Azure Cosmos DB is Microsoft’s globally-distributed, distributed database service. It enables developers to securely store, access, and process huge amounts of data, anywhere in the world. One of the essential features of Azure Cosmos DB is the Change Feed. It lists the record of changes that were made in a container in the same order they occurred. You can leverage Change Feed in conjunction with Azure Functions to efficiently archive data.
Understanding Change Feed
Change feed in Azure Cosmos DB is enabled by default and doesn’t require any additional charging. It operates on a persistent log of changes occurring in an Azure Cosmos container. The changes on Change Feed are sorted in the order of their occurrence, ensuring your applications have a hassle-free experience while processing them.
Azure Cosmos DB provides secured access to change feed through APIs like SQL (Core) API, MongoDB API, Cassandra API, etc. Change feed’s support options, like point-in-time restore, short term backup policy, and change feed pull model, make it flexible and user-friendly.
Azure Functions and Data Archiving
Azure Functions is a serverless computation service, enabling you to effectively run code in response to a variety of events without requiring infrastructure management. You can use Azure Functions to trigger actions such as data operations, integrations, and computations based on events occurring in Azure Cosmos DB.
Using Azure Functions in conjunction with Azure Cosmos DB’s Change Feed, you can facilitate efficient data archiving. For instance, you can capture every change made to a document in Cosmos DB on the Change Feed, and then an Azure Function can be set up to trigger whenever a new change appears on the Change Feed. This function could then archive these changes in an Azure Blob Storage for future review or analysis.
Learn how to set this up with this simple example:
- Create an Azure Cosmos DB account – Create an Azure Cosmos DB account with a container in the Azure Portal.
- Create an Azure Function App – Go to the Azure Portal, click on ‘Create a resource’, then select ‘Function App’. Fill in the necessary details and create a new Function App.
- Set up the Azure Function to trigger on Change Feed – In the newly created Function App, add a new Function. When asked for a trigger, choose ‘Azure Cosmos DB’. In the ‘New Trigger’ page, fill in the details of your Azure Cosmos DB account and the container to monitor.
- Write the Code – In the Azure Function, you write a piece of code that will process the changes logged on Change Feed and archive them to Azure Blob Storage. The name of the Blob could be the id of the document that was changed in Cosmos DB, and the content could be the changed document itself.
Note: Please, refer to the Azure Functions and Azure Blob Storage documentation to understand how to write the necessary code. The code language will depend on what languages the Azure Functions supports, such as C#, Java, JavaScript, etc.
By using Azure Functions in conjunction with the Change Feed feature in Azure Cosmos DB, we can easily set up a system to archive changes occurring in our data. This can be handy for keeping track of changes, maintaining backups, and even for debugging purposes. And, the best part of it all is that it is scalable and serverless, so you only pay for what you use and don’t have to worry about managing any infrastructure.
Practice Test
True or False: Azure Functions can be used to automatically trigger actions based on changes to data in Cosmos DB.
- True
- False
Answer: True.
Explanation: Azure Functions have built-in support for triggers and bindings to operations, making it easier to process Cosmos DB data.
Multiple Select: Which of the following are benefits of using Change Feed with Azure Functions?
- a) Real-time response to data changes
- b) Direct handling of large volumes of data
- c) More secure data storage
- d) Improved data transformation and movement
Answer: a, b, d.
Explanation: Change Feed with Azure Functions allow for real-time response, can handle large volumes of data, and supports data transformation and movement. However, it doesn’t directly improve data security.
True or False: Change Feed in Azure Cosmos DB stores changes and updates made to a container in chronological order.
- True
- False
Answer: True.
Explanation: Change Feed maintains a persistent record of changes, which are sorted chronologically and can be processed asynchronously and incrementally.
Single Select: The Azure function to process changes in the Change Feed of an Azure Cosmos DB container is triggered by:
- a) HTTP calls
- b) Timer-based triggers
- c) Cosmos DB triggers
- d) Blob storage triggers
Answer: c) Cosmos DB triggers
Explanation: Azure Function for processing changes in the Change Feed of a Cosmos DB container uses Cosmos DB triggers.
True or False: Change Feed support in Azure Cosmos DB works by listening to an Azure Cosmos DB container for any changes. It then outputs the sorted list of documents that were changed, in the order in which they were modified.
- True
- False
Answer: True.
Explanation: This is indeed how Change Feed works—it provides a sorted list of documents in the order they were modified.
Single Select: Which of the following functionality does the Lease container in Azure Cosmos DB provide?
- a) Data partitioning
- b) Failure recovery
- c) Data encryption
- d) Compression
Answer: b) Failure recovery.
Explanation: The Lease container provides a mechanism for failure recovery when reading changes from the Change Feed in Azure Cosmos DB.
True or False: Change Feed Processor, which simplifies the distribution and processing of partitioned items across multiple consumers, is available in SDK and can be used with Azure functions.
