Juju Compose

[Benjamin Saller]

Charm Composition is a small tool facilitating a new pattern for developing
and maintain Juju Charms. In its simplest form it lets you combine
directories, called Layers, of files to create a charm. Rather than just
copying files around however it keeps track of which files came from which
Layer so that at a later time the process can be run again. This means that
if any of the Layers have changed those bits will be reflected in the newly
regenerated charm.

Imaging that you have a charm you’ve forked to add some simple tweak or
customization. You might be able to maintain your changes in a new Layer
allowing the base you depend on to continue to evolve.

Suppose we have layer ‘a’.

trusty/a/

├── hooks/install

├── metadata.yaml

└── README.md

And a layer ‘b’

trusty/b/

├── hooks/db-relation-changed

├── composer.yaml

├── metadata.yaml

└── README.md

If ‘b’ included ‘a’ (specified in the composer.yaml file) then composer
might combine the layers in the following way.

trusty/b/

├── hooks/install

├── hooks/db-relation-changed

├── composer.yaml

├── metadata.yaml

└── README.md

That is just a ‘cp -r’ you’re saying, and it would be except the compose
process is able to support a variety of methods for combining layers. By
default compose knows about Charm specific files like metadata.yaml and
config.yaml it is able to combine them in sensible ways. For files like
this the default is a merge with the ability to remove add/replace keys
within the data of the file in the lower layer (in this case ‘a’ would be
overridden if it had a key in common with ‘b’ when composed).

Composer itself is extensible so if you’re layers need special rules
associated with how certain types of files should be combined there is an
interface for doing that.

Composer has code for doing much more interesting things than just
combining layers, but we can save that for another post. If you’re
interested in this now and want to start thinking about what nice base
layers might look like, check it out at
https://github.com/bcsaller/juju-compose



Thanks,
Ben

There is more that should be said about how this relates to the work with
Relation Stubs and so on but that can be another thread.
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