Start to wrap up all thoughts on what should happen for rsync 3.

[rsync/rsync.git] / rsync3.txt

-*- indented-text -*-

Notes towards a new version of rsync 
Martin Pool <mbp@samba.org>


Good things about the current implementation:

  - Widely known and adopted.

  - Fast/efficient, especially for moderately small sets of files over
    slow links (transoceanic or modem.)

  - Fairly reliable.

  - The choice of runnning over a plain TCP socket or tunneling over
    ssh.

  - rsync operations are idempotent: you can always run the same
    command twice to make sure it worked properly without any fear.
    (Are there any exceptions?)

  - Small changes to files cause small deltas.

  - There is a way to evolve the protocol to some extent.

  - rdiff and rsync --write-batch allow generation of standalone patch
    sets.  rsync+ is pretty cheesy, though.  xdelta seems cleaner.

  - Process triangle is creative, but seems to provoke OS bugs.

  - "Morning-after property": you don't need to know anything on the
    local machine about the state of the remote machine, or about
    transfers that have been done in the past.

  - You can easily push or pull simply by switching the order of
    files.


Bad things about the current implementation:

  - Persistent and hard-to-diagnose hang bugs remain

  - Protocol is sketchily documented, tied to this implementation, and
    hard to modify/extend

  - Both the program and the protocol assume a single non-interactive
    one-way transfer 

  - A list of all files are held in memory for the entire transfer,
    which cripples scalability to large file trees

  - Opening a new socket for every operation causes problems,
    especially when running over SSH with password authentication.

  - Renamed files are not handled: the old file is removed, and the
    new file created from scratch.

  - The versioning approach assumes that future versions of the
    program know about all previous versions, and will do the right
    thing.

  - People always get confused about ':' vs '::'

  - Error messages can be cryptic.


Protocol philosophy:

   *The* big difference between protocols like HTTP, FTP, and NFS is
    that their fundamental operations are "read this file", "delete
    this file", and "make this directory", whereas rsync is "make this
    directory like this one".


Questionable features:

  These are neat, but not necessarily clean or worth preserving.  

  - The remote rsync can be wrapped by some other program, such as in
    tridge's rsync-mail scripts.  The general feature of sending and
    retrieving mail over rsync is good, but this is perhaps not the
    right way to implement it.


Desirable features:

  These don't really require architectural changes; they're just
  something to keep in mind.
 
  - Synchronize ACLs and extended attributes

  - Anonymous servers should be efficient

  - Code should be portable to non-UNIX systems

  - Should be possible to document the protocol in RFC form

  - --dry-run option

  - IPv6 support.  Pretty straightforward.

  - Allow the basis and destination files to be different.  For
    example, you could use this when you have a CD-ROM and want to
    download an updated image onto a hard drive.

  - Efficiently interrupt and restart a transfer.  We can write a
    checkpoint file that says where we're up to in the filesystem.
    Alternatively, as long as transfers are idempotent, we can just
    restart the whole thing.  [NFSv4]

  - Scripting support.  

  - Propagate atimes and do not modify them.  This is very ugly on
    Unix.  It might be better to try to add O_NOATIME to kernels, and
    call that.

  - VFS.  Useful?

  - Unicode.  Probably just use UTF-8 for everything.


Hard links:

  At the moment, we can recreate hard links, but it's a bit
  inefficient: it depends on holding a list of all files in the tree.
  Every time we see a file with a linkcount >1, we need to search for
  another known name that has the same (fsid,inum) tuple.  We could do
  that more efficiently by keeping a list of only files with
  linkcount>1, and removing files from that list as all their names
  become known.


Scripting issues:

  - Perhaps support multiple scripting languages: candidates include
    Perl, Python, Tcl, Scheme (guile?), sh, ...

  - Simply running a subprocess and looking at its stdout/exit code
    might be sufficient, though it could also be pretty slow if it's
    called often.

  - There are security issues about running remote code, at least if
    it's not running in the users own account.  So we can either
    disallow it, or use some kind of sandbox system.


Scripting hooks:

  - Whether to transfer a file

  - What basis file to use

  - Logging
  
  - Whether to allow transfers (for public servers)

  - Authentication

  - Locking


Interactive interface:

  - Something like ncFTP, or integration into GNOME-vfs.  Probably
    hold a single socket connection open.

