http://wiki.apache.org/hadoop/FAQ
3.10. What happens when two clients try to write into the same HDFS file?
HDFS supports exclusive writes only.
When the first client contacts the name-node to open the file for writing, the name-node grants a lease to the client to create this file. When the second client tries to open the same file for writing, the name-node will see that the lease for the file is already granted to another client, and will reject the open request for the second client.
