Add chunk management notes to DESIGN.txt

DESIGN.txt

@@ -137,26 +137,51 @@ Chunk Management:
     servers need to forget some copies. If they are under-replicated,
     some chunk servers need to copy chunks from elsewhere.
 
-    Event 1: A chunk server connects
-        If the chunk server is holding some chunks, for each chunk
-        one of the following must be true:
-            - The chunk is not used by the file system
-                => Mark the chunk for removal
-            - The chunk is used and, with this copy, perfectly replicated
-                => Do nothing
-            - The chunk is used and under-replicated
-                => ???
-            - The chunk is used and over-replicated
-                => Mark the chunk for removal
+    The metadata server holds an authoritative list of hashes that each
+    chunk server needs to hold. These lists are enforced periodically and
+    automatically during the state updates, therefore the metadata server
+    only needs to reorganize the chunks in its in-memory lists. The changes
+    will propagate over the chunk servers in time. The metadata server's
+    in-memory representation is the ideal state the system is converging
+    to, which means every chunk if always replicated correctly. Given this
+    setup, a chunk can only be under-replicated when there are not enough
+    chunk servers.
 
-    Event 2: A write operation on metadata (adding chunks)
+    Case 1: The first chunk server connects with no chunks
+        CS connects
+        MS accepts
+        CS sends auth message
+        MS validates auth
+        MS requests list of chunks
+        CS sends the chunk list
+        MS notices it's empty
+
+    Case 2: The first chunk server connects with some chunks
+        CS connects
+        MS accepts
+        CS sends auth message
+        MS valudates auth
+        MS requests list of chunks
+        CS sends the chunk list
+        MS iterates over each chunk. For each chunk it determines
+           whether it's useless, under-replicated, over-replicated,
+           or properly replicated.
+        MS adds all useless or over-replicated chunks to the rem_list
+           of the newly connected chunk server. Note that it's only
+           possible for a chunk to be over-replicated by 1, so removing
+           it from the new chunk server will fix the issue.
+        MS If a chunk is under-replicated, it means that there are not
+           enough chunk servers to hold all the replicas, so there is
+           nothing to be done.
+
+    Case 3: Chunk server with some chunks disconnects
         TODO
 
     Event 3: A chunk server disconnects
         A number of chunks are lost, but there is a chance the server will come back
         in a short while (10s or so).
-
-        TODO
+            => Wait for about 10s. If the chunk server doesn't come back, distribute
+               its hashes to other chunk servers
 
     Event 4: A write operation on metadata occurs (overwriting old chunks)
         Assuming the write overwrites some chunks: