Implementing referential integrity is a crucial part of managing and designing databases. In a relational database management system (RDBMS), referential integrity ensures that relationships between tables remain consistent, meaning that the connection between the tables is maintained and any action that may disrupt this consistency is prevented.
Although it’s not built natively into Azure Cosmos DB – a globally-distributed, multi-model database service provided by Microsoft – you can implement referential enforcement using a change feed.
Azure Cosmos DB Change Feed
Azure Cosmos DB change feed is a persistent record of changes to a collection. Once you enable the change feed in a container, it records every operation that affects the container (like inserts and updates) and outputs an infinite feed of these operation logs sorted chronologically. You can read this change feed at any point in time to see an item’s change history.
Implementing Referential Enforcement using Change Feed
To implement referential enforcement in Azure Cosmos DB using a change feed, you need to have two things in place:
- An Azure Function that can process feed changes.
- A Cosmos DB Trigger to generate a feed of changes.
The Azure Function represents the business logic that you want to influence the process of maintaining referential integrity. This function triggers whenever a change is detected on your Cosmos DB collection. The function can then handle the change and enforce the referential integrity rules you have defined.
Illustration
To illustrate this, let us assume that you have two collections in an Azure Cosmos DB:
- The parent collection called “Orders”
- The child collection called “OrderDetails”
The goal is to ensure that every entry in the “OrderDetails” collection refers to an existing order in the “Orders” collection.
[FunctionName("ReferentialEnforcementByChangeFeed")]
public static async Task Run(
[CosmosDBTrigger(
databaseName: "OrderDatabase",
collectionName: "OrderDetails",
ConnectionStringSetting = "CosmosDBConnection",
LeaseCollectionName = "leases")]
IReadOnlyList
[CosmosDB(
databaseName: "OrderDatabase",
collectionName: "Orders",
ConnectionStringSetting = "CosmosDBConnection",
Id = "{CosmosDBTriggeredId}",
PartitionKey ="{CosmosDBTriggeredPK}")]
dynamic document)
{
if (input != null && input.Count > 0)
{
//If the reference Order Id does not exist in Orders collection, delete this "OrderDetails" item.
if(document == null)
{
var client = new DocumentClient(new Uri("https://localhost:8081"), "Your Key");
await client.DeleteDocumentAsync(input[0].SelfLink);
log.Info($"Item {input[0].GetPropertyValue
}
else
{
log.Verbose("No invalid reference found.");
}
}
}
This code listens to changes in the “OrderDetails” collection. Whenever a new document is created or updated, the code checks if the corresponding order exists in the “Orders” collection. If it doesn’t, the code deletes this document to ensure referential integrity.
Remember, Azure Functions are serverless, so they scale dynamically to handle the load. This means that as your data grows and the load on your Cosmos DB container increases, the function will scale accordingly to keep enforcing referential integrity.
Note
Take note that Azure Cosmos DB does not native support transactions across multiple containers. If you need to modify data that’s spread across containers in a transactional manner, you should consider using the saga pattern or two-phase commit pattern to orchestrate and coordinate those operations.
To use the change feed feature in an Azure Cosmos DB, it should be Azure Cosmos DB SQL API or MongoDB API, as it is only supported in these two types of database accounts for now.
Conclusion
So, this was a very brief introduction to how you can use the Azure Cosmos DB change feed to enforce referential integrity. It’s an efficient and scalable way to ensure that your Azure Cosmos DB data stays well-structured and consistent across collections.
Practice Test
True or False: Change Feed in Azure Cosmos DB is a mechanism to get a sorted list of all items in an Azure Cosmos container in the order they were modified.
- True
- False
Answer: True
Explanation: The Change Feed in Azure Cosmos DB gives a sorted list of items in the order of their modification. This mechanism allows applications to process or respond to each change.
True or False: Referential enforcement in Azure Cosmos DB can be implemented using a Change Feed.
- True
- False
Answer: True
Explanation: With Change Feed, an application can respond to each insert, replace, and delete operation on items in an Azure Cosmos container and enforce referential integrity.
