The Hadoop Distributed File System (HDFS) implements a permissions model for files and directories that shares much of the POSIX model. Each file and directory is associated with an owner and a group. The file or directory has separate permissions for the user that is the owner, for other users that are members of the group, and for all other users. For files, the r permission is required to read the file, and the w permission is required to write or append to the file. For directories, the r permission is required to list the contents of the directory, the w permission is required to create or delete files or directories, and the x permission is required to access a child of the directory.
In contrast to the POSIX model, there are no sticky, setuid or setgid bits for files as there is no notion of executable files. For directories, there are no setuid or setgid bits directory as a simplification. Collectively, the permissions of a file or directory are its mode. In general, Unix customs for representing and displaying modes will be used, including the use of octal numbers in this description. When a file or directory is created, its owner is the user identity of the client process, and its group is the group of the parent directory (the BSD rule).
Each client process that accesses HDFS has a two-part identity composed of the user name, and groups list. Whenever HDFS must do a permissions check for a file or directory foo accessed by a client process,
- If the user name matches the owner of foo, then the owner permissions are tested;
- Else if the group of foo matches any of member of the groups list, then the group permissions are tested;
- Otherwise the other permissions of foo are tested.
If a permissions check fails, the client operation fails. HDFS has permissions model for files and directories similar to POSIX, with three types of permission: the read permission (r), the write permission (w), and the execute permission (x).
The read permission is required to read files or list the contents of a directory. The write permission is required to write a file or, for a directory, to create or delete files or directories in it. The execute permission is ignored for a file because you can’t execute a file on HDFS (unlike POSIX), and for a directory this permission is required to access its children.
Each file and directory has an owner, a group, and a mode. The mode is made up of the permissions for the user who is the owner, the permissions for the users who are members of the group, and the permissions for users who are neither the owners nor members of the group. By default, Hadoop runs with security disabled, which means that a client’s identity is not authenticated.
