svcDefaults.md

Managing OpenStack Service Version Incompatibilities

OpenStack services can exhibit incompatibilities between versions that require careful handling during updates to prevent services from entering a CrashLoopBackOff state. An example is Glance, which changed its default deployment model from httpd+ProxyPass to httpd+WSGI between feature releases. In brownfield environments, this creates a challenge: transitioning Glance from one deployment model to another without breaking the existing ControlPlane's ability to reach a Ready state.

The solution addresses two critical aspects:

1. Backward Compatibility in Service Operators: when breaking changes are introduced, service operators must maintain the ability to orchestrate both legacy and current deployment models simultaneously.

2. Informed Decision Making: the openstack-operator must provide a mechanism for building custom resources (CRs) with sufficient information to enable services to make informed deployment decisions.

Implementation Pattern

The solution that addresses this problem leverages an annotation-based pattern centered on the OpenStackVersion Kubernetes CR. This CR triggers updates and provides the necessary data for version transitions. A new ServiceDefaults field has been added to the OpenStackVersion Status:

// ServiceDefaults - struct that contains defaults for OSP services that can
// change over time but are associated with a specific OpenStack release version
type ServiceDefaults struct {
	GlanceWsgi *string `json:"glanceWsgi,omitempty"`
}

ServiceDefaults values vary across discovered releases. Each value in .Status.AvailableVersion can have its own associated defaults. Rather than relying on specific version numbers, this mechanism allows arbitrary values to be associated with particular OpenStack releases, providing flexibility in version management. During CR reconciliation, the service defaults are used to annotate the resulting service. Service operators then process these annotations to make deployment decisions. Clearly, the service operator serves as the final component in this chain, interpreting annotations and implementing the appropriate deployment strategy based on their semantic. This approach ensures smooth transitions between incompatible service versions while maintaining system stability and operator control over deployment models.

Example

The diagram below shows how Glance has two different annotation values defined in the serviceDefaults OpenStackVersion Status field. Based on the values, which is translated into a Glance top-level CR annotation, glance-operator orchstrates the deployment of the underlying GlanceAPI resources accordingly.

+---------------------------------------+      +---------------------------------------------------+       +-------------------------------------------------+
|      +----------------------+         |      |        +----------------------+                   |       |                                                 |
|      |  openstack-version  | -------------------------| openstack-controller |                   |       |    +-------------------+                        |
|      +----------------------+         |      |        +----------------------+                   |       |    |   glance-operator |                        |
|        |                              |      |            |                                      |       |    +-------------------+                        |
|        |                              |      |            | (annotate Glance CR)                 |       |      |                                          |
|        |-> (FR2) [GlanceWSGI: false]-------------------------> [glance.openstack.org/wsgi: false]------->|      +-> if (glance.openstack.org/wsgi) == true |
|        |                              |      |            |                                      |       |          then                                   |
|        |                              |      |            |                                      |       |             deploy: httpd+wsgi                  |
|        |                              |      |            |                                      |       |          else                                   |
|        |                              |      |            | (annotate Glance CR)                 |       |             deploy: httpd+proxypass             |
|        |-> (FR3) [GlanceWSGI: true]--------------------------> [glance.openstack.org/wsgi: true]-------->|                                                 |
|                                       |      |            | (minor updates)                      |       |                                                 |
|                                       |      |                                                   |       +-------------------------------------------------+
+---------------------------------------+      +---------------------------------------------------+

Flow Explanation

1. ServiceDefaults: The OpenstackVersion object sets for different OpenStack releases (FR2, FR3) the corresponding GlanceWSGI values in serviceDefaults

2. Annotation Propagation: The openstack-controller reads these defaults and applies them as annotations to the Glance CR:

   • FR2: glance.openstack.org/wsgi: false
   • FR3: glance.openstack.org/wsgi: true

3. Deployment Decision: The glance-operator reads the annotation and selects the appropriate deployment model:

   • wsgi: true: Deploy GlanceAPI with httpd+WSGI
   • wsgi: false: Deploy GlanceAPI with httpd+ProxyPass

This pattern enables seamless transitions between deployment models while maintaining backward compatibility during updates.