From 050942608e70d01e5833fe5d0e5019071f95c68b Mon Sep 17 00:00:00 2001
From: Sam Anthony <sam@samanthony.xyz>
Date: Fri, 15 Nov 2024 17:03:21 -0500
Subject: reliable multicast: announce presence to group

---
 src/main/java/derms/net/rmulticast/Announce.java   | 47 ++++++++++++++++++++++
 .../java/derms/net/rmulticast/AnnounceMessage.java | 10 +++++
 .../java/derms/net/rmulticast/NullPayload.java     | 10 +++++
 src/main/java/derms/net/rmulticast/Receive.java    |  7 +++-
 .../derms/net/rmulticast/ReliableMulticast.java    |  7 ++++
 5 files changed, 79 insertions(+), 2 deletions(-)
 create mode 100644 src/main/java/derms/net/rmulticast/Announce.java
 create mode 100644 src/main/java/derms/net/rmulticast/AnnounceMessage.java
 create mode 100644 src/main/java/derms/net/rmulticast/NullPayload.java

(limited to 'src/main/java/derms/net/rmulticast')

diff --git a/src/main/java/derms/net/rmulticast/Announce.java b/src/main/java/derms/net/rmulticast/Announce.java
new file mode 100644
index 0000000..08d817d
--- /dev/null
+++ b/src/main/java/derms/net/rmulticast/Announce.java
@@ -0,0 +1,47 @@
+package derms.net.rmulticast;
+
+import derms.net.ConcurrentMulticastSocket;
+import derms.net.Packet;
+
+import java.io.IOException;
+import java.net.DatagramPacket;
+import java.net.InetAddress;
+import java.net.InetSocketAddress;
+import java.time.Duration;
+import java.util.logging.Logger;
+
+/**
+ * Periodically announce the presence of the local process to the group.
+ * Allows processes to populate their set of group members, even if not
+ * all members would normally send messages to the group.
+ */
+class Announce implements Runnable {
+    private static final Duration period = Duration.ofSeconds(30);
+
+    private final DatagramPacket pkt;
+    private final ConcurrentMulticastSocket outSock;
+    private final Logger log;
+
+    Announce(InetSocketAddress group, InetAddress laddr, ConcurrentMulticastSocket outSock) throws IOException {
+        AnnounceMessage msg = new AnnounceMessage(laddr);
+        this.pkt = Packet.encode(msg, group);
+        this.outSock = outSock;
+        this.log = Logger.getLogger(this.getClass().getName());
+    }
+
+    @Override
+    public void run() {
+        try {
+            for (;;) {
+                try {
+                    Wait.forDuration(period);
+                    outSock.send(pkt);
+                } catch (IOException e) {
+                    log.warning(e.getMessage());
+                }
+            }
+        } catch (InterruptedException e) {
+            log.info("Announce thread interrupted: " + e.getMessage());
+        }
+    }
+}
diff --git a/src/main/java/derms/net/rmulticast/AnnounceMessage.java b/src/main/java/derms/net/rmulticast/AnnounceMessage.java
new file mode 100644
index 0000000..bfe4b40
--- /dev/null
+++ b/src/main/java/derms/net/rmulticast/AnnounceMessage.java
@@ -0,0 +1,10 @@
+package derms.net.rmulticast;
+
+import java.net.InetAddress;
+
+/** A message whose only purpose is to announce the presence of the sender in the group. */
+class AnnounceMessage extends Message<NullPayload> {
+    AnnounceMessage(InetAddress laddr) {
+        super(new NullPayload(), laddr, new MessageID[0], new MessageID[0]);
+    }
+}
diff --git a/src/main/java/derms/net/rmulticast/NullPayload.java b/src/main/java/derms/net/rmulticast/NullPayload.java
new file mode 100644
index 0000000..1eee5d6
--- /dev/null
+++ b/src/main/java/derms/net/rmulticast/NullPayload.java
@@ -0,0 +1,10 @@
+package derms.net.rmulticast;
+
+import java.io.Serializable;
+
+class NullPayload implements Serializable, Hashable {
+    @Override
+    public int hash() {
+        return -1;
+    }
+}
diff --git a/src/main/java/derms/net/rmulticast/Receive.java b/src/main/java/derms/net/rmulticast/Receive.java
index f20f833..29ee305 100644
--- a/src/main/java/derms/net/rmulticast/Receive.java
+++ b/src/main/java/derms/net/rmulticast/Receive.java
@@ -50,12 +50,15 @@ class Receive<T extends Serializable & Hashable> implements Runnable {
     }
 
     private void receive(Message<T> msg) {
+        groupMembers.add(msg.sender);
+
+        if (msg instanceof AnnounceMessage)
+            return;
+
         acks.add(msg.id());
         received.add(msg);
         delivered.add(msg);
 
-        groupMembers.add(msg.sender);
-
         nacks.remove(msg.id());
         retransmissions.remove(msg);
 
diff --git a/src/main/java/derms/net/rmulticast/ReliableMulticast.java b/src/main/java/derms/net/rmulticast/ReliableMulticast.java
index 528bb41..a9e1fa4 100644
--- a/src/main/java/derms/net/rmulticast/ReliableMulticast.java
+++ b/src/main/java/derms/net/rmulticast/ReliableMulticast.java
@@ -53,6 +53,13 @@ public class ReliableMulticast<T extends Serializable & Hashable> {
         (new Thread(new Retransmit<T>(retransmissions, outSock, group))).start();
 
         (new Thread(new Prune<T>(received, groupMembers))).start();
+
+        try {
+            (new Thread(new Announce(group, laddr, outSock))).start();
+        } catch (IOException e) {
+            log.severe("Failed to start announce thread: " + e.getMessage());
+            throw e;
+        }
     }
 
     public void send(T payload) throws IOException {
-- 
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