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•  package neko

  ScalaNeko is a framework designed to help with the prototyping of distributed algorithms.

  ScalaNeko Framework

  ScalaNeko is a framework designed to help with the prototyping of distributed algorithms. It is loosely based on the Neko framework [1] which was programmed in Java more than a decade earlier, mainly by Péter Urbán.

  Whereas the original Neko framework was designed for performance evaluation and modeling, the main focus of ScalaNeko is to serve as a support for teaching distributed algorithms. Hence, the current version of ScalaNeko only supports simulated execution. However, we still have the intention to support actual distributed execution in a future version, and hence provide a full replacement of the original Neko.

  1. Architecture

  In order to effectively use ScalaNeko, it is helpful to understand its general architecture, which can be described as follows:

  There are several important entities in ScalaNeko:

  • The system is what handles the execution engine within the virtual machine and the initialization procedure. There is exactly one instance running for every virtual machine. The system also holds a discrete event simulator. See neko.Main and neko.kernel.NekoSystem.
  • The network simulates the behavior of a network, and is responsible for transmitting messages between processes. In the current version, it is running over a discrete-event simulation. See neko.network.Network and neko.kernel.sim.Simulator.
  • The processes are the basic unit of concurrency, and represent a virtual computer connected through a network. Every process has a unique identity represented by a neko.PID. A process does nothing by itself and is merely a shell for protocols. See neko.NekoProcess and neko.ProcessConfig.
  • The protocols are the actual logic of the system and implement the algorithms. A process holds one or many protocols, which are organized as a stack. There are two kinds of protocols: active and reactive ones. While active protocols carry their own flow of execution, that is, act as a thread, concurrently with the system, the reactive protocols only execute code as a reaction to incoming events. See neko.ActiveProtocol, neko.ReactiveProtocol, neko.Protocol, and neko.ProtocolUtils.
  • Protocols and processes exchange information through events. There are two types of events: signals and messages. Signals allow protocols within the same process to notify each other. In contrast, messages allow protocol instances to communicate across different processes. In other words, only messages are transmitted through the network. See neko.Event, neko.Signal, neko.UnicastMessage, neko.MulticastMessage, and neko.Wrapper.

  A simplified view of the architecture of an execution of ScalaNeko is depicted below:

  +-------------------------------------------------------+
  |       process p1                    process pn        |
  |  +-------------------+         +-------------------+  |
  |  | +---------------+ |         | +---------------+ |  |
  |  | | protocol p1:A | |         | | protocol pn:A | |  |
  |  | +-------------+-+ |         | +-------------+-+ |  |
  |  |   |           |   |   ...   |   |           |   |  |
  |  | +-+-----------V-+ |         | +-+-----------V-+ |  |
  |  | | protocol p1:B | |         | | protocol pn:B | |  |
  |  | +-------------+-+ |         | +-------------+-+ |  |
  |  +---|-----------|---+         +---|-----------|---+  |
  |      |           |                 |           |      |
  |  +---+-----------V-----------------+-----------V---+  |
  |  |                      network                    |  |
  |  +-------------------------------------------------+  |
  |                  +------------------+                 |
  |                  |     simulator    |                 |
  |                  +------------------+       system    |
  +-------------------------------------------------------+

  Creating a ScalaNeko application typically requires to implement the following steps:

  1. Implement the protocols. At least, an application will require to implement an active protocol, but also possibly a number of reusable reactive ones.
  2. Each protocol is likely to define its own message types. The most appropriate location for doing so is in a companion object of the protocol. Messages are best defined as a case class so that they are ensured to be immutable and code for pattern matching is automatically generated by the compiler.
  3. Creating a process initializer that instantiates and connects the protocols of the processes.
  4. Creating a main object which provides the basic parameters of the execution, such as the total number of processes to create and their initializer.

  The initialization proceeds roughly as illustrated below:

       creates            creates
  Main ------> NekoSystem ------> Network
                          creates
                    ''    ------> ProcessInitializer
                          creates             creates
                    ''    =====>> NekoProcess =====>> Protocol

  2. Creating protocols

  A protocol can be either active or reactive. An active protocol is one that executes its own thread, concurrently with that of the other protocols or processes. In contrast, a reactive protocol only executes as a reaction to events, and does not do anything otherwise.

  2.1 Active protocols

  An active protocol is typically defined as a subclass of neko.ActiveProtocol.

  An active protocol has its own thread of control. The code of the protocol is implemented in its method neko.ActiveProtocol.run, which must be defined in the subclass. This code is executed concurrently with the rest of the system.

  An active protocol has access to operations for sending and receiving message. New messages are sent with the method neko.ActiveProtocol.SEND. While messages are received through blocking calls to neko.ActiveProtocol.Receive, as illustrated below. Note that, in order to receive messages of a certain type, the protocol must register by calling neko.ActiveProtocol.listenTo for this type.

  class PingPong(c: ProcessConfig) extends ActiveProtocol(c, "ping-pong")
  {
    val next = me.map{i => (i+1) % N}
    var record = Set.empty[Event]
  
    listenTo(classOf[Ping])
    listenTo(classOf[Pong])
    def run(): Unit =
    {
      SEND(Ping(me, next))
  
      Receive {
        case Ping(from, _) => SEND(Pong(me, from))
        case Pong(from, _) => SEND(Ping(me, from))
      }
  
      Receive { m =>
        record += m
      }
    }
  }

  It is also possible to override the method neko.ActiveProtocol.onReceive. By doing so, messages that are matched by onReceive are processed reactively upon arrival, while those that are not matched by onReceive are stored into the receive queue and must be handled by a blocking call to neko.ActiveProtocol.Receive.

