In the single-process mode, the server receives requests from web clients to a single process. Inside the single server process, acceptor threads are running that are waiting for new requests to arrive. When a request arrives, an acceptor thread accepts the connection and puts the request into the connection queue. A request processing thread picks up the request from the connection queue and handles the request.
Because the server is multi-threaded, all NSAPI extensions written to the server must be thread-safe. This means that if the NSAPI extension uses a global resource, like a shared reference to a file or global variable, then the use of that resource must be synchronized so that only one thread accesses it at a time. All plug-ins provided with the Web Server are thread-safe and thread-aware, providing good scalability and concurrency. However, your legacy applications might be single-threaded. When the server runs the application, it can only execute one at a time. This leads to server performance problems when put under load. Unfortunately, in the single-process design, there is no real workaround.