Implement optimistic concurrency control using ETags

Optimistic concurrency control is essential for managing concurrent access and updates to data in distributed systems like Microsoft Azure Cosmos DB. By utilizing entity tags (ETags), you can implement optimistic concurrency control to ensure data consistency. In this article, we will explore how to effectively use ETags for optimistic concurrency control in Cosmos DB.

What are ETags?

ETags are unique identifiers associated with each document in Cosmos DB. They represent the state of the document at a specific point in time. By comparing ETags during updates, you can determine if the document has been modified concurrently by another process. If the ETag of the document being updated does not match the ETag in the database, it indicates that a concurrent update has occurred, and the update request can be rejected.

Implementing Optimistic Concurrency Control Using ETags

Follow these steps to implement optimistic concurrency control using ETags in Cosmos DB:

1. Enable ETag support: Ensure that ETag support is enabled for the collection or container where you want to implement concurrency control. You can enable it during the creation of the container or by modifying the container properties.
2. Retrieve the document and its ETag: When fetching a document for updating, make sure to retrieve the ETag associated with it. This can be done by including the "_etag" property in the retrieved document.
   GET /dbs/{databaseId}/colls/{collectionId}/docs/{documentId} HTTP/1.1
Host: {databaseAccount}.documents.azure.com
3. Prepare the update operation: Before performing the update, ensure that you have the updated document ready. Make any necessary changes to the document based on your business logic.
4. Specify the ETag in the update request: When sending the update request, include the expected ETag value in the If-Match header. This ensures that the update will only succeed if the current ETag matches the expected value.
   PUT /dbs/{databaseId}/colls/{collectionId}/docs/{documentId} HTTP/1.1
   Host: {databaseAccount}.documents.azure.com
   Content-Type: application/json
   If-Match: "{documentEtag}"

   {
// Updated document content
}

5. Handle ETag mismatch errors: If the ETag in the database does not match the expected ETag in the If-Match header, Cosmos DB will respond with a 412 Precondition Failed error. Catch this error in your code and handle it accordingly. You can choose to retry the update operation or notify the user about the concurrent modification.

By utilizing ETags and implementing optimistic concurrency control in Cosmos DB, you can ensure that data modifications are managed consistently in a distributed environment. This approach enables concurrent updates without causing conflicts or data inconsistencies.

Remember to always validate and test your implementation thoroughly to ensure it meets your specific requirements and works effectively in your application scenario.

In conclusion, optimistic concurrency control using ETags is an important technique when designing and implementing native applications using Microsoft Azure Cosmos DB. By following the steps outlined in this article, you can effectively manage concurrent updates and maintain data consistency in your Cosmos DB collections.