It can be difficult to understand how to see how Juju has made its decisions. There are several commands that affect Juju’s behaviour, each with a different scope. Each command controls dozens of settings. This post aims to simplify navigating this.
Juju has lots of knobs to adjust. Almost every option has a bug report (or several) attached to them. Juju is a system that has implemented functionality in response to user requests.
Terminology
Constraints affect which machine is provisioned. They’re constraints on the shape of the machine that are provisioned. The smallest instance type that satisfies the constraints is chosen.
Configuration affects everything else. Configuration options affect Juju’s behaviour, outside of selecting machines to provision.
Additionally, you may also see series options as command-line arguments for a few commands. They affect the operating system that’s deployed to the newly provisioned machine and behave as a configuration option.
Precedence Rules
Highest position wins.
Constraints
- command-line options (
--constraints) - application constraints
- model constraints
Configuration
- command-line options (
--config) - application config
- model config, which is initially set by
- model defaults
Types of Settings
Bootstrap Constraints
juju bootstrap --bootstrap-constraints --bootstrap-series
The bootstrap process deploys the Juju controller onto a newly provisioned machine on a cloud. Bootstrap constraints only affect the machine that the Juju controller is deployed to.
Application Constraints
get-constraints set-constraints
Users can define constraints to apply whenever a unit of an application is deployed.
Application constraints are combined with model constraints to determine which instance is provisioned.
Where model and application constraints overlap, the application constraints take precedence.
Model Constraints
get-model-constraints set-model-constraints
Model constraints affect all machines that are created within the model, irrespective which charm is being deployed or application being added to.
They work with application constraints to determine which instance type to provision.
Model Defaults
juju model-defaults
Model defaults specify the model config settings that will be used for any future models.
Model Config
juju model-config
Model-level configuration settings affect anything under the model’s control.
Application Config
juju model-config
Application-level configuration settings affect any new models within the application.