smokserwis/zookeeper-volume

A Docker volume as a ZooKeeper tree volume plugin

Installation

To install just type:

docker plugin install smokserwis/zookeeper-volume
docker plugin enable smokserwis/zookeeper-volume

Note that if you keep your Docker root in other place than
/var/lib/docker, best install from source, before which
change the path in config.json, or alternatively symlink this location to your target.

If you logs are stored somewhere else than /var/log, you best
do it too. Or symlink, it’s your choice.

Building from source

Just check this repo out on a normal UNIX platform (sorry, only UNIXes supported for the time being)
and invoke build.sh.
build.sh will automatically install and enable the plugin.

If you set the env DEBUG during build.sh‘s run,
debugging mode will be enabled on the plugin. You can later change it.

Debugging

To enable debug logs just type:

docker plugin set smokserwis/zookeeper-volume DEBUG=1

They will be dumped at /var/log/daemon.log or the usual place where your Docker drops it’s logs.
Note that setting the env DEBUG during a run of build.sh will
automatically set the variable to 1.
The variable is kept at default 0 in config.json.

Note that the output from zookeeper-fuse is in:

• non-debug cases redirected to /dev/null.
• in debug cases it will be redirected to
  /var/log/zookeeper-volume/volumename.stdout.txt and
  /var/log/zookeeper-volume/volumename.stderr.txt

Usage

To define a volume just type

docker volume create -d smokserwis/zookeeper-volume -ohosts=192.168.2.237:2181 -opath=/zk-child zookeeper

The thing that you provide in hosts option is a ZooKeeper connection string.
This is the lists of hosts with ports separated with a comma.
As so, it is mandatory to provide port number.

Creating volumes

Volume must have at least a single option:

• either host for an address and a port nameorhostsfor multiple of those, separated by a,`

Other options can be optionally given:

• path: zookeeper path to mount as root
• mode: zookeeper-fuse access mode
  (default is HYBRID), HYBRID is recommended for best impersonation of a filesystem
  due to how zookeeper’s filesystem behaves. Read the appropriate README
  to figure out how exactly HYBRID works. You can alternatively choose DIR or FILE.
• auth: zookeeper authentication string (by default none given)

Thanks and credits

Special thanks to borowskk’s zookeeper-fuse,
without which this plugin wouldn’t happen to exist.

HYBRID mode

Since zookeeper-fuse’s default access modes of DIR and FILE do not permit to use ZooKeeper systems as a valid filesystem (ie. once you create a file it becomes a directory) and does not support symlinks I have extended zookeeper-fuse to add symlink support and if you create a file it stays a file. Basically it remembers what you have done while creating that file. I’ve also filed a pull request.

Note however that cache invalidation is not yet supported, so if you process files with new names you are going to run into trouble. If you however keep on processing the same files, you should be OK.

What hybrid mode supports:

• files once created as files stay files. Directories created as directories start as directories. Note that this however applies to a single machine only, so if you create an empty file on one machine it’s going to be seen as a directory on another.
• Symlinks are supported. However, in HYBRID mode __symlinks__ is an invalid file name, since this is the file in which symlink information is stored at the root.
• cp is supported
• mv is supported for files only. Moving a directory is a ENOSYS.

Please note that the default mode in which
smokserwis/zookeeper-volume mounts volumes is
HYBRID not DIR or FILE.

In HYBRID mode syscalls lock and flock are mapped to no-ops. They
will create a file for all it’s worth.

It works much better if you ls -R the entire mounted directory prior to using files from there.

Keep in mind that the maximum file size is limited by
zookeeper-fuse
and is currently set to 128 kB.

Changelog

v1.8

• renaming (moving) empty directories is supported

v1.7

• dropped ujson requirements
• refactored the code
• in HYBRID mode the chmod of directories will be 777 instead of 755

v1.6

• the solution should segfault far less

v1.5

• added support for mv-ing files

v1.4

• fixed a bug related with writing symlink name to too short a buffer
• fixed a bug wherein a filesystem required as ls -R /mount/zoo to load all symlinks
• Maximum file size increased to 128 kB

v1.3

• fixed a bug wherein directories would
  be listed as files

v1.2

• support for lock, flock, release,
  releasedir and access added
• the filesystem is enough for certbot to put it’s certificates onto this

v1.1

• HYBRID mode is fully supported
• default moe in which a volume is mounted switched to HYBRID instead of DIR