Will my server be able to run only one client if its a single-threaded process. If yes, why?

Question

I have googled decent enough to understand threads and processes. One thing I am confused is about single-threaded process. The scenario is the Server-Client application process where each client is treated as a single process in the server that is single-threaded. Some blogs are saying that single-threaded program will not be able to handle multiple clients at a time ie., it can start one process and only after finishing it, another process can be started. But some blogs saying its possible.

According to my understanding, in a single-core processor system, if its programmed for multi-tasking, depending on the time-slice allocated for each processes, more than one process can be concurrently handled. In multi-processor system also, more than one client process can be handled in parallel. Is it just the web servers that does not handle more than one process at a time because it is iterative server? If its anyother concurrent server, will it be handling more than one process, without waiting for each process to get completed to handle the next one..?

As I am confused by many different explanations in different blogs, I am expecting a very accurate answer just for the above mentioned scenario, in either a single-processor with multi-tasking environment (or) a multi-processor environment. Also, would like to know if there are any benefits of using a single-threaded process over a process without any threads.

Sorry If my understanding is wrong. Kindly respond.

Thanks in advance..

Answer 1

If your server does this:

while(true)
    listen()
    accept()
    read()
    process()
    write()
    close()

And each of the calls to accept, read, process, write and close are single-threaded, then the server cannot listen for new requests again until those calls complete. The requests might queue up or simply get dropped if the server is not listening for them. If accept/read/process/write/close take a long time compared to listen(), then your server will not be available to service requests frequently. Moreoever, if some requests take 100 seconds and some requests take 1 second, and a lot of that time is blocking on I/O to e.g. a database or a web service, then you're spending most of your time in the server doing nothing. Here's the alternative:

while(true)
    listen()
    start new thread()
        accept()
        read()
        process()
        write()
        close()

Now all you're doing synchronously is kicking off the new thread to handle the incoming request, and then immediately beginning to listen again. The OS will handle scheduling the threads and if some of them end up waiting a long time for some I/O to occur, your server can still process the fast requests in the meantime.