re-organize code directory structure

note/replication.txt

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+1. each file can choose the replication factor
+2. replication granularity is in volume level
+3. if not enough spaces, we can automatically decrease some volume's the replication factor, especially for cold data
+4. plan to support migrating data to cheaper storage
+5. plan to manual volume placement, access-based volume placement, auction based volume placement
+
+When a new volume server is started, it reports 
+  1. how many volumes it can hold
+  2. current list of existing volumes and each volume's replication type
+Each volume server remembers:
+  1. current volume ids
+  2. replica locations are read from the master
+
+The master assign volume ids based on
+  1. replication factor
+     data center, rack
+  2. concurrent write support
+On master, stores the replication configuration
+{
+  replication:{
+    {type:"00", min_volume_count:3, weight:10},
+    {type:"01", min_volume_count:2, weight:20},
+    {type:"10", min_volume_count:2, weight:20},
+    {type:"11", min_volume_count:3, weight:30},
+    {type:"20", min_volume_count:2, weight:20}
+  },
+  port:9333,
+}
+Or manually via command line
+  1. add volume with specified replication factor
+  2. add volume with specified volume id
+
+
+If duplicated volume ids are reported from different volume servers, 
+the master determines the replication factor of the volume,
+if less than the replication factor, the volume is in readonly mode
+if more than the replication factor, the volume will purge the smallest/oldest volume
+if equal, the volume will function as usual
+
+
+Use cases:
+  on volume server
+    1. weed volume -mserver="xx.xx.xx.xx:9333" -publicUrl="good.com:8080" -dir="/tmp" -volumes=50
+  on weed master
+    1. weed master -port=9333
+      generate a default json configuration file if doesn't exist
+      
+Bootstrap
+  1. at the very beginning, the system has no volumes at all.
+When data node starts:
+  1. each data node send to master its existing volumes and max volume blocks
+  2. master remembers the topology/data_center/rack/data_node/volumes
+     for each replication level, stores
+       volume id ~ data node
+       writable volume ids
+If any "assign" request comes in
+  1. find a writable volume with the right replicationLevel
+  2. if not found, grow the volumes with the right replication level
+  3. return a writable volume to the user
+
+  
+Plan:
+  Step 1. implement one copy(no replication), automatically assign volume ids
+  Step 2. add replication
+
+For the above operations, here are the todo list:
+  for data node:
+    0. detect existing volumes DONE
+    1. onStartUp, and periodically, send existing volumes and maxVolumeCount  store.Join(), DONE
+    2. accept command to grow a volume( id + replication level)  DONE
+       /admin/assign_volume?volume=some_id&replicationType=01
+    3. accept setting volumeLocationList  DONE
+       /admin/set_volume_locations_list?volumeLocationsList=[{Vid:xxx,Locations:[loc1,loc2,loc3]}]
+    4. for each write, pass the write to the next location, (Step 2)
+       POST method should accept an index, like ttl, get decremented every hop
+  for master:
+    1. accept data node's report of existing volumes and maxVolumeCount ALREADY EXISTS /dir/join
+    2. periodically refresh for active data nodes, and adjust writable volumes
+    3. send command to grow a volume(id + replication level)  DONE
+    4. NOT_IMPLEMENTING: if dead/stale data nodes are found, for the affected volumes, send stale info
+       to other data nodes. BECAUSE the master will stop sending writes to these data nodes
+    5. accept lookup for volume locations    ALREADY EXISTS /dir/lookup
+    6. read topology/datacenter/rack layout
+
+TODO:
+  1. replicate content to the other server if the replication type needs replicas