Etags Concept

Introduction

ETag (Entity Tag) is an HTTP header used for caching and optimistic concurrency control in web applications. It provides a mechanism to identify different versions of a resource and manage updates efficiently.

Purpose

The main purpose of ETag is to reduce server load and improve performance by allowing clients to cache resources and validate whether they have the latest version.

Where are we using it

In Google calendar extension for Scrims ETag is used when upsertGoogleEvents function is triggered which compared the ETag of existing event with the updated event coming from Scrims and skips if they are the same.

Note: we do not check ETag if event is coming from Google.

if (existingEvent) {
  if (localEvent.etag && localEvent.etag !== "") {
    if (existingEvent.raw_json.etag === localEvent.etag) {
      console.info("[upsert-google-event] skipping google upsert; same etag found");
      return;
    }
  }
}

Key Concepts

1. Generation: ETags are typically generated based on the content of the resource using hashing algorithms (e.g., MD5, SHA-1, SHA-256).
2. Usage:
   • Client-Side Caching: Clients store the ETag received from the server. When requesting the resource again, they include the ETag in the If-None-Match header to check if the resource has changed.
   • Server-Side Validation: Servers compare the client-provided ETag with the current version of the resource. If they match, the server returns a 304 Not Modified response, indicating the client's cached version is up-to-date.
3. Concurrency Control: ETags can be used for optimistic concurrency control. Clients include the ETag when updating a resource (If-Match header), allowing the server to reject updates if the resource has been modified since the client last retrieved it.

Implementation Guidelines

1. Generating ETags:
   • Use strong hashing algorithms to ensure uniqueness and security of ETags.
2. Handling Requests:
   • Include ETag in the response headers when serving resources.
   • Process If-None-Match and If-Match headers in requests to validate and manage resource updates.

Example Scenario

Scenario: Using ETag for Cache Validation

1. Prerequisites:
   • Understanding of HTTP headers and caching mechanisms.
   • API endpoints that serve resources with ETag headers.
2. Generating ETag:
   • Calculate ETag based on the content of the resource (e.g., content hash).
   • Include ETag in response headers (ETag: "abcdef123456").
3. Client-Side Handling:
   • Client receives ETag in the response.
   • Subsequent requests include If-None-Match: "abcdef123456" header to validate cached resource.
4. Server-Side Validation:
   • Server compares client-provided ETag with current resource version.
   • If match (ETag unchanged), return 304 Not Modified; else, serve updated resource.

Conclusion

ETag enhances performance and ensures data consistency in web applications by enabling efficient caching and optimistic concurrency control. By implementing ETag headers in API responses, applications can optimize resource delivery and reduce unnecessary data transfers.