Which of the following are benefits of using Change Feed in Azure Cosmos DB? (Multiple select)
- a. Real-time processing
- b. Increased data redundancy
- c. Intelligent caching
- d. Event-driven architecture
Answer: a, d
Explanation: Change Feed allows real-time processing as it can react to changes in data as soon as they happen, and it supports event-driven architectures by enabling reactive programming models.
True or False: Azure Cosmos DB change feed can only be used for simple applications with limited data.
- True
- False
Answer: False
Explanation: Azure Cosmos DB’s change feed feature can handle massive amounts of data, allowing applications to react to each insert, replace, and delete operation on items in an Azure Cosmos container.
What feature does Change Feed provide that is crucial for implementing referential enforcement?
- a. Data partitioning
- b. Real-time analytics
- c. Network optimization
- d. Event-triggered processing
Answer: d
Explanation: The Change Feed feature enables event-triggered processing, which is critical for implementing referential enforcement, as it allows applications to respond and enforce integrity on each data modification event.
True or False: With the Change Feed feature, an application can only see the final state of a modified item, not the intermediate states.
- True
- False
Answer: True
Explanation: The Change Feed only provides the final state of the modified item in Azure Cosmos DB without any intermediate states.
True or False: Change Feed supports both point-in-time and continuous modes.
- True
- False
Answer: False
Explanation: As of now, Azure Cosmos DB’s Change Feed does not support point-in-time modes. It only supports continuous modes.
Which process does Azure Cosmos DB’s Change Feed utilize to ensure low latency?
- a. Sharding
- b. Indexing
- c. Polling
- d. Partitioning
Answer: c
Explanation: The Change Feed uses a polling mechanism to maintain low latency in data changes.
True or False: Change Feed in Azure Cosmos DB is an optional feature that can be turned on or off based on requirements.
- True
- False
Answer: True
Explanation: Change Feed is an optional feature in Azure Cosmos DB and can be enabled or disabled as per the needs of the application.
Which of the following is supporting to the distributed and reliable event-driven programming model?
- a. Backup and restore
- b. Cosmos DB’s Change Feed
- c. Data partitioning
- d. Decreased data redundancy
Answer: b
Explanation: Cosmos DB’s Change Feed provides distributed and reliable event-driven programming model by enabling applications to react to data changes. This makes it ideal for implementing referential enforcement in a distributed system.
Interview Questions
What does the term “change feed” refer to in Microsoft Azure Cosmos DB?
The change feed in Cosmos DB is a persistent record of changes to a container in the order they occur. It enables applications to react to new events in real-time or even retrospectively.
How can you access the change feed in Cosmos DB?
You can access the change feed through Azure Functions, or programmatically via the ChangeFeedProcessor or Cosmos DB SDKs.
What is the main purpose of implementing referential enforcement using a change feed?
Referential enforcement using a change feed allows for the propagation of changes in the source data to other related databases or systems. It helps maintain data integrity and consistency across various platforms.
What is the ChangeFeedProcessor class in Azure Cosmos DB?
The ChangeFeedProcessor class is used to read from the change feed in a scalable and fault-tolerant manner. It provides methods for starting, stopping, and checking the status of the change feed processing.
In which scenarios could the change feed be useful for implementing referential enforcement?
The change feed is useful in scenarios such as syncing a database with a cache, triggering events or computations based on changes, moving data to other systems, or maintaining backups.
Can a change feed in Azure Cosmos DB be used for real-time processing?
Yes, change feeds allow for real-time processing as it provides a sorted list of documents within a container in the order in which they were modified.
How does the ChangeFeedProcessor handle changed data?
The ChangeFeedProcessor reads the change feed and invokes a delegate with the changed data. It monitors changes and there is no need to write code to poll for changes.
What is the “start from beginning” feature of change feed?
This feature allows the change feed to read changes from the beginning of time. This is useful for scenarios like initial data migration or audit scenarios where historical data processing is important.
How does Azure Cosmos DB manage change feed checkpoints?
Cosmos DB maintains a cursor in a separate container to keep track of the last item that was read in the change feed. This is called a checkpoint. Cosmos DB automatically manages these checkpoints.
Is it possible to filter changes by partition key in Cosmos DB change feed?
Yes, you can filter changes by partition key. However, it’s important to note that filtering only applies to new changes after the time of the filter application and not to historical data.
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