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Distributed Systems Simulator - Part 3: Advanced Examples and Protocol API

Table of Contents

• Distributed Systems Simulator - Part 3: Advanced Examples and Protocol API
• ⇢ Additional Examples
• ⇢ ⇢ Lamport and Vector Timestamps
• ⇢ ⇢ Simulating Slow Connections
• ⇢ ⇢ Raft Consensus Failover
• ⇢ Protocol API
• ⇢ ⇢ Class Hierarchy
• ⇢ ⇢ Implementing a Custom Protocol
• ⇢ ⇢ Available API Methods
• ⇢ ⇢ Example: Reliable Multicast Implementation
• ⇢ Project Statistics

Additional Examples

Lamport and Vector Timestamps

t_l(j) := 1 + max(t_l(j), t_l(i))

for (i := 0; i < n; i++) {
    if (i = j) {
        v(i)++;
    } else if (v(i) < w(i)) {
        v(i) := w(i);
    }
}

Simulating Slow Connections

1/2 * (rand(500,2000) + rand(2000,8000)) = 1/2 * rand(2500,10000) = rand(1250,5000)ms

Raft Consensus Failover

009395ms: PID: 2; ... Leader elected by majority vote: process 2 (term 1)

• Stable startup leadership
• Heartbeats and follower acknowledgments
• Log replication
• Leader failure detection
• Majority-based reelection
• Safe reintegration of a recovered former leader
• Continued service after a later follower crash

Protocol API

Class Hierarchy

VSAbstractEvent
  +-- VSAbstractProtocol (base class for all protocols)
        +-- VSDummyProtocol
        +-- VSPingPongProtocol
        +-- VSBroadcastProtocol
        +-- VSInternalTimeSyncProtocol
        +-- VSExternalTimeSyncProtocol
        +-- VSBerkeleyTimeProtocol
        +-- VSOnePhaseCommitProtocol
        +-- VSTwoPhaseCommitProtocol
        +-- VSBasicMulticastProtocol
        +-- VSReliableMulticastProtocol

Implementing a Custom Protocol

• A public constructor: Must specify whether the client or the server initiates requests, using VSAbstractProtocol.HAS_ON_CLIENT_START or VSAbstractProtocol.HAS_ON_SERVER_START.
• onClientInit() / onServerInit(): Called once before the protocol is first used. Used to initialize protocol variables and attributes via the VSPrefs methods (e.g. initVector, initLong). Variables initialized this way appear in the process editor and can be configured by the user.
• onClientReset() / onServerReset(): Called each time the simulation is reset.
• onClientStart() / onServerStart(): Called when the client/server initiates a request. Typically creates and sends a VSMessage object.
• onClientRecv(VSMessage) / onServerRecv(VSMessage): Called when a message arrives.
• onClientSchedule() / onServerSchedule(): Called when a scheduled alarm fires.
• toString(): Optional. Customizes log output for this protocol.

Available API Methods

• sendMessage(VSMessage message): Sends a protocol message (automatically updates Lamport and Vector timestamps)
• hasOnServerStart(): Whether the server or client initiates requests
• isServer() / isClient(): Whether the current process has the protocol activated as server/client
• scheduleAt(long time): Creates an alarm that fires at the given local process time, triggering onClientSchedule() or onServerSchedule()
• removeSchedules(): Cancels all pending alarms in the current context
• getNumProcesses(): Returns the total number of processes in the simulation

• getTime() / setTime(long): Get/set the local process time
• getGlobalTime(): Get the current global simulation time
• getClockVariance() / setClockVariance(float): Get/set the clock drift
• getLamportTime() / setLamportTime(long): Get/set the Lamport timestamp
• getVectorTime() / updateVectorTime(VSVectorTime): Get/update the vector timestamp
• getProcessID(): Get the process PID
• isCrashed() / isCrashed(boolean): Check or set crash state
• getRandomPercentage(): Get a random value between 0 and 100

• new VSMessage(): Create a new message
• getMessageID(): Get the message NID
• setBoolean(key, value) / getBoolean(key): Set/get boolean data
• setInteger(key, value) / getInteger(key): Set/get integer data
• setLong(key, value) / getLong(key): Set/get long data
• setString(key, value) / getString(key): Set/get string data
• getSendingProcess(): Get a reference to the sending process
• isServerMessage(): Whether it's a server or client message

Example: Reliable Multicast Implementation

public class VSReliableMulticastProtocol extends VSAbstractProtocol {
    public VSReliableMulticastProtocol() {
        // The client initiates requests
        super(VSAbstractProtocol.HAS_ON_CLIENT_START);
        super.setClassname(super.getClass().toString());
    }

    private ArrayList<Integer> pids;

    // Initialize protocol variables (editable in the process editor)
    public void onClientInit() {
        Vector<Integer> vec = new Vector<Integer>();
        vec.add(1); vec.add(3);
        super.initVector("pids", vec, "PIDs of participating processes");
        super.initLong("timeout", 2500, "Time until resend", "ms");
    }

    // Send multicast to all servers that haven't ACKed yet
    public void onClientStart() {
        if (pids.size() != 0) {
            long timeout = super.getLong("timeout") + process.getTime();
            super.scheduleAt(timeout);
            VSMessage message = new VSMessage();
            message.setBoolean("isMulticast", true);
            super.sendMessage(message);
        }
    }

    // Handle ACK from a server
    public void onClientRecv(VSMessage recvMessage) {
        if (pids.size() != 0 && recvMessage.getBoolean("isAck")) {
            Integer pid = recvMessage.getIntegerObj("pid");
            if (pids.contains(pid))
                pids.remove(pid);
            super.log("ACK from Process " + pid + " received!");
            if (pids.size() == 0) {
                super.log("ACKs from all processes received!");
                super.removeSchedules();
            }
        }
    }

    // Retry on timeout
    public void onClientSchedule() { onClientStart(); }
}

Project Statistics

• 61 source files across 12 Java packages
• Approximately 15,710 lines of code
• 2.2 MB of generated Javadoc documentation
• 142 KB compiled JAR file
• 10 built-in protocols
• 163 configurable settings

• 146 source files (117 main + 29 test) across 19 Java packages
• Approximately 27,900 lines of code (22,400 main + 5,500 test)
• 12 built-in protocols
• 208 unit tests
• 269 configurable settings

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