Learn Netty (codes + notes)

Netty Introduction

• NIO framework
• supports protocols: FTP, SMTP, HTTP
• big data, gaming, telcom
• elastic search, hadoop(Avro), Dubbo

BIO

• Blocking I/O
• Key class: SocketServer, Socket, BufferedReader(InputStreamReader), PrintWriter
• pros
  • easy to model and code
  • request -> create new thread -> handle
• cons
  • performance bottleneck, 1 request : 1 thread
  • hign concurrency scenario, CPU context switch costs
• before tomcat 7 uses BIO, after uses NIO

I/O models in network programming

• blocking vs non-blocking
• sync vs async
• 5 I/O models in Linux
  • blocking I/O
  • non-blocking I/O
  • multiplexing I/O: multi requests(threads)
  • signal-driven I/O: similar to events, SIGIO
  • async I/O: POSIX aio_xxx functions, Future-Listener
• I/O 2 steps
  • initialize I/O request, wait for data ready
  • actual I/O operation
• multiplexing
  • network I/O, multiple TCP connections(socket/channel), reuse one or many threads
  • use one or more threads to handle multiple TCP connections
  • reduce overheads, too many threads creation/destruction/maintain
  • select
    • how?
      • monitor file descriptors: writedfs, readfds, exceptfds
      • block after call select() till data is readable, writble, exception or timeout.
      • after select() returns, by iterating over fdset array to find out which hanldes triggered events. then get the fd for I/O operations
      • support on all platforms
      • performance descrease for more fds
      • for every select() call, fd is copied from user state to kernal state
      • limit on max fd per thread: 1024
  • poll
  • epoll
    • since kernel 2.6. no fds restrictions. copy once from user state to kernel state
    • via event notification
    • register fd via epoll_ctl. once fd is ready, kernel will use callback to activated fd.
    • max fd will be OS max file handle. 1G mem -> 100k handle
    • performance enhance. Callback
    • kernel and user space store same memory space.
    • epoll_create(), epoll_ctl(), epoll_wait()
  • example
    • 1M connections. 10k are active
    • select: need 1000 threads to support
    • poll: fd interation is stuck. need lots of space
    • epoll
• Java I/O history
  • before jdk1.4, BIO
  • jdk1.4, NIO
  • jdk1.7, NIO 2.0. support AIO. support file, socket AIO

Netty echo service demo

• EventLoop, EventLoopGroup
  • thread, thread pool
• Bootstrap
• Channel
  • connection between client and server.
• ChannelHandler, ChannelPipeline