DHX: A Customized Hook Debugging Environment Plugin

DHX is a plugin -- scripts that extend the functionality of Juju -- which allows you to fully and automatically customize the machines created by Juju, making developing and debugging hooks as painless as possible. It is drop in replacement for juju debug-hooks, recommended for more advanced users who need to debug hooks repetitively.

Bugs, feature requests, and pull requests can be submitted against the Juju Plugins project.


The machines created by Juju are completely standard, based on the Ubuntu Cloud Image. While this consistency is great for running charms and while developing and debugging those charms, many developers prefer to tweak things such as their editor or bash configurations, or install additional tools and debugging libraries. Doing this each time on a new machine spun up by Juju is repetitive and tedious.

DHX allows you to customize the environment and tools available when you start a debug session on a new machine. Additionally, it has options to aid debugging, such as automatically syncing changes made to the charm on a remote machine or restarting the failed hook and automatically dropping you into a debug session for that hook.

Here is an overview of the features:

  • Option to use default tmux key bindings instead of screen bindings
  • Upload files, such as custom bash, vim, etc. configs
  • Execute custom init scripts upon first connect to a new environment
  • Improved selection of unit to debug
  • Automatic restart and debug a failed hook
  • Automatic sync of changes made to charm during debug session
  • Support for easy paired debug sessions

Installation and Setup

First, follow the instructions in the README to install the plugin bundle. This should be straightforward and will give you access to all of the Juju plugins, DHX included.

Next, you'll want to create a configuration file to define what customizations to perform. For example:

cat > ~/.juju/debug-hooks-rc.yaml <EOF
use_tmux_keybindings: true
- '~/.vimrc'
- '~/.vim'
init: '~/.juju/my-dhx-init.sh'
- '.*'
- '*.pyc'
import_ids: []
auto_sync: false
auto_restart: false

This configuration will use the standard keybindings (Ctrl-b) for tmux instead of the screen keybindings (Ctrl-a) that debug-hooks normally uses. It will also upload your VIM configuration and execute my-dhx-init.sh on the remote machine upon the first connect to perform any additional customizations (e.g., installing ipdb for improved debugging of charms written in python).

Running DHX

DHX is a drop-in replacement for juju debug-hooks. So, whenever you would use juju debug-hooks to start a debugging session, you should instead use juju dhx (or juju debug-hooks-ext, if you want to be verbose). This will automatically detect if the environment you're connecting to has been customized and, if not, apply your customizations.

Improved Unit Selection

Instead of typing out the full unit name, in the form service/0, you can let DHX figure out which unit you want to debug. It will use cues such as which units are in error state, the charm in your current directory, or service name.

If DHX can't unambiguously choose a unit, it will present you with its best guess along with a list of units that you can choose from by number or name, so that just pressing Enter to accept the default will usually do the right thing.

For example, in a case where you had three units running, two of which were currently in an error state. Running:

juju status --format=oneline

might return:

- chamilo/0: (error)
- mysql/0: (started)
- mysql/1: (error)

To start a debug session, you could enter:

juju dhx

The plugin will then list the known units and prompt for your choice, defaulting to the first unit found to be in the error state:

0: chamilo/0 (error)
1: mysql/0
2: mysql/1 (error)

Select a unit by number or name: [chamilo/0]

You can also specify a service, like so:

juju dhx mysql

in this case only units running the given service are listed, and again, the default choice is the first one found in the error state:

0: mysql/0
1: mysql/1 (error)

Select a unit by number or name: [mysql/1]

Retrying Failed Hooks

The most common reason why you need to start a debug-hooks session is because a hook failed and you want to figure out why. Once you are in the debug-hooks session, you want to restart the failed hook and start debugging it.

Instead of manually running juju resolved $unit, you can just add the --retry (or just -r) option to DHX:

juju dhx -r

You will then be connected to the unit, it will be customized if necessary, and the hook will be immediately restarted and paused for you to debug.

You can also set auto_retry: true in your config file to always assume the --retry to be given.

Syncing Changes from the Remote Unit

When debugging hooks, you will almost always make changes to the charm to figure out what went wrong and resolve the problem. However, it is easy to forget to apply all of those changes to your local copy of the charm once you're done.

DHX integrates with the sync-charm plugin, which downloads changes made to the charm on the remote unit and applies them to your local codebase. Just invoke DHX with the --sync (or just -s) option, and any changes you make during your debugging session will be automatically pulled back down when you are done:

juju dhx -s

You can also set auto_sync: true in the config file to always assume the --sync option to be given.

When using this option, however, you may pick up files that are created by the charm while running which should not be pulled back down. Any file you add to the list of sync_excludes in your config will be skipped when performing the sync. The list also supports the use of wildcards.

Remote Paired Debugging

Paired programming can be useful for applying another set of eyes to a problem. DHX supports this by allowing you to import another developer's Launchpad ID, allowing them to join your session.

Let's say you want to have a paired session with Bob. You would start your session with:

juju dhx -i bob

For Bob to join your session, you will need to tell him the public address of the unit, which you can get from juju status. Then Bob can join your session using the --join (or just -j) option:

juju dhx -j $public_address

Bob will be connected and immediately brought into your tmux session.

Bob may also join the session without using DHX, using the following:

ssh -t ubuntu@$public_address 'sudo tmux attach'

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