The change feed in Azure Cosmos DB is an automatic function that records all the data changes within a container. This function tracks the modifications and writes them to a change log in the same sequence that they occurred.
What is Aggregation Persistence?
Aggregation persistence, or materialized view patterns, is a design technique where pre-computed views are created in a database to allow complex queries to access data more efficiently. It’s beneficial when the database stores vast, complex data that’s frequently queried or analyzed.
How does Aggregation Persistence work with Change Feed?
Cosmos DB’s change feed coupled with aggregation persistence can create a powerful system that ensures data accuracy and maintains high performance. For instance, as soon as new data is written in a container, the change feed mechanism detects the changes. Any persistent aggregations connected to the change feed are then updated instantly, making sure the views are always up-to-date.
Implementing Aggregation Persistence using Change Feed
Here are the key steps to implement aggregation persistence using change feed:
1. Enable the Change Feed in Azure Cosmos DB
The change feed is enabled by default when you create a new Azure Cosmos DB container. All update and delete operations in the container would be recorded accordingly.
2. Implement Persistent Aggregations
You should then implement persistent aggregations dependent on your data querying requirements. The persistent aggregations should hold data from the change feed, making it possible to compute the aggregate values.
3. Set up Change Feed Processors
Change Feed Processors are the way to read from the change feed. They provide a delegate method ProcessChangesAsync that you implement with your aggregation logic.
To Make it clearer, consider the example below:
public class StockAggregator : IChangeFeedObserver
{
public Task OpenAsync(ChangeFeedObserverContext context)
{
Console.WriteLine("Worker opened", context.PartitionKeyRangeId);
return Task.CompletedTask;
}
public Task CloseAsync(ChangeFeedObserverContext context, ChangeFeedObserverCloseReason reason)
{
Console.WriteLine("Worker closing, {0}", context.PartitionKeyRangeId);
return Task.CompletedTask;
}
public Task ProcessChangesAsync(ChangeFeedObserverContext context, IReadOnlyList
{
Console.WriteLine("Change feed: partition {0} count {1}", context.PartitionKeyRangeId, docs.Count);
foreach (var doc in docs)
{
// Perform your aggregation operations here
// ...
}
return Task.CompletedTask;
}
}
In this code, we implemented a “StockAggregator” worker that reads from the change feed and performs some aggregation operations on the data.
Understanding the concept of implementing aggregation persistence by using change feed is crucial for anyone preparing to take the DP-420 Designing and Implementing Native Applications Using Microsoft Azure Cosmos DB Exam. With the proper knowledge and practical application, you can improve database query responses while maintaining data accuracy.
Practice Test
True/False: Change Feed support in Azure Cosmos DB works by listening to an Azure Cosmos DB container for any changes.
- True
- False
Answer: True.
Explanation: It does this by pulling the changes. This can be extremely valuable in multiple scenarios, like synchronization, denormalization, data movement, and many more.
When using query processing, an aggregation query pipeline is required. Is this True or False?
- True
- False
Answer: True.
Explanation: The aggregation query pipeline is vital because it summarizes the data across multiple documents.
What is the primary function of Azure Cosmos DB Change Feed?
- A. Data archival only
- B. Data movement only
- C. Triggers and computing events
- D. Synchronization and denormalization only
Answer: C. Triggers and computing events
Explanation: While Change Feed does support data movement, synchronization, and denormalization, it is primarily used for triggering and computing events with any changes in Azure Cosmos DB.
True/False: Implementing aggregation persistence by using a change feed offsets the cost of query computation by storing the result of the aggregation for fast lookups.
- True
- False
Answer: True.
Explanation: Change Feed used in conjunction with aggregation persistence can deliver real-time processing and lower latencies during querying.
Change Feed Processor simplifies incremental data load scenarios. True or False?
- True
- False
Answer: True.
Explanation: Change Feed Processor simplifies the distribution of change feed events and reliably consumes change feed across multiple workers and instances.
You should use the CosmosClient.ChangeFeedProcessorBuilder instead of IChangeFeedProcessor when implementing change feed. True/False?
- True
- False
Answer: False.
Explanation: The recommended way to implement change feed in .NET Core is using IChangeFeedProcessor.
How can a developer implement the aggregation pipeline in Azure Cosmos DB?
- A. Using SQL language
- B. Using NoSQL language
- C. Using JavaScript language
- D. Using Python language
Answer: A. Using SQL language
Explanation: The database query engine runs the SQL queries and completes each pipeline stage.
The Change Feed feature in Azure Cosmos DB only supports inserts, not updates or deletes. True/False?
- True
- False
Answer: False.
Explanation: Change Feed in Azure Cosmos DB supports all types of operations – inserts, updates, and deletes.
You can pause and resume reading of changes anytime when using Change Feed. True/False?
- True
- False
Answer: True.
