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The PORT_ACK message is the router's response to the application's NEW_PORT message. After receiving PORT_ACK, the application is safe to process requests using this port. This message avoids a racing condition when the application starts processing a request from the shared queue and sends REQ_HEADERS_ACK. The REQ_HEADERS_ACK message contains the application port ID as reply_port, which the router uses to send request data. When the application creates a new port, it immediately sends it to the main router thread. Because the request is processed outside the main thread, a racing condition can occur between the receipt of the new port in the main thread and the receipt of REQ_HEADERS_ACK in the worker router thread where the same port is specified as reply_port.
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