Multi-Backend Abstraction

[Junde Yhi]

• Abstraction layer upon database backends

[Junde Yhi]

The abstraction takes form of FUSE, where the implementer pack all function pointers to one structure, and pass it to the caller to register.

In FUSE, we have

struct fuse_operations {
  int (*getattr) (const char *, struct stat *, struct fuse_file_info *fi);
  int (*readlink) (const char *, char *, size_t);
  int (*mknod) (const char *, mode_t, dev_t);
  int (*mkdir) (const char *, mode_t);
  int (*unlink) (const char *);
  int (*rmdir) (const char *);
  int (*symlink) (const char *, const char *);
  int (*rename) (const char *, const char *, unsigned int flags);
  int (*link) (const char *, const char *);
  int (*chmod) (const char *, mode_t, struct fuse_file_info *fi);
  int (*chown) (const char *, uid_t, gid_t, struct fuse_file_info *fi);
  int (*truncate) (const char *, off_t, struct fuse_file_info *fi);
  int (*open) (const char *, struct fuse_file_info *);
  int (*read) (const char *, char *, size_t, off_t, struct fuse_file_info *);
  int (*write) (const char *, const char *, size_t, off_t, struct fuse_file_info *);
  int (*statfs) (const char *, struct statvfs *);
  int (*flush) (const char *, struct fuse_file_info *);
  int (*release) (const char *, struct fuse_file_info *);
  int (*fsync) (const char *, int, struct fuse_file_info *);
  int (*setxattr) (const char *, const char *, const char *, size_t, int);
  int (*getxattr) (const char *, const char *, char *, size_t);
  int (*listxattr) (const char *, char *, size_t);
  int (*removexattr) (const char *, const char *);
  int (*opendir) (const char *, struct fuse_file_info *);
  int (*readdir) (const char *, void *, fuse_fill_dir_t, off_t, struct fuse_file_info *, enum fuse_readdir_flags);
  int (*releasedir) (const char *, struct fuse_file_info *);
  int (*fsyncdir) (const char *, int, struct fuse_file_info *);
  void *(*init) (struct fuse_conn_info *conn, struct fuse_config *cfg);
  void (*destroy) (void *private_data);
  int (*access) (const char *, int);
  int (*create) (const char *, mode_t, struct fuse_file_info *);
  int (*lock) (const char *, struct fuse_file_info *, int cmd, struct flock *);
  int (*utimens) (const char *, const struct timespec tv[2], struct fuse_file_info *fi);
  int (*bmap) (const char *, size_t blocksize, uint64_t *idx);
  int (*ioctl) (const char *, unsigned int cmd, void *arg, struct fuse_file_info *, unsigned int flags, void *data);
  int (*poll) (const char *, struct fuse_file_info *, struct fuse_pollhandle *ph, unsigned *reventsp);
  int (*write_buf) (const char *, struct fuse_bufvec *buf, off_t off, struct fuse_file_info *);
  int (*read_buf) (const char *, struct fuse_bufvec **bufp, size_t size, off_t off, struct fuse_file_info *);
  int (*flock) (const char *, struct fuse_file_info *, int op);
  int (*fallocate) (const char *, int, off_t, off_t, struct fuse_file_info *);
  ssize_t (*copy_file_range) (const char *path_in, struct fuse_file_info *fi_in, off_t offset_in, const char *path_out, struct fuse_file_info *fi_out, off_t offset_out, size_t size, int flags);
};

... in which some of the functions will not be implemented (e.g. access, we always use default_permission). Thus, the "abstraction" is just a strip of the above structure:

struct mdbfs_operations {
  /* FS operations */
  void *(*init) (struct fuse_conn_info *conn, struct fuse_config *cfg);
  void (*destroy) (void *private_data);
  int (*statfs) (const char *, struct statvfs *);

  /* File Object Manipulation */
  int (*create) (const char *, mode_t, struct fuse_file_info *);
  int (*mknod) (const char *, mode_t, dev_t);
  int (*unlink) (const char *);
  int (*mkdir) (const char *, mode_t);
  int (*rmdir) (const char *);

  /* I/O */
  int (*open) (const char *, struct fuse_file_info *);
  int (*read) (const char *, char *, size_t, off_t, struct fuse_file_info *);
  int (*read_buf) (const char *, struct fuse_bufvec **bufp, size_t size, off_t off, struct fuse_file_info *);
  int (*write) (const char *, const char *, size_t, off_t, struct fuse_file_info *);
  int (*write_buf) (const char *, struct fuse_bufvec *buf, off_t off, struct fuse_file_info *);
  int (*opendir) (const char *, struct fuse_file_info *);
  int (*readdir) (const char *, void *, fuse_fill_dir_t, off_t, struct fuse_file_info *, enum fuse_readdir_flags);

  /* Link */
  int (*symlink) (const char *, const char *);
  int (*readlink) (const char *, char *, size_t);
};

Database back-ends should provide a function (e.g. register()), which returns a structure described above. Then the file system driver will register these functions directly to FUSE. Then, FUSE will take the responsibility of calling them at proper time.

[Junde Yhi]

Initial implementation in 8dcb900, closing.