The HTTP response status code 102 Processing is an informative message from the server to the client, indicating a substantial delay before the final HTTP response is delivered.

This code serves as a heads-up to the client, informing them of the extended waiting period, ensuring that the client does not time out prematurely.

By receiving this status, the client can patiently wait for the eventual response without interrupting the communication prematurely.

When receiving the 102 Processing status code, the client will keep the HTTP connection open, patiently awaiting the final HTTP response. Then the server is responsible for delivering the ultimate response; otherwise, a genuine timeout will ensue.

Despite it's no longer a part of the official HTTP specifications, the 102 Processing status code is still used in practical scenarios.

Fortunately, modern HTTP clients are designed to overlook informational status messages they don't recognize, thereby ensuring that clients without explicit support for this code are not adversely affected. Note that the server has the flexibility to send multiple 102 Processing status updates ahead of the final HTTP response.

In this specific sample, the client starts a request for the "sample.pdf" file, and the server responds with a 102 Processing HTTP status code, signifying a longer processing time than usual.

After a brief interval, the server reiterates the processing status. Eventually, the server concludes the process with the final HTTP response, indicating success with a 200 OK status code and providing the requested "sample.pdf" file as the message body.

Request:
GET /sample.pdf HTTP/1.1
Host: www.example.com

Response:
HTTP/1.1 102 Processing
< short pause in transmission >
HTTP/1.1 102 Processing
< short pause in transmission >
HTTP/1.1 200 OK
Content-Type: application/pdf
Content-Length: 10000
< message body containing the "sample.pdf" content will follow >

The HTTP 102 Processing status code, while no longer part of the official HTTP specifications, is still used in practice.

It serves to inform the client that there might be a significant delay in processing the HTTP request, allowing the client to maintain the connection and wait for the final HTTP response to arrive without prematurely timing out.