The fundamental purpose of Juju is to deploy and manage software applications in a way that is fast and easy. All this is done with the help of charms, which are bits of code that contain all the necessary intelligence to do these things. Charms can exist online (in the Charm Store) or on your local filesystem (previously downloaded from the store or written locally).
Charms use the concept of series analogous as to how Juju does with Ubuntu series ('Trusty', 'Xenial', etc). For the most part, this is transparent as Juju will use the most relevant charm to ensure things "just work". The default series can be configured at a model level, see Configuring models for further details. In the absence of this setting, the default is to use the series specified by the charm.
Typically, applications are deployed using the online charms. This ensures that you get the latest version of the charm. Deploying in this way is straightforward:
juju deploy mysql
This will create a machine in your chosen backing cloud within which the MySQL application will be deployed.
Assuming that the Xenial series charm exists and was used above, an equivalent command is:
juju deploy cs:xenial/mysql
Where 'cs' denotes the Charm Store.
Note: A used charm gets cached on the controller's database to minimize network traffic for subsequent uses.
The charm store offers charms in different stages of development. Such stages are called channels. Some users may want the very latest features, or be part of a beta test; others may want to only install the most reliable software. The channels are:
- stable: (default) This is the latest, tested, working stable version of the charm.
- candidate: A release candidate. There is high confidence this will work fine, but there may be minor bugs.
- beta: A beta testing milestone release.
- edge: The very latest version - expect bugs!
As each new version of a charm is automatically versioned, these channels serve as pointers to a specific version number. It may be that after time a beta version becomes a candidate, or a candidate becomes the new stable version.
The default channel is 'stable', but you can specify a different channel easily. Here, we choose the 'beta' channel:
juju deploy mysql --channel beta
In the case of there being no version of the charm specified for that channel, Juju will fall back to the next 'most stable'; e.g. if you were to specify the 'beta' channel, but no charm version is set for that channel, Juju will try to deploy from the 'candidate' channel instead, and so on. This means that whenever you specify a channel, you will always end up with something that best approximates your choice if it is not available.
See Upgrading applications for how charm upgrades work.
Some charms support more than one series. It is also possible to force a charm to deploy to a different series. See the documentation on Multi-series charms to learn more.
It is possible to deploy applications using local charms. See Deploying charms offline for further guidance.
Deployed applications usually start with a sane default configuration. However, for some applications it may be desirable (and quicker) to configure them at deployment time. This can be done whether a charm is deployed from the Charm Store or from a local charm. See Application configuration for more on this.
The Juju operator can specify which machine (or container) an application is to be deployed to. See Deploying to specific machines for full coverage of this topic.
Using network spaces the operator is able to create a more restricted network topology for applications at deployment time. See Deploying to network spaces for more information.
A common enterprise requirement, once applications have been running for a while, is the ability to scale out (and scale back) one's infrastructure. Fortunately, this is one of Juju's strengths. The Scaling applications page offers in-depth guidance on the matter.