Explanation: The Change Feed feature is a persistent record of changes, and developers can choose to pause and resume the reading of these changes as needed.
Which of the following is not a real-world use of Change Feed in Azure Cosmos DB?
- A. Materialized view pattern
- B. Triggering a serverless function
- C. Visualizing data in real time
- D. Encrypting sensitive data
Answer: D. Encrypting sensitive data
Explanation: Change Feed supports real-time processing and applications but does not have inherent capabilities to encrypt data.
Can you modify the size of the page in the change feed?
- A. Yes
- B. No
Answer: A. Yes
Explanation: It is easy to modify the size of the page in the Change Feed by adjusting the maximal item count.
True/False: You can only read the change feed of a single Azure Cosmos container and not multiple containers at the same time.
- True
- False
Answer: False.
Explanation: By leveraging a host of libraries, developers can read the change feed of multiple Azure Cosmos containers in parallel.
Do Change Feed and Aggregation persistence work together to provide a real-time analytics solution by reducing costs and complexities?
- A. Yes
- B. No
Answer: A. Yes
Explanation: Combining real-time change feed and aggregation persistence can reduce storage costs and complexity when performing real-time analytical operations.
True/False: The Azure Cosmos DB change feed is enabled by default on all Cosmos DB accounts.
- True
- False
Answer: True.
Explanation: The Azure Cosmos DB change feed feature does not need to be enabled explicitly as it is already enabled by default.
Can the order of changes in Change Feed be guaranteed to be in chronological order?
- A. Yes
- B. No
Answer: A. Yes
Explanation: The Change Feed in Azure Cosmos DB preserves the order of the changes, and the changes are sorted by the modification time of the items.
Interview Questions
What is aggregation persistence in the context of Microsoft Azure Cosmos DB?
Aggregation persistence refers to a design pattern which involves the computation and storing of cumulated or aggregated data as a separate entity in the database, typically in response to an event or change in another part of the database.
What is a change feed in Microsoft Azure Cosmos DB?
In Cosmos DB, a change feed is a sorted list of documents within a container that have been modified. It enables enhancements such as triggering an action, implementing a queue, or creating projections of data based on the modifications made.
How do change feeds in Azure Cosmos DB support the implementation of aggregation persistence?
Change feeds can be used to monitor data changes in a container, calculate aggregations based on those changes, and then persist these aggregations in the same or different container. This allows instantaneous data processing and maintains the aggregated data without the need for re-computation or re-processing.
What kind of API can be used with change feed in Azure Cosmos DB to persist aggregations?
You can use Azure Functions, Stream processing APIs like Spark Structured Streaming or Change Feed Processor library to process items from change feed and persist the aggregations.
Does the change feed in Azure Cosmos DB report changes in the order of their occurrence?
Yes, the change feed in Azure Cosmos DB retains changes in the exact order of their occurrence and delivers them in the same order.
What is the purpose of the Lease container while working with change feed in Azure Cosmos DB?
The lease container is used to store the checkpoint information about the change feed processing. It maintains the state of the ongoing read operations making it possible to continue reading from where it left off after a failure or restart.
What data operation types are surfaced in the change feed in Cosmos DB?
The change feed includes inserts and update operations. Delete operations are not currently included in the change feed.
In the context of Microsoft Azure Cosmos DB, what is Eventual Consistency?
Eventual Consistency is a consistency model where the system guarantees that any read operations will eventually return the most recent write. It offers lower latency and higher availability.
Why is Azure Functions a popular choice for reading from the change feed?
Azure Functions is serverless, thus it can scale dynamically to accommodate the load and process changes as they happen. It automatically checkpoints the progress making it resilient to failures.
Can you partition data using the change feed feature in Cosmos DB?
Yes, the change feed supports feed processing across multiple partitions and distributes the changes accordingly. Each logical partition has its own change feed.
How can you enable the Change Feed in Azure Cosmos DB?
The Change Feed in Azure Cosmos DB is enabled by default and cannot be disabled, so you don’t need to do anything to enable it.
What is the role of StartFromBeginning in processing change feed in Cosmos DB?
The StartFromBeginning property, when set to true, directs the change feed processor to read changes from the beginning of the change history, rather than starting with the current changes.
Can you use the change feed together with the multi-region writes capability?
Yes, you can consume the change feed in the context of multi-region writes. However, the order of the changes in the change feed in this case is the order of the writes within each region, which might be different across regions.
Is it possible to filter out specific types of changes from the change feed?
Directly, no. The change feed includes all insert and update operations on the items in the container. However, you can implement filtering logic in the consumer of the change feed such as in your Azure Function or other processing system.
Can you use time-to-live (TTL) with change feed in Azure Cosmos DB?
Yes, but using TTL on items in your container does not affect the time those items remain in the change feed. The change feed will contain details of an operation as long as needed, irrespective of the TTL of the item itself in the container.
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