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Tablesnap

Theory of Operation

Tablesnap is a script that uses inotify to monitor a directory for IN_MOVED_TO events and reacts to them by spawning a new thread to upload that file to Amazon S3, along with a JSON-formatted list of what other files were in the directory at the time of the copy.

When running a Cassandra cluster, this behavior can be quite useful as it allows for automated point-in-time backups of SSTables. Theoretically, tablesnap should work for any application where files are written to some temporary location, then moved into their final location once the data is written to disk. Tablesnap also makes the assumption that files are immutable once written.

Installation

This distribution provides a debian/ source directory, allowing it to be built as a standard Debian/Ubuntu package and stored in a repository. The Debian package includes an init script that can run and daemonize tablesnap for you. Tablesnap does not daemonize itself. This is best left to tools like init, supervisord, daemontools, etc.

There are pre-build binaries for Ubuntu Maverick amd64 and i386 in this PPA: https://launchpad.net/~synack/+archive/tablesnap

# cat /etc/apt/sources.list.d/tablesnap.list << EOF
> deb http://ppa.launchpad.net/synack/tablesnap/ubuntu maverick main
> deb-src http://ppa.launchpad.net/synack/tablesnap/ubuntu maverick main
> EOF
# aptitude update

If you are not a Debian/Ubuntu user or do not wish to install the tablesnap package, you may copy the tablesnap script anywhere you'd like and run it from there. Tablesnap depends on the pyinotify and boto Python packages. These are available via "pip install pyinotify; pip install boto;", or as packages from most common Linux distributions.

Configuration

All configuration for tablesnap happens on the command line. If you are using the Debian package, you'll set these options in the DAEMON_OPTS variable in /etc/default/tablesnap.

Usage: tablesnap [options] <bucket> <path> [...]
Options:
  -h, --help            show this help message and exit
  -k AWS_KEY, --aws-key=AWS_KEY
  -s AWS_SECRET, --aws-secret=AWS_SECRET
  -r, --recursive       Recursively watch the given path(s)s for new SSTables
  -a, --auto-add        Automatically start watching new subdirectories within path(s)
  -B, --backup          Backup existing SSTables to S3 if they're not already there

For example:

$ tablesnap -k AAAAAAAAAAAAAAAA -s BBBBBBBBBBBBBBBB me.synack.sstables /var/lib/cassandra/data/GiantKeyspace

This would cause tablesnap to use the given Amazon Web Services credentials to backup the SSTables for my GiantKeyspace to the S3 bucket named me.synack.sstables.

Questions, Comments, and Help

The fine folks in #cassandra-ops on irc.freenode.net are an excellent resource for getting tablesnap up and running, and also for solving more general Cassandra issues.