Upgrading applications
An application is upgraded with the upgrade-charm command, where the upgrade
candidate is known as the revision. The default channel is naturally the
channel of the currently deployed charm.
Note: It is possible, though unlikely, that the revision is older than that of the deployed application. In this case, the application will actually be downgraded (to the revision).
The notion of a channel is related to revision numbers. See Deploying applications for an overview of channels.
The Charm developer guide provides low-level details on the mechanics of charm upgrades.
You can query for charm information, such as channels and revisions, with the
charm utility, which is available via the Charm Tools software. See the
Charm Tools page for guidance.
Upgrade mechanics
Here is a summary of how charm upgrades work.
The agent (running on each unit of the application being upgraded) unpacks the new version of the charm to a staging directory next to the original charm. It then points itself to the newly unpacked charm and deletes the old one, after which the expected hooks are fired using the newly unpacked charm code.
The original charm continues to run until the new charm is successfully
downloaded and unpacked. Only then does the original one stop running and the
new one begin to run (by executing the install and upgrade-charm hooks).
To be clear, the logic of the upgrade itself is contained within the charm. Juju simply unpacks the new charm and fires the hooks.
Crossgrading an application
Crossgrading an application refers to upgrading an application by switching out
the current charm and replacing it with a new local charm. This is accomplished
by way of the --switch option. This differs from upgrading an existing charm
locally (with the --path option) due to the charm being considered entirely
new.
This is considered a dangerous operation since Juju has only limited information with which to determine compatibility. The operation will succeed so long as the following conditions are met:
- The new charm must support all relations that the application is currently participating in.
- Each configuration setting shared by the original and new charm must be of the same type.
The new charm may add new relations and configuration settings.
Forced upgrades
A charm upgrade may require the use of the --force-series option.
Consider the case where an application is initially deployed using a charm that supports Precise and Trusty. If a new version of the charm is released that only supports Trusty and Xenial then applications already deployed on Precise can only be force-upgraded.
Examples
To upgrade WordPress based on the current channel:
juju upgrade-charm wordpress
To upgrade MySQL based on the 'edge' channel:
juju upgrade-charm mysql --channel edge
To upgrade Git to a specific revision number:
juju upgrade-charm git --revision 2
To upgrade Apache using a local updated charm:
juju upgrade-charm apache2 --path ~/charms/apache2
Note: The above implies the application was originally deployed locally. The path must be the same in both cases (deployed and upgraded).
To crossgrade MySQL with a local charm:
juju upgrade-charm --switch ~/charms/new-mysql mysql
To force an upgrade of PostgreSQL that would otherwise fail:
juju upgrade-charm postgresql --force-series