Reference: https://www.udemy.com/java-non-blocking-io-with-javanio-and-design-patterns/
Reference: https://github.com/kabutz/Transmogrifier
Reference: http://www.java2s.com/Tutorials/Java/Socket/How_to_use_Java_SocketChannel_create_a_HTTP_client.htm
Reference: https://www.youtube.com/watch?v=3m9RN4aDh08

• the main goal of this project is to show how to implement single-threaded and multi-threaded multiplexed non-blocking server using java.nio
  • please refer firstly: https://github.com/mtumilowicz/java12-nio-non-blocking-polling-server-workshop
• in the workshop we will try to fix failing tests from test/server/workshop package by following steps and hints in java/server/workshop and java/hander/workshop packages
• answers: java/server/answers, java/hander/answers packages

• a key represents the registration of a particular channel object with a particular selector object - moreover, we have methods: channel(), selector()
• SelectionKey object contains two sets
  • the interest set - operations we are interested in
  • the ready set - operations the channel is ready to perform (time the selector last checked the states of the registered channels)
• operations:
  • isReadable(),
  • isWritable(),
  • isConnectable(),
  • isAcceptable()
• good practice: we should use one selector for all selectable channels and delegate the servicing of ready channels to other threads
  • therefore we have a single point to monitor channel readiness and a decoupled pool of worker threads to handle the incoming data

• provide the ability to do readiness selection, which enables multiplexed I/O
  • I/O multiplexing is the capability to tell the kernel that we want to be notified if one or more I/O conditions are ready, like input is ready to be read
• provide the capability to ask a channel if it's ready to perform an I/O operation of interest to you
  • for example - check if ServerSocketChannel has any incoming connections ready to be accepted
• manages information about a set of registered channels and their readiness states
• channels are registered with selectors, and a selector can be asked to update the readiness states of the channels currently registered with it
• simple analogy
  • each pneumatic tube (channel) is connected to a single teller station inside the bank
  • station has three slots where the carriers (data buffers) arrive, each with an indicator (selection key) that lights up when the carrier is in the slot
  • teller (worker thread) once for a couple of minutes glances up at the indicator lights (invokes select()) to determine if any of the channels are ready (readiness selection)
  • teller (worker thread) can perform another task while the drive-through lanes (channels) are idle yet still respond to them in a timely manner when they require attention
• invoking select() on a selector object causes that the associated keys are updated by checking all the channels registered with that selector
• by iterating over these keys, we can service each channel that has become ready since the last time we invoked select()
• large number of channels can be checked for readiness simultaneously
  • true readiness selection is performed by operating system
  • one of the most important functions performed by an operating system is to handle I/O requests and notify processes when their data is ready
  • abstractions by which Java code can request readiness selection service from the underlying operating system
• given channel can be registered with more than one selector and has no idea which Selector objects it's currently registered with
• data never passes through selectors
• maintains three sets of keys:
  • Registered key set
    • currently registered keys associated with the selector
    • not every registered key is necessarily still valid
    • returned by the keys() method
  • Selected key set
    • Selected key set c Registered key set
    • key whose associated channel was determined by the selector to be ready for at least one of the operations in the key's interest set
    • returned by the selectedKeys()
    • selected key set vs the key's ready set
      • each key has an embedded ready set, and each key can be in selected key set
  • Cancelled key set
    • Cancelled key set c Registered key set
    • contains keys whose cancel() methods have been called (the key has been invalidated), but they have not been deregistered
• selector.select()
  • blocks indefinitely if no channels are ready
  • this method returns a nonzero value since it blocks until a channel is ready
  • it can return 0 if the wakeup() method of the selector is invoked
  • select() - return value is not a count of ready channels, but the number of channels that became ready since the last invocation of select()
• wakeup() provides the capability to gracefully break out a thread from a blocked select() invocation