- True
- False
Answer: True
Explanation: Change Feed Processor is part of Azure Cosmos DB SDK that simplifies the distribution and processing of Change Feed event data across multiple consumers.
Multiple Select: Which of the following are valid scenarios in which Change Feed can be useful?
- a) Moving data to a secondary data store
- b) Creating materialized views from main container data
- c) Triggering a text message each time data is added
- d) Real time processing and analytics of container data
Answer: a, b, d
Explanation: Change Feed is suitable for scenarios that involve moving data, creating views, or real-time processing and analytics. However, triggering a text message is more suitably done through Logic Apps or Azure Functions directly.
True or False: Change Feed in Azure Cosmos DB can be enabled or disabled for each database in your Cosmos DB account.
- True
- False
Answer: False.
Explanation: The Change Feed in Azure Cosmos DB cannot be disabled; it is enabled by default for all Azure Cosmos DB accounts and does not add extra cost.
Single Select: In a scenario where an Azure Function is used to process changes from the Change Feed of an Azure Cosmos DB container, what is the maximum delay in triggering the function after a change is made in the Cosmos DB container:
- a) There is no delay
- b) 5 seconds
- c) 30 seconds
- d) Depends on the amount of data changed
Answer: a) There is no delay
Explanation: The function is triggered almost instantly after a change is made in the Cosmos DB container since Change Feed provides real-time response to changes. It does not depend on amount of data changed.
Interview Questions
What is Azure Functions?
Azure Functions is a serverless solution that allows writing less code, maintaining less infrastructure, and saving more costs. It enables developers to focus more on business logic and it takes care of the infrastructure to run your applications in the cloud.
Describe Change Feed in the context of Azure Cosmos DB?
Change Feed in Azure Cosmos DB is a persistent log of documents within a container. This log sorts the documents in the order they get modified. By enabling Change Feed, it becomes possible to get a sorted list of documents within an Azure Cosmos container.
How does the change feed in Azure Cosmos DB work?
When documents are modified in a database, these changes get stored in the change feed. Applications can then read the change feed and react to the changes. This makes change feed a perfect solution for scenarios where the order of operations and their serialization matter—for example, stream processing or triggering events.
How is Change Feed beneficial when used with Azure Functions?
Change Feed along with Azure Functions helps in creating real-time applications. As Azure Functions can be easily bound to the Change Feed trigger, it automatically invokes an azure function as and when there are changes recorded for a specific container.
What is the purpose of Azure Function’s Change Feed trigger?
The Azure Function’s Change Feed trigger allows users to create an Azure function that gets executed whenever there are new changes in the Azure Cosmos DB container. This helps in processing or migrating Azure Cosmos DB data to other services in real-time.
How does Azure Cosmos DB ensure data integrity during migration by using Change Feed?
Azure Cosmos DB’s Change Feed maintains a persistent record of changes in the order of their occurrence, hence providing an accurate timeline of changes. During migration, the change feed ensures that all modification operations are captured in sequence, thereby ensuring data integrity.
Is it possible to pause Change Feed processing in Azure Cosmos DB?
Yes, Change Feed processing can be paused and resumed on-demand. You can stop the Change Feed Processor, and it will keep track of the last item it processed, allowing you to resume where it left off.
How to handle large volumes of data with Change Feed and Azure Functions?
Change Feed and Azure Functions can handle large volumes of data by parallelizing the data processing. This configuration allows the Function to scale and process large volumes of data efficiently.
What types of applications commonly use Change Feed with Azure Functions?
Change Feed with Azure Functions is commonly used in monitoring apps, caching, data movement, IoT, and AI scenarios. It lets applications to react to new data in real-time or archive for later processing.
How can Change Feed be enabled in Azure Cosmos DB?
Change Feed is automatically enabled in Azure Cosmos DB and does not need explicit activation. The log of changes is kept, and an application can subscribe to these changes.
What is the impact of Change Feed log storage on Azure Cosmos DB pricing?
Change Feed logs do not affect the cost of your Azure Cosmos DB resources; they’re stored within a container. Any reads from or writes to the Change Feed only incur regular Request Unit (RU) charges.
Can you fetch previous changes once they have been read from the Change Feed?
Yes, you can re-read changes from the Change Feed. The Change Feed in Azure Cosmos DB is persistent, and you have the option to read from the start of the Change Feed, essentially fetching the entire Change Feed history.
What kind of data does the Change Feed in Azure Cosmos DB return?
Change Feed returns the entire modified document and not just the difference in the before and after images of the modification. It will include inserted and updated documents, but it does not track deleted ones.
Can you process changes of multiple containers with one Azure Function using Change Feed?
Each instance of a Change Feed trigger processes changes on a specific Azure Cosmos DB container. If you need to process changes on multiple containers, you would need to create multiple Azure Functions.
How to ensure ordering while processing data with Change Feed and Azure Functions?
By default, Change Feed and Azure Functions process changes in the order they occur. To ensure ordering across multiple instances or partitions, you can handle lease distribution yourself or reduce the degree of parallelism to 1.
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