XS backup restore

Goals

• Simple, efficient (minimise processing, traffic), quick dev turnaround, debuggable
• Sane, fail-safe, atomic-ish
• Independent of the actual storage strategy (DS-agnostic)
• And yet, it must work well with the current DS (as of April 2008), and avoid restricting the evolution of the DS
• Safe for XO and XS
• The server can refuse to backup due to traffic/load
• Simple version negotiation
• Supports full homedir restore
• Supports per-document restore (via journal and/or webbased)
  • There is some interest in leveraging a webbased 'document restore' facility as 'async document share/publish' mechanism.

Notes

• All timestamps are integers representing seconds elapsed since the UNIX epoch.
• There is a REST meta-protocol versioning scheme. Outside of that initialcheck, what this page describes is the version "1" of the backup/restore protocol.

XO-initiated backup

XO side

1. Issue a HTTP GET to XS with path 
  /backup/<protocol version>/available/<this_XO_serial_number>
  
  <protocol version> is the integer representing the latest
  backup protocol version supported by this XO. In protocol version 1,
  a successful reply consists of a single integer:
  
      timestamp -- timestamp of latest backed up item for this user                                    
                   or 0 if there are no previous backups

  If the sent protocol version is not supported by the school server,
  it will return a 404 not found error, whose only body contents is 
  a comma-separated list of integers representing the backup protocol
  versions supported by this school server.

  If this school server refuses to provide backup service for this XO,
  it will return a 403 forbidden error.

  If the school server is too busy to deal with the XO's backup request,
  it will return a 503 service unavailable error. The XO will sleep 5
  minutes and retry.


2. If the request in step 1 succeeded, go to step 3. Otherwise, 
  if none of the backup system versions on the XO (multiple may be
  present) are in the 'versions' variable listed in the 404 error, abort
  until next scheduled backup time (we cannot back up to this XS). If
  a version was returned that also exists locally, go back to step 1
  and use that protocol version.

3. Write out all the metadata for all the documents available for
  backup, in CanonicalJSON format. Save it as metadata.json
  overwriting (atomically) any previously existing version.

4. Run rsync-over-ssh between the datastore and a remote directory
  called datastore-current/ in the user's home directory on
  the XS.

  The remote datastore-current directory will have a complete set of files
  so use the rsync facilities available to optimise the transfer and 
  delete stale files:

   --times
   --partial (to make retries faster)
   --delete
 
  Check the exit value from rsync. If non-zero, retry up to 3 times.
  If still non-zero, abort until next backup.

5. Store the epoch of the end time of step 4.

Note: This backup scheme is not atomic. Users of the backed-up data must be prepared for slightly inconsistent state between metadata and files - a large window exists between steps 3 and 5. Solutions to this could come from the FS (a ZFS-like implementation) or from a higher-level layer (a git-based DS for example).

XS side

On the school server, when getting a request for /backup/<protocol version>/available/<SN>:

1. Check if we support the protocol version. If not, return 404 and a list
  of supported versions. Otherwise, proceed.

2. Check if we know this machine (can find it in our registration DB on
  the XS). If not, return 403. We will not offer it backup service.
  Check if we're too busy to process another concurrent backup (e.g.
  based on transfer rate or number of rsync processes), if so, return
  503.
 
3. Check if backups for this machine exist. In protocol version 1, if
  backups don't exist, let timestamp be 0. Otherwise, find the
  timestamp of the last backed-up object for this machine and return
  it.

4. Check system and network load metrics - can we offer service to this
  client?


5. Return timestamp in the body of a 200 OK
  response.

When the rsync-over-ssh connection comes in, we need to have an rsync wrapper script that will

1. Establish a lock using flock to prevent overlaps

2. Cleanup/sanitise parameter list to rsync

3. Upon successful completion, set a success flag

XS maintenance

• A regular cronjob checks for recent success flags. Home directories that are marked as successfully backed up will be 'shadowed' with a hardcopy script similar to pdumpfs.
  • It might be a good idea to spot partial/failed backups and checkpoint/shadow them anyway. If our handling of inconsistent data is reasonably good, a partial backup might be a passable data source for per-document restores.
• A low-freq cronjob runs hardlink.py
• A cronjob removes old pdumpfs snapshots

XO-initiated full restore

XO side

1. Issue a HTTP GET to the XS with path
  /backup/<protocol version>/restore/<this_XO_serial_number>

  The response is 0 or a single absolute path on the XS, pointing to
  the location of this XO's backup files in the backup hierarchy. If
  the response is 0, abort and report to user; there are no backups
  to restore. Otherwise store the path variable for future use.

