The 102 Processing status code is an interim response used to inform the client that the server has accepted the complete request, but has not yet completed it. This status code SHOULD only be sent when the server has a reasonable expectation that the request will take significant time to complete. As guidance, if a method is taking longer than 20 seconds (a reasonable, but arbitrary value) to process the server SHOULD return a 102 Processing response. The server MUST send a final response after the request has been completed.
Methods can potentially take a long period of time to process, especially methods that support the Depth header. In such cases the client may time-out the connection while waiting for a response. To prevent this the server may return a 102 Processing status code to indicate to the client that the server is still processing the method.