What is the use of ETag property in Cosmos DB?

In Azure Cosmos DB, the ETag (entity tag) property is a system-generated property associated with each document. It serves as a mechanism for optimistic concurrency control and helps ensure data consistency and integrity. Here’s the use of the ETag property in Cosmos DB:

  1. Concurrency Control:
    • The ETag property allows Cosmos DB to implement optimistic concurrency control, which helps prevent conflicts when multiple clients attempt to modify the same document simultaneously.
    • When a document is read from Cosmos DB, the client receives the ETag value associated with that document.
    • When the client wants to modify the document, it includes the ETag value in the request. This helps ensure that the document has not been modified by another client since it was last read.
  2. Preventing Conflicts:
    • When a client attempts to update a document, Cosmos DB compares the ETag value provided by the client with the current ETag value of the document in the database.
    • If the ETag values match, it indicates that the document has not been modified by any other client, and the update operation is allowed to proceed.
    • If the ETag values do not match, it means the document has been modified by another client, and Cosmos DB returns a concurrency conflict error. The client can then choose an appropriate strategy, such as refreshing the document and reattempting the update.
  3. Data Consistency:
    • The ETag property ensures data consistency by detecting conflicts and preventing inconsistent updates from being applied to documents.
    • By using the ETag property during update operations, you can maintain the integrity and consistency of the data stored in Cosmos DB.
  4. Optimistic Concurrency:
    • Optimistic concurrency control, facilitated by the ETag property, allows multiple clients to concurrently read and modify documents without locking or blocking each other.
    • It assumes that conflicts are rare and provides a lightweight mechanism for conflict detection.

The ETag property is automatically generated and managed by Cosmos DB, and you do not need to explicitly set or modify it. It is returned with each document read operation and should be included in update or delete requests to ensure concurrency control.

By utilizing the ETag property, you can implement optimistic concurrency control, prevent conflicts, and maintain data consistency when working with documents in Azure Cosmos DB.

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