  - Can either call us as a separate process, or as a library.

  - The standalone process needs to produce output in a form easily
    digestible by a calling program, like the --emacs feature some
    have. 

  - Yow!  emacs support.  (You could probably build that already, of
    course.)


Pie-in-the-sky features:

  These might have a severe impact on the protocol, and are not
  clearly in our core requirements.  It looks like in many of them
  having scripting hooks will allow us 

  - Transport over UDP multicast.  The hard part is handling multiple
    destinations which have different basis files.  We can look at
    multicast-TFTP for inspiration.

  - Conflict resolution.  Possibly general scripting support will be
    sufficient.

  - Integrate with locking.  It's hard to see a good general solution,
    because Unix systems have several locking mechanisms, and grabbing
    the lock from programs that don't expect it could cause deadlocks,
    timeouts, or other problems.  Scripting support might help.

  - Replicate in place, rather than to a temporary file.  This is
    dangerous in the case of interruption, and it also means that the
    delta can't refer to blocks that have already been overwritten.
    On the other hand we could semi-trivially do this at first by
    simply generating a delta with no copy instructions.

  - Replicate block devices.  Most of the difficulties here are to do
    with replication in place, though on some systems we will also
    have to do I/O on block boundaries.


In favour of evolving the protocol:

  - Keeping compatibility with existing rsync servers will help with
    adoption and testing.

  - We should at the very least be able to fall back to the new
    protocol.

  - Error handling is not so good.


In favour of using a new protocol:

  - Maintaining compatibility might soak up development time that
    would better go into improving a new protocol.

  - If we start from scratch, it can be documented as we go, and we
    can avoid design decisions that make the protocol complex or
    implementation-bound. 


Error handling:

  - Errors should come back reliably, and be clearly associated with
    the particular file that caused the problem.

  - Some errors ought to cause the whole transfer to abort; some are
    just warnings.  If any errors have occurred, then rsync ought to
    return an error.


Concurrency:

  - We want to keep the CPU, filesystem, and network as full as
    possible as much of the time as possible.

  - We can do nonblocking network IO, but not so for disk.

  - It makes sense to on the destination be generating signatures and
    applying patches at the same time. 

  - Can structure this with nonblocking, threads, separate processes,
    etc.


Uses:

  - Mirroring software distributions:

  - Synchronizing laptop and desktop

  - NFS filesystem migration/replication.  See
    http://www.ietf.org/proceedings/00jul/00july-133.htm#P24510_1276764

  - Sync with PDA
 
  - Network backup systems

  - CVS filemover


Conflict resolution:

  - Requires application-specific knowledge.  We want to provide
    policy, rather than mechanism.

  - Possibly allowing two-way migration across a single connection
    would be useful.


Moved files:

  - There's no trivial way to detect renamed files, especially if they
    move between directories.

  - If we had a picture of the remote directory from last time on
    either machine, then the inode numbers might give us a hint about
    files which may have been renamed.

  - Files that are renamed and not modified can be detected by
    examining the directory listing, looking for files with the same
    size/date as the origin.


Filesystem migration:

  The NFSv4 working group wants atomic migration.  Most of the
  responsibility for this lies on the NFS server or OS.  

  If migrating a whole tree, then we could do a nearly-atomic rename
  at the end.  This ties in to having separate basis and destination
  files.

  NFSv4 probably wants to migrate file locks, but that's not really
  our problem.


Scalability:
 
  We should aim to work well on machines in use in a year or two.
  That probably means transfers of many millions of files in one
  batch, and gigabytes or terabytes of data.

  For argument's sake: at the low end, we want to sync ten files for a
  total of 10kb across a 1kB/s link.  At the high end, we want to sync
  1e9 files for 1TB of data across a 1GB/s link.

  On the whole CPU usage is not normally a limiting factor, if only
  because running over SSH burns a lot of cycles on encryption.


Streaming:

  A big attraction of rsync is that there are few round-trip delays:
  basically only one to get started, and then everything is
  pipelined.  This is a problem with FTP, and NFS (at least up to
  v3).  NFSv4 can pipeline operations, but building on that is
  probably a bit complicated.