  If the request returns a 500, abort and report to user that they
  must pick out restore files individually from the web interface.

  If the request returns a 503, wait 1 minute, then retry step 1,
  otherwise proceed.

2. Let variable index_path be the concatenation of the path variable
  from step 1 and the string restore.idx. Rsync the file whose path
  is index_path from the XS to the XO.  This file is a set of lines
  formatted like the contents of a backup.idx file -- produced
  exactly like in step 4 of the XO-side backup.  In other words, the
  first line repeats the protocol version, and every next line
  describes a single DS object. If the rsync fails, retry 3 times;
  if still failing, abort restore and report to user.

3. For every item in this list, parse out any paths to files,
  and write each one (e.g. one for the binary object, one for the
  thumbnail) to a local file called restore-files.idx, one per line.

  Note that the paths contained in 'restore.idx' that we
  received in step 2 are absolute paths on the schoolserver,
  e.g. /backups/<SN>/<filename>, and those paths MUST be preserved
  when writing to restore-files.idx.

4. Run a rsync on the XO, going from the schoolserver to the XO, and
  pass it restore-files.idx as the list of files to rsync.

5. Check the rsync exit value. If non-zero, retry 3 times. If still
  non-zero, abort and report failure to the user.

6. Go back through the list received in step 2 line by line.  For
  every file path in the current line (there might be several for
  e.g. binary object, thumbnail, etc), strip everything except the
  filename -- remove the directory components. Verify that the files
  exist locally on the XO.

  If they don't exist, rsync didn't get all the files back, but there
  should have been some (because we didn't get 0 for timestamp in
  step 1) AND rsync thinks it succeeded (because of step 5). Abort
  restore and report to user that something is wrong.

  If the files exist, issue a request to the DS to create the object
  based on the metadata in the line, and pass in stripped file paths
  for the contents/thumbnail.

  (Note: if the DS does not support setting creation timestamps or
  thumbnails through the present API, another function might have to
  be added specifically for the restore system to use, where such
  functions are allowed.)

7. If the last line in the list returned in step 1 is processed and
  stored in the DS, we have succeeded with the restore. Inform
  user. Eat some ice cream. Do the macarena.

XS side

On the school server, when getting a request for /backup/<protocol version>/restore/<SN>:

1. Check if we support the protocol version. If not, return 404 and a list
  of supported versions. Otherwise, proceed.

2. Check if backups for this machine exist. If not, return 200 OK whose
  only body contents is 0. Otherwise, proceed.

3. Check if a file called restore.idx exists in the backup hierarchy
  for the XO. If so, return absolute path to this XO's files in the
  server backup hierarchy (e.g. /backups/<SN>/) as sole body of a 200
  OK response. If it doesn't exist, proceed.
   
4. Check if a file called restore-state.idx in the backup hierarchy
  for the XO exists. If not, return error 500. For some reason we don't
  have a state file for this machine; this shouldn't happen, but it means
  the user has to pick out objects to restore individually from the
  web interface.
   
5. Return 503 service unavailable, and in the background, spawn
  a restore process that does the following:

    5.1. Check if a file called restore-state.idx.processing in the
         backup hierarchy for this XO exists. If not, proceed to next
         step. If it exists, and its timestamp is older than 10
         minutes, we tried to prepare a restore list for this machine
         already and somehow failed (e.g. database timeouts,
         etc). Check if restore-state.idx.processing.pid exists and
         is owned by us; if so, load its contents and send SIGKILL
         to the PID, then move to step 5.3. If the timestamp is
         younger than 10 minutes, exit.
     
    5.2. Move restore-state.idx in the XO's backup hierarchy to
         restore-state.processing.idx

    5.3. Write our own PID to restore-state.processing.idx.pid.

    5.4. To a temporary file, write a line containing the backup
         protocol version.

    5.5. For each line in restore-state.processing.idx
         (representing a UUID), query all the relevant metadata from
         the XS store and write it, one JSON dictionary-encoded line
         per object, to the temporary file. Paths of any referenced
         files (binary objects, thumbnails) must be absolute paths on
         the XS. If any queries fail, retry with a timeout, and if
         failure continues, exit the updater.

    5.6. When finished, move temporary file to restore.idx in the
         backup hierarchy for this XO. Unlink
         restore-state.processing.idx.