Implement data archiving by using a change feed

Effective data archiving strategies can aid in reducing storage costs, improving application performance, and data compliance. One effective way one can implement data archiving is by using a change feed. In the context of the DP-420 Designing and Implementing Native Applications using Microsoft Azure Cosmos DB exam, we’ll discuss the usage of change feed for data archiving and how this can be implemented.

An Introduction to Change Feed

Azure Cosmos DB Change Feed is a powerful tool that listens to your Azure Cosmos DB container for any changes. It then outputs the sorted list of documents that were changed, together with some metadata. Change feed support in Azure Cosmos DB works by listening to an Azure Cosmos container for any changes. It then outputs the sorted list of documents that were changed, in the order in which they were modified.

Data Archiving using Change Feed

Now, coming to how we can use this for data archiving. With the Cosmos DB change feed functionality, you can create a subscriber application that invokes a certain action whenever a change is detected in the Cosmos DB container. This action can be to move the data to a more cost-effective storage, hence effectively implementing data archiving.

Steps to Implement Data Archiving in Cosmos DB using Change Feed

Create a Cosmos DB instance

If you haven’t already, you will need to set up your Cosmos DB instance. This can be done through Azure’s portal, with the option to select your preferred API, such as SQL or MongoDB, and the region where you want the database to be hosted.

Implement Change Feed Processor

Azure Cosmos DB provides a Change Feed Processor library that simplifies the creation of a subscriber application. It distributes the events among multiple instances of the observer, checkpoints the progress, and handles the failures.

Below is a sample code snippet to demonstrate how to implement the Change Feed Processor:

DocumentCollectionInfo feedCollectionInfo = new DocumentCollectionInfo()
{
Uri = new Uri("https://cosmosDBaccount.documents.azure.com:443/"),
MasterKey = "masterKey",
DatabaseName = "databaseName",
CollectionName = "collectionName"
};

ChangeFeedProcessorBuilder.Create()
.WithHostName("hostName")
.WithFeedCollection(feedCollectionInfo)
.WithLeaseCollection(leaseCollectionInfo)
.WithObserver()
.Build().StartAsync().Wait();


Design a Subscriber Application

A subscriber application can be designed to archive or dispose of the older data on detecting a certain type of change. This application should be built based on the business need – whether you want to archive the data after a certain time period, or after the data reaches a certain volume, or when the data is not accessed often.

Use Azure Storage Services

Azure Blob Storage or Azure Data Lake can be used for long-term storage of your archived data. These services are cost-effective solutions for storing large amounts of unstructured data, such as text or binary data.

In summary, the Azure Cosmos DB change feed provides a sorted list of documents within a container in the order in which they were modified. This can be utilized to implement a data archiving strategy, by appropriately designing a subscriber application that listens to these changes and moves the data to a cost-effective storage like Azure Blob Storage or Azure Data Lake to store the archived data. Implementing such data archiving solutions can greatly help in reducing costs, improving performance, compliance, and overall manageability of applications.

Practice Test

True/False: Data archiving is the process of moving data that is no longer actively used to a separate storage device for long-term retention.

• True
• False

Answer: True

Explanation: Data archiving is typically done to free up valuable space on systems that are designed for speed, not storage.

Multiple Select: Which of the following are methods to implement data archiving using a change feed?

• A. Exporting the data to a CSV file.
• B. Using Azure Functions to trigger with each data change.
• C. Keeping a log file of all changes.
• D. Storing data in Azure Blob storage.

Answer: B, D

Explanation: While you can technically implement any of these methods, the most common methods recommended by Microsoft for Azure Cosmos DB are using Azure Functions to implement serverless architectures and storing data in Azure Blob storage.

True/False: Change feed in Azure Cosmos DB only works with SQL API.

• True
• False

Answer: False

Explanation: Change feed support is available for SQL API, MongoDB API, Cassandra API, and Gremlin API.

Single Select: Which Azure service can you use to process change feed and implement data archiving?

• A. Azure Functions
• B. Azure Logic Apps
• C. Azure Data Factory
• D. Azure Machine Learning

Answer: A. Azure Functions

Explanation: Azure Functions is an event-driven serverless compute platform. You can use it to process and archive change feed data.

Multiple Select: What are the benefits of using a change feed in data archiving?

• A. Synchronization with secondary databases.
• B. Real-time data processing.
• C. Lower cost.
• D. In-memory storage.

