okofw

openstack-k8s-operators-framework

The goal of okofw is to implement a framework of a generic reconcile engine that helps implementing openstack service operators without duplicating logic like:

• loading and saving the state of the reconciled instance
• initializing status conditions and automatically calculating the overall Ready condition
• ...

Concepts

Top of the concepts introduced by the controller-runtime the framework introduces additional building blocks

Reconcile request (Req)

A type encapsulating the data related to a single request to Reconcile a CR instance. A new request is created for each Reconcile() call and individual reconcile Steps can use it to store data and pass them to other Steps in the same Reconcile run.

Reconcile request handler (Handler)

The engine that executes the reconcile Steps according to the request, handle step results, loads and saves the CR instance, etc.

Step

A single piece of work to be done to reconcile a CR instance run by the Reconcile request handler. It has access to the CR instance state, and the Reconcile request and it can manipulate both.

There are different phases of a step execution implemented by different functions:

• Do(): Normal reconciliation
• Cleanup(): implement any cleanup action needed during CR deletion
• Post(): implement tasks that always needs to be run right before the CR is persisted even if a previous step failed.

Reconcile flow

        ┌───────────┐
        │Reconcile()│
        └─────┬─────┘
          ┌───▽───┐
          │Load CR│
          └───┬───┘
  ____________▽_____________
 ╱                          ╲    ┌─────────────────────┐
╱ DeletionTimestamp.isZero() ╲___│Ensure self finalizer│
╲                            ╱yes└──────────┬──────────┘
 ╲__________________________╱     ┌─────────▽─────────┐
              │no                 │For each Step: Do()│
 ┌────────────▽───────────┐       └─────────┬─────────┘
 │For each Step in reverse│                 │
 │order: Cleanup()        │                 │
 └────────────┬───────────┘                 │
   ┌──────────▽──────────┐                  │
   │Remove self finalizer│                  │
   └──────────┬──────────┘                  │
              └───────┬─────────────────────┘
           ┌──────────▽──────────┐
           │For each step: Post()│
           └──────────┬──────────┘
                  ┌───▽───┐
                  │Save CR│
                  └───────┘

For each Reconcile() call a new Req and Handler is created based on the request from the controller-runtime (i.e which CR to reconcile) and based on the programmer defined Steps (i.e. how to reconcile).

Implementation

To keep the engine and some common steps (i.e. condition handling) generic the Req is a generic type where the type parameter T represents the type of the CR instance (i.e Req[T] means a reconcile request for the T CR type). The Handler takes an Req instance so it is also generic with type parameter T and Req[T].

This allows to create both generic and specific Steps. For example Conditions is a generic step where the T CR type is only restricted to support GetConditions and SetCondition calls. An example for a type specific step is EnsureNonZeroDivisor from the simple_controller where T is replaced with the specific CR type v1beta1.Simple. So this step can directly access all the CR specific Spec and Status fields.

The implementation strategy is to use interfaces to specify the need of the generic steps towards the CR type. This way the generic step will be reusable for multiple CRs. But also allow implementing steps that are only useful for a single CR type without the need to write boilerplate interfaces (or do type casting) to access all the CR specific fields.

Available generic steps

• Conditions: This step ensure that the every condition is initialized and the Ready condition is always recalculated before the CR is saved. It requires that the CRD type implements the InstanceWithConditions interface. If another Step wants to manage a condition then that Step needs to implement the ConditionManager interface to declare which conditions it manipulates. Then the Conditions step will know what conditions needs to be initialized. Note that Conditions step should be added to the Handler before any other steps manipulating conditions.

Examples

• v1beta1.Simple + simple_controller: Shows the basic Reconcile setup without any external dependencies but with Condition handling. The reconciler only reads its Spec and writes its Status.
• v1beta1.RWExternal + rwexternal_controller: The reconciler reads an external input (a Secret) and create external output (another Secret) with additional cleanup logic for the output during CR deletion.