Azure Cosmos DB Change Feed is a powerful feature that enables you to track and respond to changes in your Cosmos DB containers in real-time. The change feed provides a sorted list of documents that were modified in the order they were changed, making it ideal for building reactive applications and…Azure Cosmos DB Change Feed is a powerful feature that enables you to track and respond to changes in your Cosmos DB containers in real-time. The change feed provides a sorted list of documents that were modified in the order they were changed, making it ideal for building reactive applications and data pipelines.
To implement change feed notifications, you have several approaches:
**1. Azure Functions Trigger:**
The simplest method is using Azure Functions with a Cosmos DB trigger. The function automatically executes whenever documents are inserted or updated in your container. You configure this through the function.json bindings or using attributes in your code, specifying the database name, container name, and a lease container for tracking progress.
**2. Change Feed Processor Library:**
For more control, use the Change Feed Processor in the .NET SDK. This involves creating a ChangeFeedProcessor instance that monitors your container. You define a delegate handler that processes each batch of changes. The processor handles partitioning, checkpointing, and load balancing across multiple instances automatically.
**3. Pull Model:**
The SDK also supports a pull-based approach where you explicitly request changes using FeedIterator. This gives you fine-grained control over when and how you read changes from specific partitions.
**Key Implementation Steps:**
- Create a lease container to store checkpoints and coordinate between multiple readers
- Configure the start time or continuation token to specify where to begin reading
- Implement error handling and retry logic for resilience
- Process changes idempotently since the same change might be delivered multiple times
**Common Use Cases:**
- Real-time data synchronization between services
- Triggering notifications or workflows based on data changes
- Maintaining materialized views or search indexes
- Event sourcing and audit logging
The change feed captures inserts and updates but does not track deletions. For delete scenarios, implement soft deletes using a TTL property or deletion flag.
Implement Cosmos DB Change Feed Notifications
Why It Is Important
The Cosmos DB change feed is a critical feature for building reactive, event-driven applications in Azure. It enables real-time processing of data changes, which is essential for scenarios like maintaining synchronized caches, triggering workflows, real-time analytics, and implementing event sourcing patterns. For the AZ-204 exam, understanding change feed is vital because it represents a core competency in building modern, scalable cloud applications that respond to data changes efficiently.
What Is Cosmos DB Change Feed?
The change feed is a persistent, ordered log of changes made to items in an Azure Cosmos DB container. It records inserts and updates to documents in the order they occurred. Key characteristics include:
• Sorted by modification time within each logical partition • Does not capture deletes - you must implement soft delete patterns using a flag or TTL • Works with all Cosmos DB APIs (SQL, MongoDB, Cassandra, Gremlin, Table) • Provides a pull model where consumers read changes at their own pace • Changes are available for reading almost instantly after they occur
How Change Feed Works
The change feed operates by maintaining a log of all modifications. Here is how you can consume it:
1. Change Feed Processor Library The recommended approach using the SDK. It handles partition management, checkpointing, and load balancing automatically.
2. Azure Functions Trigger The simplest method using a Cosmos DB trigger binding that automatically scales based on workload.
3. Change Feed Pull Model For scenarios requiring fine-grained control over reading changes using FeedIterator.
Key Components
• Monitored Container: The container being watched for changes • Lease Container: Stores state and coordinates processing across multiple consumers • Compute Instance: The host running the change feed processor • Delegate: The code that processes each batch of changes
Common Use Cases
• Real-time data synchronization between services • Triggering notifications or alerts • Maintaining materialized views • Data migration and ETL pipelines • Event sourcing and CQRS implementations
Exam Tips: Answering Questions on Implement Cosmos DB Change Feed Notifications
1. Remember deletes are not captured: If a question asks about tracking deletions, look for answers involving soft delete with a TTL property or an "isDeleted" flag.
2. Lease container is mandatory: For the Change Feed Processor, a separate lease container is required to track progress. Questions often test this requirement.
3. Azure Functions is the simplest option: When asked for the least code or easiest implementation, choose Cosmos DB triggered Azure Functions.
4. Ordering is per partition: Changes are guaranteed to be in order only within a logical partition key, not across the entire container.
5. Latest version mode vs All versions and deletes: The default mode only shows the latest version of changed documents. Know that "All versions and deletes" mode is available for SQL API in certain scenarios.
6. StartFromBeginning: By default, change feed starts from the current time. Use StartFromBeginning option to read all historical changes.
7. Multiple consumers: The processor automatically distributes partitions among multiple instances for horizontal scaling. Each instance uses the same processor name but different instance names.
8. Watch for trigger vs processor questions: Functions triggers are ideal for serverless scenarios; the processor library offers more control for hosted applications.