A Beginners Guide on ETags


Etags, or Entity Tags, are a critical component in many web applications and APIs, providing a mechanism to ensure data consistency and cache optimisation. They are widely used in modern web development and can be found in various databases, like Azure Cosmos DB.

In this guide, we will explore what Etags are, their advantages and disadvantages, and as an example we will understand how they are used in Azure Cosmos DB.

What is an Etag?

An Etag is a unique identifier generated by a server for a specific version of a resource, such as a document, image, or web page.

Etags are typically used in HTTP headers to allow clients and servers to communicate efficiently and ensure data consistency.

When a client requests a resource, the server generates an Etag for the current version of that resource and includes it in the response header. The client can then store the Etag and use it in subsequent requests to the server, allowing the server to determine if the client has the most up-to-date version of the resource.

Advantages of Etags

Etags offer several benefits, including:

a. Data consistency: Etags help prevent data loss and conflicts when multiple clients are trying to update the same resource simultaneously. By using Etags, the server can ensure that a client's update is based on the most recent version of the resource.

b. Cache control: Etags can be used to optimize cache usage by allowing clients to determine if their cached version of a resource is still valid. If the Etag for a resource has not changed since the client last requested it, the client can continue to use its cached version, reducing server load and improving performance.

c. Bandwidth reduction: Etags enable clients to make conditional requests, which only return data if the client's version of the resource is outdated. This can help reduce bandwidth usage and improve overall application performance.

Disadvantages of Etags

Despite their benefits, Etags also have some drawbacks:

a. Overhead: Etags add some overhead to server responses, as they require additional processing to generate and compare Etag values.

b. Complex implementation: Implementing Etags correctly can be challenging, especially when dealing with distributed systems and multiple caching layers.

c. Weak Etags: In some cases, servers might generate "weak" Etags that do not guarantee complete uniqueness, which can lead to false positives when comparing resource versions.

Applications of Etags

Etags are commonly used in various web applications and APIs, such as:

a. Content management systems (CMS): Etags can help ensure data consistency when multiple users are editing the same content simultaneously.

b. RESTful APIs: Etags are often used in APIs to enable clients to make conditional requests, improving performance and reducing bandwidth usage.

c. Web caching: Etags play a crucial role in web caching strategies, helping clients determine when to cache, update, or invalidate resources.

Etags in Cosmos DB

Azure Cosmos DB, a NoSQL database service, makes extensive use of Etags for concurrency control and cache management.

Here's how ETags are used in Cosmos DB:

  1. When a client reads a document from Cosmos DB, the service includes an ETag in the response headers. This ETag represents the current version of the document.

  2. When a client wants to update the document, it sends a request to Cosmos DB, including the ETag value it received earlier in the "If-Match" header.

  3. Cosmos DB compares the ETag value in the request with the current ETag value of the document in the database. If the values match, it means the document hasn't been modified by another process since the client last read it, so the update can proceed. Cosmos DB then updates the document and assigns a new ETag value to the updated version.

  4. If the ETag values don't match, it indicates that the document has been modified by another process since the client last read it. In this case, Cosmos DB rejects the update request, and the client must re-read the document to get the latest version and its corresponding ETag before attempting the update again.

This approach allows multiple clients to work concurrently on the same document without the need for locking mechanisms. It also reduces the chance of data corruption or loss due to simultaneous updates.

In addition to concurrency control, ETags can also help improve the performance of Cosmos DB by allowing clients to cache data and use conditional requests to fetch only the latest version of a document if the cached data is outdated.


Etags are a powerful tool for maintaining data consistency and optimizing cache usage in web applications and APIs. By understanding the basics of Etags and their use in systems like Cosmos DB, developers can create more efficient, high-performing applications that deliver a better user experience.

This is it for this article, if you found this helpful do give it a like and share it with your colleagues
If you have any suggestions, do comment them below, would love to work on them.