Implement a custom conflict resolution policy for Azure Cosmos DB for NoSQL

Azure Cosmos DB is a powerful NoSQL database service that offers a flexible and scalable solution for managing large volumes of data. One important aspect of working with Cosmos DB is handling conflicts that may arise when multiple clients attempt to modify the same document simultaneously. In this article, we will explore how to implement a custom conflict resolution policy for Azure Cosmos DB.

Conflict Resolution Overview

Conflict resolution is the process of determining which version of a document should be preserved when conflicts occur. By default, Cosmos DB uses last-writer-wins (LWW) as its conflict resolution policy. This means that when conflicting versions of a document are detected, the version with the latest timestamp will be preserved, while the other versions will be discarded. However, this policy may not always be suitable for all scenarios.

Implementing a Custom Conflict Resolution Policy

To implement a custom conflict resolution policy, you can leverage the Cosmos DB Change Feed feature. The Change Feed provides an easy way to consume the real-time changes happening in a Cosmos DB container. By using the Change Feed, you can write custom logic to handle conflicts based on your specific requirements.

Here’s a step-by-step guide to implementing a custom conflict resolution policy:

1. Create an Azure Cosmos DB account and configure a container

Start by creating an Azure Cosmos DB account and configure a container to store your documents. Set the conflict resolution mode to “Custom” when configuring the container.

2. Enable the Change Feed

Enable the Change Feed feature for your Cosmos DB container. This can be done through the Azure portal or by using the Azure Cosmos DB SDK.

3. Register a Change Feed processor

Write code to register a Change Feed processor that will process the changes in the container. The Change Feed processor will read the changes from the Change Feed and allow you to implement your custom conflict resolution logic.

// Register the Change Feed processor
var processor = new ChangeFeedProcessorBuilder()
.WithHostName(hostName)
.WithFeedContainer(myFeedContainer)
.WithLeaseContainer(myLeaseContainer)
.WithProcessorOptions(new ChangeFeedProcessorOptions{
StartFromBeginning = true
})
.WithDelegate(ProcessChangesAsync)
.Build();

await processor.StartAsync();

4. Implement conflict resolution logic

In the delegate method (e.g., ProcessChangesAsync), you can implement your custom conflict resolution logic based on the changes received from the Change Feed. This logic can include comparing the properties of the conflicting documents, applying business rules, or even involving external systems for conflict resolution.

async Task ProcessChangesAsync(IReadOnlyCollection changes, CancellationToken cancellationToken)
{
foreach (var change in changes)
{
// Handle conflict resolution based on your requirements
if (change.Conflict)
{
// Apply your custom conflict resolution logic here
// For example, you can choose to merge the conflicting documents or apply a specific resolution strategy
}

// Update or process the document
await ProcessDocumentAsync(change.Document);
}
}

5. Update or process the document

After handling the conflict resolution, you can update or process the document as needed. This can involve modifying its properties, performing calculations, or triggering actions based on its content.

6. Monitor and fine-tune the conflict resolution

Continuously monitor the conflict resolution process to ensure it meets your requirements. You may need to fine-tune the logic based on the feedback and behavior observed in your specific scenario.

By implementing a custom conflict resolution policy, you have the flexibility to handle conflicts in Azure Cosmos DB according to your specific needs. Whether it is merging document versions, applying complex resolution strategies, or involving external systems, the Change Feed feature allows you to implement your own logic seamlessly.

Remember to test your custom conflict resolution logic thoroughly to ensure it performs as expected in different scenarios. The Change Feed feature provides an excellent foundation for building conflict resolution capabilities that align with your application requirements.

Conclusion

Azure Cosmos DB is a powerful NoSQL database service that offers a customizable conflict resolution policy through the Change Feed feature. By leveraging the Change Feed and implementing a custom conflict resolution logic, you can handle conflicts in Cosmos DB based on your specific requirements.