several aspects such as your application’s read and write patterns, and the overall size and distribution of your data.
- Uniform Data Distribution: Choose a key that distributes data evenly across all partitions. This ensures that no single partition (known as a hot partition) becomes a bottleneck and impacts performance.
- Uniform Workload Distribution: Similar to data, it’s also key to evenly distribute read and write operations across all partitions. A skew towards certain partitions can lead to higher latencies and throttling.
- Storage Capacity: Each partition in Cosmos DB has a limit of 20GB. Choose a partition key that will allow your data to remain below this threshold when distributed across the partitions.
Effective Strategies for Choosing Partition Keys
Normalize High Cardinality Values
High cardinality values make excellent partition keys as they naturally distribute data uniformly across all partitions. For instance, using `orderID` instead of `orderStatus` for an e-commerce application, because the number of potential values for `orderID` is much higher.
Choose Write and Read Evenly Distributed Keys
To ensure a uniform distribution of read and write workload, avoid keys that may have bursts of read or write operations. In a social media app, a `userId` might be an effective partition key because reads and writes to a user’s data are expected to be fairly uniform.
Use Composite Partition Key to Handle Multi-Tenant Applications
In multi-tenant applications, it’s advisable to use a composite partition key that combines `tenantId` with `orderId` to achieve better distribution of workload.
Balance between Capacity and Throughput
It’s crucial to strike a balance between the storage capacity and the workload. A large number of partitions can dilute the provisioned RU/s, leading to less throughput per partition. You should consider your application’s anticipated data volume and request rate while choosing the partition key.
Conclusion
In conclusion, choosing the right partition key in Azure Cosmos DB is critical for scaling and optimizing your application’s performance. Consider the factors like your application’s read and write patterns, data storage capacity, and throughput needs to make this key design decision. Remember, once a partition key is decided and data is stored, it cannot be changed, making it all the more important to choose wisely.
Practice Test
True or False: The partition key can be a property of your choice.
• True
• False
Answer: True.
Explanation: The choice of partition key is critically important to distribute the load evenly to each partition across the Cosmos DB.
What is the purpose of choosing a partition key in Azure Cosmos DB?
• a. To increase the speed of your application
• b. To provide a filtering mechanism in your application
• c. To distribute data evenly across all physical partitions of a container
• d. To improve the security of your application
Answer: c. To distribute data evenly across all physical partitions of a container
Explanation: The main purpose of partition key selection is to ensure an even distribution of data and allow for efficient scaling.
True or False: The partition key chosen should lead to a large number of partition keys.
• True
• False
Answer: True.
Explanation: This is important to evenly distribute data across various partitions and reduce hotspots in Cosmos DB.
What is the maximum size of a single logical partition in Azure Cosmos DB?
• a. 10 GB
• b. 20 GB
• c. 30 GB
• d. 40 GB
Answer: b. 20 GB
Explanation: A single logical partition has a limit of 20 GB.
True or False: Cosmos DB allows you to change or update your Partition Key after your database has been set up.
• True
• False
Answer: False.
Explanation: Currently, Azure Cosmos DB doesn’t support the update of a partition key after it has been set and data has been stored using it.
Which of the following can not be used as a partition key in Cosmos DB?
• a. null
• b. undefined
• c. Boolean
• d. Number
Answer: b. undefined
Explanation: Undefined values cannot be used as a partition key in Cosmos DB.
True or False: The Partition key should result in an even distribution of requests and storage for high throughput and storage scenarios
• True
• False
Answer: True.
Explanation: Indeed, an even distribution allows for keeping latencies low and preventing any partition from getting overwhelmed with requests, hence resulting in optimal performance.
What is the problem of choosing a date as the partition key?
• a. Difficult to divide the data
• b. Hot partition
• c. Security risk
• d. None of the above
Answer: b. Hot partition
Explanation: Choosing a date(Datetime.UtcNow) as the partition key may result in a hot partition, which is a partition that gets all the requests, which can lead to throttling.
Can you use the ‘/’ character in a partition key path?
