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+How to use Replication
+
+## Introduction
+
+SeaweedFS can support replication. The replication is implemented not on file level, but on volume level.
+
+## How to use Replication
+
+Basically, the way it works is:
+
+1. start weed master, and optionally specify the default replication type
+
+   ```
+   ./weed master -defaultReplicationType=001
+   ```
+
+2. start volume servers as this:
+
+   ```
+   ./weed volume -port=8081 -dir=/tmp/1 -max=100
+   ./weed volume -port=8082 -dir=/tmp/2 -max=100
+   ./weed volume -port=8083 -dir=/tmp/3 -max=100
+   ```
+
+Submitting, Reading, Deleting files has the same steps.
+
+## The meaning of replication type
+
+Note: This subject to change.
+
+```
+000: no replication, just one copy
+001: replicate once on the same rack
+010: replicate once on a different rack in the same data center
+100: replicate once on a different data center
+200: replicate twice on two other different data center
+110: replicate once on a different rack, and once on a different data center
+...
+```
+
+So if the replication type is xyz
+
+```
+x: number of replica in other data centers
+y: number of replica in other racks in the same data center
+z: number of replica in other servers in the same rack
+```
+
+x,y,z each can be 0, 1, or 2. So there are 27 possible replication types, and can be easily extended. Each replication type will physically create x+y+z+1 copies of volume data files.
+
+## Example Topology Configuration File
+
+The WeedFS master server tries to read the topology configuration file. It default to /etc/weedfs/weedfs.conf if not specified. The topology setting to configure data center and racks file format is as this.
+
+```
+  <Configuration>
+    <Topology>
+      <DataCenter name="dc1">
+        <Rack name="rack1">
+          <Ip>192.168.1.1</Ip>
+        </Rack>
+      </DataCenter>
+      <DataCenter name="dc2">
+        <Rack name="rack1">
+          <Ip>192.168.1.2</Ip>
+        </Rack>
+        <Rack name="rack2">
+          <Ip>192.168.1.3</Ip>
+          <Ip>192.168.1.4</Ip>
+        </Rack>
+      </DataCenter>
+    </Topology>
+  </Configuration>
+```
+
+## Configure Topology via Volume Server
+Instead of the weed.conf file, volume servers can start with a specific data center name.
+
+```
+ weed volume -dir=/tmp/1 -port=8080 -dataCenter=dc1
+ weed volume -dir=/tmp/2 -port=8081 -dataCenter=dc2
+```
+
+### Allocate File Key on specific data center
+
+When requesting a file key, an optional "dataCenter" parameter can limit the assigned volume to the specific data center. For example, this specify
+
+```
+ http://localhost:9333/dir/assign?dataCenter=dc1
+```