Appending Files in Hadoop

Can I really append a file in HDFS OR do i need to always replace the file? http://hadoop4mapreduce.blogspot.com/2012/08/two-methods-to-append-content-to-file.html We had a client that wanted to merge data coming from same source into a single large file. The challenge was how to handle concurrency. The append process is not thread safe. The solution we used was to batch all the small files into a larger file periodically.