A fuse file system cooperated with LemonDB to handle queries and tables. (extension of VE482 Lab8)

Compile:

• cd src
• make
• cd ..

Run & Test:

• ./src/lemondbfs -f dbdir/ mountdir/
• ls -laR mountdir/
• cd mountdir/mTable0/
• echo "SWAP ( c16 c9 ) FROM mTable0 WHERE ( c0 <= -20302 ) ;" > .query
• ls -a
• head res.tbl
• cd ../..
• fusermount -u mountdir

Currently no real file system operation is operated on mountdir/. Instead, they are all handled/translated/mirrored to real local file system operations or LemonDB instructions acted on dbdir/ and tables/files inside.

Goals:

• access lemonDB files and run queries without accessing the software
• extend UNIX "everything is a file" philosophy: a query is a file

Characteristics of lemonDBfs:

• usage: lemondbfs db_dir mount_dir

• strategy:

  1. for each .tbl file show a dir in mount_dir eg. for file mTable10.tbl show dir mTable10

  2. inside each dir show the corresponding table in file orig.tbl eg. for file mTable10.tbl show mtable10/orig.tbl

  3. inside each dir show an empty file called .query eg. for mTable10.tbl show the empty file mTable10/.query

  4. when a query is written to a .query file, apply it the orig.tbl and create the file res.tbl containing the result of the query eg. upon a write on mTable10/.query run the query (error on wrong query) and create mTable10/res.tbl with the result of the query

Getting started:

• prepare a skeleton of the code (eg. based on hello.c example?)
• add a simple printf to understand how/when the functions are called eg. printf("getattr(\"%s\"\n", path);
• keep the code short and simple, no long functions

Useful resources:

• libfuse source code: https://github.com/libfuse/libfuse

• fuse documentation: https://libfuse.github.io/doxygen/

• real examples:

  • fuse examples: https://github.com/libfuse/libfuse/tree/master/example
  • sshfs: https://github.com/libfuse/sshfs/blob/master/sshfs.c
  • fuse tutorial (the most useful one): https://www.cs.nmsu.edu/~pfeiffer/fuse-tutorial/

• toy examples:

  • https://engineering.facile.it/blog/eng/write-filesystem-fuse/
  • http://www.maastaar.net/fuse/linux/filesystem/c/2016/05/21/writing-a-simple-filesystem-using-fuse/

• lemonDBfs must be more stable (cf. BUGS)

• Allow more than one query in the .query file

• When many queries are input allow temporary result tables to be created. Use a mount flag? Take advantage of the DUMP query or use another approach?

• Allow the creation .tbl files: a dir in mount_dir + orig.tbl + .query file should create a corresponding .tbl file in db_dir

• Allow the deletion of .tbl files: when a dir is deleted in mount_dir the corresponding .tbl file should disappear

• Update this TODO. Creation + deletion are on the way, but not stable at all yet

!!DO NOT RUN AS ROOT!!

Could potentially corrupt the DB, or even crash lemonDB. New creation and deletion are very unstable and can have terrible consequences...

Already handled by uid check to forbid root user.