Keystone Charm Goal State Support¶
Charms install and configure a piece of software on a unit in a model. How the software should be set up is determined by a set of information sources.
Unit operating environment
Application level configuration options
Set in the model and available to the charm at a early stage
Subordinate relations to charms installed on the same unit
Peer/Cluster relation to other units of same application in same model
Relations to units of other applications in the same model
Cross-model relations
In its simplest form a charm’s sole goal is to as quickly as possible install a piece of software, write its configuration and fire it up enabling the end user to make use of the software.
Problem Description¶
As the deployment progresses the charm learns more about its broader environment in pace with its relation-joined and relation-changed hooks being fired. Some of these relations may fundamentally change how the software should be set up or behave. Which dependencies and requirements a piece of software needs met before it can be brought to a operational state also relies heavily on which relations the charm has.
At present you may find that by the time a deployment is complete a charm has reconfigured and restarted it’s services several times, each time it may also have notified it’s peers and relations to do the same. These activities prolong the deployment time, and causes race conditions and transient errors on depending services.
Proposed Change¶
To address some of these issues Juju has introduced a goal-state command to the hook environment. This can be used by a charm to get a upfront view of what Juju is going to deploy.
Scope¶
Introduce goal-state support in Keystone charm
When available, make use of the early view of deployment to streamline charm operation
Limitations¶
The feature has dependency on support from Juju. At the time of this writing this feature is only supported by Juju 2.4.
When charm is deployed with versions of Juju without goal state support it falls back to inferring information about the deployment from configuration options and as relations appear.
Goal state gives us a peek into a future, the reality of that future can change beneath us if modifications are applied to the model before deployment completes.
In the event of goal state changing mid-deployment, the benefits of the goal state optimizations may be minimized or in worst case make deploy time longer.
Alternatives¶
Some of these issues can be alleviated by introducing configuration options that give the charm a hint of expected scale. But this approach is static and contradicts the dynamic use and ease of scale out properties of Juju.
Implementation¶
Assignee(s)¶
- Primary assignee:
fnordahl
Gerrit Topic¶
Use Gerrit topic “goal-state” for all patches related to this spec.
git-review -t goal-state
Work Items¶
Write test charm and associated test specs to exercise and monitor how identity-* relations are exercised by the keystone charm in complex deployment scenarios.
Delay initial run of identity-admin, identity-service and identity-credentials relations until:
all keystone units have joined and completed the charms peer/cluster relation
shared-db relation is ready and settled
If HA, make sure it is really up and ready
Limit the number of times identity-admin, identity-service and identity-credentials relations are fired after initial run
Limit the number of times credentials are added/updated to the database
Repositories¶
No new repositories required.
Documentation¶
This is mainly a under the hood change and documentation will be added as part of the code in docstrings.
Security¶
No changes affecting security.
Testing¶
Unit tests will be written alongside any code changes.
Functional tests will be expanded or written as necessary.
A test charm and associated test specs will be written to exercise and monitor how identity-* relations are exercised by the keystone charm in complex deployment scenarios.
Comparison of deployment times prior to and after the proposed change will be documented as part of the review process.
Dependencies¶
Juju 2.4 is required for goal-state support, charm will fall back to current behaviour when this is not available.
