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Lyrebird Assignment #4

Overview

This program is used to decrypt large amounts of encrypted tweet files. To reduce the overall running time and take advantages of both multiple machines and multiple CPU cores, it used a distributed system with client-server architecture. Server and client communicate using sockets, server regularly accept new client and assign decryption tasks to clients, client uses fork function to spread tasks across multiple processes and pipes to pass messages between parent and child processes, the result message of decryption will be send back to server and be recorded in the log file. As for scheduling, both of server and client use "first come first served" with select function.

Design && Implementation

  • Server side

    • The server first get its ip address using getifaddrs (filtered '0.0.0.0' and '127.0.0.1'), then it uses this ip address to create, bind and listen at a socket with a random port number, if all goes well, server will print its welcome message to the terminal which contains its ip address and port number (use getsockname to get value).

    • After that, the server begins a while loop and keep running select function to see if there is a new client ask for connect or new message received. If there is a new client, server will accept it and record its information to log file. If there is a new message received from client, the server will handle it in different way according to the message type:

      • DISPATCH_MSG: send a new pair of encrypt and decrypt file name to client and let it do decryption.
      • SUCCESS_MSG: record the successful decrypted file name, the client ip address and pid to log file.
      • FAILURE_MSG: record the error message, the client ip address and pid to log file.
      • DISCONNECT_MSG: record the disconnect message for one client to log file and close the socket which connected to that client.

    • Once all the decryption tasks has been assigned to clients, the server will break the while loop and send each connected client a CLIENT_EXIT_MSG message to ask them exit. After that, server will wait for all clients' remained messages and handle it the same way like above until there is no client. Finally, server prints a end message and quit.

  • Client side

    • The client first try to use fork function to create child processes with the number of CPU cores, each time a child process created, client will send a DISPATCH_MSG to server to ask for task, and then:
      • In parent process, it creates a socket and try to connect to server using the input ip address and port number, once connected with server, it begins to receive both server and child processes' messages using select function. It will assign the task that received from server to its child processes (using recv and write function) or send the decryption feedback message from child process to server side (using read and send function).
      • In child process, it receive the decryption task from parent process, finish it and uses pipe to pass the decryption feedback message to parent process, if doesn't have fatal error, it will wait for new task again, always do like this until pipe closed by parent process.
    • When parent process receive the CLIENT_EXIT_MSG from server side, it will close all the write pipes to child processes, read all remaining messages from child processes and send them to server side, then it wait for all child processes exit and finally exit along with DISCONNECT_MSG send to server.

Testing

During my testing, to make sure that my decryption is still right, I tested it with the data in assignment #2 #3 which includes empty tweet, maximal size of tweet, vary long name file, in order to test the robustness of my program, I tried with wrong number of parameters in main function, wrong ip address, wrong port number, empty config file and nonexistent config file.

After that, I tested the socket function through printing each times message in recv and send, As I expected, all messages (input & output file name, client message) passed well.

Besides that, I also tested:

  • multiple clients
  • cross platform (ubuntu + MacOS)
  • huge amounts of decrypt files (10000+)
  • log file line number (should equals to tasks number * 2 + clients number * 2 + 2)

Last but not least, there should be no zombie process after my program finished, so I used ps and kill to check it and result shows everything is well.