Contact Management

- There was an improvement in the paper page 64 to set a contact as stale and replace it if it's stale, but only if there is a replacement, otherwise just keep a stale count. The stale count is the amount of time a contact failed to respond to a request.
- The above feature is now implemented now.
- Added the management of stale contacts code to insert and removal

src/kademlia/routing/Bucket.java

@@ -45,7 +45,13 @@ public interface Bucket
     public boolean containsNode(Node n);
 
     /**
-     * Remove a contact from this bucket
+     * Remove a contact from this bucket.
+     * 
+     * If there are replacement contacts in the replacement cache,
+     * select the last seen one and put it into the bucket while removing the required contact.
+     *
+     * If there are no contacts in the replacement cache, then we just mark the contact requested to be removed as stale.
+     * Marking as stale would actually be incrementing the stale count of the contact.
      *
      * @param c The contact to remove
      *

src/kademlia/routing/Contact.java

@@ -19,6 +19,15 @@ public class Contact implements Comparable<Contact>
     private final Node n;
     private long lastSeen;
 
+    /**
+     * Stale as described by Kademlia paper page 64
+     * When a contact fails to respond, if the replacement cache is empty and there is no replacement for the contact,
+     * just mark it as stale.
+     *
+     * Now when a new contact is added, if the contact is stale, it is removed.
+     */
+    private int staleCount;
+
     /**
      * Create a contact object
      *
@@ -59,6 +68,22 @@ public class Contact implements Comparable<Contact>
         return c.getNode().equals(this.getNode());
     }
 
+    /**
+     * Increments the amount of times this count has failed to respond to a request.
+     */
+    public void incrementStaleCount()
+    {
+        staleCount++;
+    }
+
+    /**
+     * @return Integer Stale count
+     */
+    public int staleCount()
+    {
+        return this.staleCount;
+    }
+
     @Override
     public int compareTo(Contact o)
     {

src/kademlia/routing/KadBucket.java

@@ -57,11 +57,38 @@ public class KadBucket implements Bucket
         }
         else
         {
-            /* If the bucket is filled, we put the contacts in the replacement cache */
+            /* If the bucket is filled, so put the contacts in the replacement cache */
             if (contacts.size() >= this.config.k())
             {
-                /* Bucket is filled, place this contact in the replacement cache */
-                this.insertIntoCache(c);
+                /* If the cache is empty, we check if any contacts are stale and replace the stalest one */
+                Contact stalest = null;
+                for (Contact tmp : this.contacts.keySet())
+                {
+                    if (tmp.staleCount() > this.config.stale())
+                    {
+                        /* Contact is stale */
+                        if (stalest == null)
+                        {
+                            stalest = tmp;
+                        }
+                        else if (tmp.staleCount() > stalest.staleCount())
+                        {
+                            stalest = tmp;
+                        }
+                    }
+                }
+
+                /* If we have a stale contact, remove it and add the new contact to the bucket */
+                if (stalest != null)
+                {
+                    this.contacts.remove(stalest);
+                    this.insert(c);
+                }
+                else
+                {
+                    /* No stale contact, lets insert this into replacement cache */
+                    this.insertIntoReplacementCache(c);
+                }
             }
             else
             {
@@ -97,15 +124,19 @@ public class KadBucket implements Bucket
             return false;
         }
 
-        this.contacts.remove(c);
-
-        /* If there are replacement contacts in the replacement cache, lets put them into the bucket */
         if (!this.replacementCache.isEmpty())
         {
+            /* Replace the contact with one from the replacement cache */
+            this.contacts.remove(c);
             Contact replacement = this.replacementCache.firstKey();
             this.contacts.put(replacement, replacement);
             this.replacementCache.remove(replacement);
         }
+        else
+        {
+            /* There is no replacement, just increment the contact's stale count */
+            this.contacts.get(c).incrementStaleCount();
+        }
 
         return true;
     }
@@ -137,7 +168,7 @@ public class KadBucket implements Bucket
     /**
      * When the bucket is filled, we keep extra contacts in the replacement cache.
      */
-    private void insertIntoCache(Contact c)
+    private void insertIntoReplacementCache(Contact c)
     {
         /* Just return if this contact is already in our replacement cache */
         if (this.replacementCache.containsKey(c))