1. refactoring, merge "replication" logic into "topology" package

2. when growing volumes, additional preferred "rack" and "dataNode"
paraemters are also provided. Previously only "dataCenter" paraemter is
provided.

go/topology/allocate_volume.go

@@ -0,0 +1,32 @@
+package topology
+
+import (
+	"code.google.com/p/weed-fs/go/storage"
+	"code.google.com/p/weed-fs/go/util"
+	"encoding/json"
+	"errors"
+	"net/url"
+)
+
+type AllocateVolumeResult struct {
+	Error string
+}
+
+func AllocateVolume(dn *DataNode, vid storage.VolumeId, collection string, rp *storage.ReplicaPlacement) error {
+	values := make(url.Values)
+	values.Add("volume", vid.String())
+	values.Add("collection", collection)
+	values.Add("replication", rp.String())
+	jsonBlob, err := util.Post("http://"+dn.PublicUrl+"/admin/assign_volume", values)
+	if err != nil {
+		return err
+	}
+	var ret AllocateVolumeResult
+	if err := json.Unmarshal(jsonBlob, &ret); err != nil {
+		return err
+	}
+	if ret.Error != "" {
+		return errors.New(ret.Error)
+	}
+	return nil
+}