If-Match - HTTP

The If-Match header is an HTTP request header used to ensure that the requested resource matches a specific version before performing an action on the server, such as updating or deleting a resource. This header is particularly useful for avoiding conflicts and maintaining consistency when multiple clients are working on the same resource concurrently.

The primary purpose of the If-Match header is to prevent the “lost update problem.” This problem occurs when two or more clients retrieve the same resource, make changes to it, and then attempt to update the resource on the server. Without proper synchronization, one of the updates may be lost, leading to data inconsistency.

The If-Match header helps to avoid this problem by ensuring that the server only processes the request if the resource’s current state matches the client’s expected state. If the resource has been modified by another client since it was last retrieved, the server will reject the request, allowing the client to handle the conflict accordingly.

Using the If-Match Header

To use the If-Match header, include it in an HTTP request along with an ETag (Entity Tag) value, which is an identifier representing the version of the resource. The ETag value is usually provided by the server in the response header when the resource is initially retrieved.

If the resource’s current ETag matches the one provided in the If-Match header, the server will process the request. If the ETag values do not match, the server will return a 412 Precondition Failed status code, indicating that the precondition set by the If-Match header has not been met.

Request Example

Here’s an example of an HTTP request using the If-Match header:

PUT /example-resource/123 HTTP/1.1
Host: example.com
Content-Type: application/json
If-Match: "abc123"

In this example, the client is attempting to update the resource at /example-resource/123 with the ETag value abc123. If the server finds that the resource’s current ETag matches abc123, it will process the update. Otherwise, it will return a 412 Precondition Failed status code.

Response Example

Here’s an example of an HTTP response when the If-Match precondition has been met:

HTTP/1.1 200 OK
Content-Type: application/json
ETag: "def456"

In this example, the server has successfully processed the update and returned a 200 OK status code. The new ETag value def456 is provided in the response header, which the client can use for future requests involving this resource.

If the precondition has not been met, the server will return a 412 Precondition Failed status code:

HTTP/1.1 412 Precondition Failed
Content-Type: application/json

In this case, the client should handle the conflict, such as by retrieving the latest version of the resource and resolving any discrepancies before attempting to update the resource again.

Summary

The If-Match header is an essential tool for maintaining data consistency and avoiding conflicts when multiple clients are working on the same resource. By providing an ETag value in the If-Match header, clients can ensure that their updates are only processed if the resource’s current state matches their expected state. This helps to prevent the “lost update problem” and allows clients to handle conflicts gracefully.

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