Answer: A, B

Explanation: Change feed helps in maintaining synchronization with secondary databases. It also supports real-time processing of the data. However, it doesn’t facilitate in-memory storage or cost reduction.

Single Select: Change feed is designed to provide transactional consistency. What type of consistency does it provide?

• A. Eventual consistency.
• B. Bounded staleness.
• C. Consistent prefix.
• D. Session consistency.

Answer: C. Consistent prefix.

Explanation: In the context of Azure Cosmos DB, change feed provides the order of changes (inserts and updates) in the order they were actually written (committed) to the database. This is known as ‘consistent prefix’ consistency.

True/False: Change feed in Azure Cosmos DB provides the changes in the chronological order they were made.

• True.
• False.

Answer: True.

Explanation: Change feed reflects the changes in the order they were made to the database which helps in maintaining historical data changes.

Single Select: To implement data archiving using change feed and Azure Functions, your Azure Functions should be triggered by which type of trigger?

• A. HTTP Trigger.
• B. Queue Trigger.
• C. Cosmos DB Trigger.
• D. Blob Trigger.

Answer: C. Cosmos DB Trigger.

Explanation: To implement data archiving using change feed, your Azure Functions should be triggered by Cosmos DB triggers. This way the function will be executed once there is any change in the data.

True/False: One of the limitations of using a change feed for data archiving in Azure Cosmos DB is that it does not capture deleted items.

• True.
• False.

Answer: True.

Explanation: Change feed provides the add and update operation logs but it does not capture the delete operation.

Multiple Select: Change feed in Azure Cosmos DB can be processed using which of the following?

• A. Azure Functions.
• B. Azure Logic Apps.
• C. Azure Databricks.
• D. All of the above.

Answer: D. All of the above.

Explanation: Change feed in Azure Cosmos DB can be processed with Azure Functions, Azure Logic Apps, and Azure Databricks. This gives you a flexible range of options to fit your business logic and architecture requirements.

Interview Questions

What is a change feed in Azure Cosmos DB?

A change feed in Azure Cosmos DB is a log of all the created, updated, or deleted documents within a container. Change feed supports reading change log events sorted by the times they were written and performed.

What is the primary use case for change feed in Azure Cosmos DB?

The primary use case for change feed is to provide real-time data movement within Azure Cosmos DB, allowing you to listen to data changes and respond accordingly.

What is data archiving in Azure Cosmos DB?

Data archiving in Azure Cosmos DB is the process of storing copies of data that is no longer actively used in a secure, affordable, and reliable storage system for long-term accessibility.

What mechanism does Azure Cosmos DB provide to implement data archiving?

Azure Cosmos DB uses change-feed mechanism to capture data changes and then archive them in a separate storage for long term access.

Can the change feed of Azure Cosmos DB be used with Azure Functions?

Yes, change feed can be integrated with Azure Functions allowing you to create serverless architectures that react to changes in data.

How does the change feed processor help in Azure Cosmos DB data management?

A change feed processor provides a robust and scalable framework for distributing the changes to multiple consumers and managing partitions and leasing.

What functionalities does the change feed provide in Azure Cosmos DB?

Change feed provides functionalities such as data replication, data movement, event-driven architecture, and real-time data processing.

Is the change feed data stored indefinitely in Azure Cosmos DB?

Change feed retains change history for 7 days and does not keep data indefinitely.

How is the order of operation maintained in Azure Cosmos DB change feed?

Change feed sorts all changes by operation’s timestamp in the order they were written to the database, thus maintaining the order.

How do you access the change feed in Azure Cosmos DB?

Change feed access in Azure Cosmos DB is achieved either using the ChangeFeedProcessor class in the .NET SDK, or the change feed support in Azure Functions.

Is change feed supported in all APIs of Azure Cosmos DB?

Yes, change feed is supported in all Azure Cosmos DB APIs including SQL API, MongoDB API, Cassandra API, Gremlin API, and Table API.

What factors should be considered in the designing phase of data archiving in Azure Cosmos DB?

Factors such as the type and size of data to archive, retention period of the data, and the cost involved in archiving should be considered while designing data archiving in Azure Cosmos DB.

How does the Azure Cosmos DB change feed handle concurrency?

Azure Cosmos DB change feed handles concurrency by using a lease container that maintains concurrency control and checkpoints.

How can we use Azure Functions with Azure Cosmos DB to implement data archiving?

Azure Functions can be connected to the Azure Cosmos DB change feed to react to changes. Data can be then archived in Azure Blob Storage or Data Lake Store for long term storage.