I was trying loading file using hadoop API as an experiment.
I want to set replication to minimum as this one is for experiment. I first tried this with FileSystem.setReplication():
Configuration config = new Configuration();
config.set("fs.defaultFS","hdfs://192.168.248.166:8020");
FileSystem dfs2 = FileSystem.get(config);
Path src2 = new Path("C:\\Users\\abc\\Desktop\\testfile.txt");
Path dst2 = new Path(dfs2.getWorkingDirectory()+"/tempdir");
dfs2.copyFromLocalFile(src2, dst2);
dfs2.setReplication(dst2, (short)1); /**setting replication**/The replica was shown as 1, but it was available on 3 datanodes.
When I tried it with Configuration.set():
Configuration config = new Configuration();
config.set("fs.defaultFS","hdfs://192.168.248.166:8020");
config.set("dfs.replication", "1"); /**setting replication**/
FileSystem dfs2 = FileSystem.get(config);
Path src2 = new Path("C:\\Users\\abc\\Desktop\\testfile.txt");
Path dst2 = new Path(dfs2.getWorkingDirectory()+"/tempdir");
This gave the desired outcome (1 replica available on 1 datanode)
Why there are two APIs for the same thing? What is the difference between these two?
The difference is that Filesystem‘s setReplication() sets the replication of an existing file on HDFS. In your case, you first copy the local file testFile.txt to HDFS, using the default replication factor (3) and then change the replication factor of this file to 1. After this command, it takes a while until the over-replicated blocks get deleted. (source)
On the other hand, when you use the config.set("dfs.replication", "1"); command to set the replication, you can copy the local file after that, so its blocks get copied just once, from the first time.
In other words, I believe (but I might be wrong) that both commands have the same final result, but you have to wait a little bit until the first one is carried out.