I want to create a file in HDFS that has a bunch of lines, each generated by a different call to map. I don't care about the order of the lines, just that they all get added to the file. How do I accomplish this?
If this is not possible, then is there a standard way to generate unique file names to put each line of output into a separate file?
There is no way to append to an existing file in hadoop at the moment, but that's not what it sounds like you want to do anyway. It sounds like you want to have the output from your Map Reduce job go to a single file, which is quite possible. The number of output files is (less than or) equal to the number of reducers, so if you set your number of reducers to 1, you'll get a single file of output.
Before you go and do that however, think if that's what you really want. You'll be creating a bottle neck in your pipeline where it needs to pass all your data through a single machine for that reduce. Within the HDFS distributed file system, the difference between having one file and having several files is pretty transparent. If you want a single file outside the cluster, you might do better to use getmerge from the file system tools.