  2.2 Reactive protocols

  Most protocols in a process are reactive. A reactive protocol is usually sandwiched between a network and an application (or a lower-level protocol and a higher-level one). The simplest way to implement one is by extending neko.ReactiveProtocol. The information has two flows: downstream and upstream. This is illustrated in the figure below.

           application
    |                      ^
    V                      |
  +----------------------------+
  | onSend        DELIVER(...) |
  |                            | Reactive protocol
  | SEND(...)        onReceive |
  +----------------------------+
    |                      ^
    V                      |
            network

  For the downstream flow (from application to network), the code of the protocol is implemented in the method neko.ReactiveProtocol.onSend, usually implemented as a scala.PartialFunction which reacts as appropriate to each event. The protocol can itself send messages through the neko.ReactiveProtocol.SEND method.

  For the upstream flow (from network to application), the code of the protocol is implemented in the method neko.ReactiveProtocol.onReceive, also implemented as a scala.PartialFunction which reacts appropriately to each incoming events. Events of a certain type are delivered to the protocol only if it registers to the event type by calling the neko.ReactiveProtocol.listenTo method on that event type. The protocol can deliver a message to the application through the method neko.ReactiveProtocol.DELIVER.

  Note that the two flows are not mutually exclusive. It is perfectly valid, and even frequent, for a protocol to call neko.ReactiveProtocol.DELIVER in neko.ReactiveProtocol.onSend, or to call neko.ReactiveProtocol.SEND in neko.ReactiveProtocol.onReceive .

  3. Defining new events (messages and signals)

  Let's start with a little bit of terminology. An event denotes anything that happens in the system and is represented by the abstract class neko.Event. Events can be of two types:

  • A signal is an event that occurs within one process, and can go from one protocol to another, but never cross process boundaries. It is represented by the subclasses of neko.Signal.
  • A message is an event that crosses process boundaries, but is typically (but not necessarily) interpreted by the same protocol in the target process. It is represented by the subclasses of neko.Message.

  A message can be "top-level" or a "wrapper". A top-level message is one that is created by the sending protocol. It has its own identity, as well as a source and destinations. In contrast, a wrapper is simply a shell that extends the information of an existing message. It retains the same identity, source, and destinations, but provides a shell to the message and can add its own information. This results into messages of three types:

  • A neko.MulticastMessage is a top-level message with multiple destinations. See the example below on how to define a new message:

  case class Snapshot(
      from: PID,
      to: Set[PID])
    extends MulticastMessage

  NB: The arguments *must* be named from and to.

  • A neko.UnicastMessage is a top-level message with a single destination process.

  case class Token (
      from: PID,
      to: PID)
    extends UnicastMessage

  NB: The arguments *must* be named from and to.

  • A neko.Wrapper is a shell that wraps an existing message. A wrapper can also extend another wrapper; not only top-level messages. A wrapper preserves the identity, the source and the destinations of the message it wraps.

  case class SequencedMessage(msg: Message, sn: Int) extends Wrapper(msg)

  4. Initialization of a process

  While processes are created automatically, their protocols are not, and must be initialized and connected. This is done through a process initializer, by providing an instance of neko.ProcessInitializer, whose sole role is to create the protocols of a process and combine them.

  ProcessInitializer { p =>
      val app  = new PingPong(p)
      val fifo = new FIFOChannel(p)
      app --> fifo
    }

  In the above example, each process is initialized by executing the above code. The code creates two protocols while registering them into the object p given as argument (which represents the process being initialized). Then, the two protocols are connected such that all SEND operations of protocol app are handed to protocol fifo. The send operations of protocol fifo use the default target which is the network interface of the process.

  It is also possible to initialize processes differently, by discriminating based on the identifier of the process to initialize. That identifier is obtained from the argument with p.pid.

  5. Setting up a new system

  A new instance of a ScalaNeko system is created and configured by creating an object that extends neko.Main. The resulting object becomes a main object and is thus executable (neko.Main is a subclass of scala.App).

  Class neko.Main requires to set parameters, such as the network topology and the process initializer, as illustrated below:

  object PingPongApp extends Main(topology.Clique(3))( ProcessInitializer { p=> ... } )

  Future planned versions of ScalaNeko will make it possible to define many more parameters, such as the network topologyDescriptor, etc...

  References

  1. Péter Urbán, Xavier Défago, André Schiper: Neko: A Single Environment to Simulate and Prototype Distributed Algorithms. J. Inf. Sci. Eng. 18(6): 981-997 (2002).

  Contributors

  Lead architect: Xavier Défago

  Other contributors:

  • Naoyuki Onuki (trace system; integration with NekoViewer)

  Definition Classes

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•  object WaveClient

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