• a. Yes
• b. No
Answer: a. Yes
Explanation: Cosmos DB allows ‘/’ in a partition key path to indicate a hierarchy.
True or False: The partition key value that’s used to create the documents is case-sensitive.
• True
• False
Answer: True.
Explanation: The partition keys in Cosmos DB are indeed case-sensitive. This implies ‘ABC’ and ‘abc’ would result in two different partitions.
When creating a container in Azure Cosmos DB without specifying a partition key, What will be the storage capacity of the container?
• a. 20 GB
• b. 40 GB
• c. 100 GB
• d. Unlimited
Answer: a. 20 GB
Explanation: Even without specifying a partition key, the container would be created with a single partition and the maximum limit for a single partition is 20 GB.
True or False: Choosing the correct partition key can help to maximize the throughput of your application.
• True
• False
Answer: True.
Explanation: A well-chosen partition key can help to distribute the load across various physical partitions, thus maximizing the provisioned throughput and ultimately the performance of your application.
What happens when a logical partition in Cosmos DB exceeds 20 GB?
• a. The data is automatically deleted.
• b. The data is automatically moved to another partition.
• c. It throws an error and you cannot create more items on this logical partition.
• d. It automatically creates a new logical partition.
Answer: c. It throws an error and you cannot create more items on this logical partition.
Explanation: If a logical partition exceeds 20 GB, Azure Cosmos DB service returns an error (HTTP status code 413) and you can’t write more data to this logical partition.
Interview Questions
What is a partition key in Azure Cosmos DB?
The partition key is a property within the data that Azure Cosmos DB uses to distribute the data across multiple partitions. It is a JSON property within each document used to partition or distribute your container’s data among multiple physical partitions.
Why is it crucial to choose a correct partition key in Azure Cosmos DB?
Choosing the correct partition key is essential to ensure a uniform distribution of data and requests, which maximizes throughput, decreases latency, and optimally uses storage.
How does Azure Cosmos DB use the partition key?
Azure Cosmos DB uses the partition key to distribute data among multiple logical and physical partitions. The partition key maps related data items to the same logical partition.
What are the factors to consider when choosing a partition key?
The factors to consider are a high degree of cardinality, balancing the distribution of workloads, and the capacity to cope with future growth.
Which operations in Cosmos DB require specifying the partition key?
Operations like Read, Replace, Upsert, Delete, and some queries require the provision of a partition key.
How do you select a partition key that provides a uniform distribution of writes?
Choose a key that has a wide range of values and is accessed randomly. Avoid keys with concentrated write/deletion activities to prevent hotspot partitions.
What does high cardinality mean in the context of choosing a partition key?
High cardinality means a partition key that has a large number of distinct values. A key with high cardinality helps distribute data evenly across various partitions.
What can result from poorly chosen partition keys in Cosmos DB?
Poorly chosen partition keys can lead to uneven data distribution, hot partition problems, reduced throughput, and increased latency.
How should the partition key be chosen if the data access patterns are not evenly distributed?
If data access patterns aren’t evenly distributed, the partition key should involve a compound of multiple properties to distribute multiple hot partitions.
What is the maximum storage limit for a single logical partition in Azure Cosmos DB?
The maximum limit for a single logical partition in Azure Cosmos DB is 20 GB.
What happens if a logical partition key exceeds the storage limit?
If a logical partition key exceeds the storage limit, it will cause the DB service to read, write, or update data in that partition to be throttled.
Is it possible to change the partition key after creating the Cosmos DB container?
No, it’s not possible to change the partition key once the Cosmos DB container has been created.
Can you use multiple partition keys in Azure Cosmos DB?
You cannot use multiple partition keys in Azure Cosmos DB. However, you can create a composite key from multiple attributes, which can be used as a single partition key.
How is the partition key used to partition data across multiple regions?
Data gets distributed across multiple regions based on the partition keys. Each key corresponds to a specific range of partition key values. Within every region, each partition is replicated to ensure high availability and global distribution.
Can one locate data in Azure Cosmos DB without a partition key?
Yes, it’s possible using a cross-partition query. However, it’s not recommended because it may result in a high request unit (RU) charge and increased latency.
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