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Assuming “platform API” means the Kubernetes API, application updates are reconciled by repeatedly comparing the desired state in a resource’s spec with the state currently observed in the cluster, then making safe changes to close the gap. A controller should respond to relevant resource events, tolerate concurrent updates, and make retries safe. The title does not identify a specific platform, so this guide focuses on Kubernetes controllers and operators rather than a proprietary API.
What reconciliation means in Kubernetes
A reconciler takes a resource’s desired configuration and works to make actual system state match it. In an operator, the custom resource commonly holds the application’s desired settings in spec; the controller reads that resource and manages the related Kubernetes resources. The Operator SDK reconciler documentation describes this desired-versus-actual-state role and explains that reconciliation runs as events occur on watched resources.
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This is a loop, not a one-time update command. A user can change the custom resource, a managed object can be altered or removed, or an earlier operation can fail. The controller observes a change, evaluates what should be true, and takes the next safe action. The goal is convergence: after repeated reconciliations, managed resources reflect the declared intent.
How to build an application update reconciler
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Put intent in the primary resource
Define the application’s desired settings in the custom resource’s
spec. Keep the reconciler responsible for translating that intent into changes to the resources it manages, rather than relying on an operator user to update each dependent object by hand.#1 Best Overall
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Watch resources that can affect the result
Watch the primary custom resource and relevant dependent resources. Events on watched resources can enqueue reconciliation, allowing the controller to respond both to requested application changes and to changes in objects it manages. The Operator SDK tutorial documents reconciliation in response to watched-resource events: Operator SDK Go tutorial.
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Read current state before deciding what to change
Compare the observed object with the intended values before writing. If it is already correct, avoid an unnecessary update. This follows from the Operator SDK’s recommendation to make reconciliation idempotent: processing the same state again should not create harmful repeated effects.
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Choose an update method that matches the change
Use a full update when replacing the representation is appropriate and the client can handle version conflicts. Use a patch when the intended change is partial or needs a condition. The choice depends on which fields the operation owns and whether it must detect concurrent changes.
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Handle conflicts by rereading and reevaluating
If another client updates an object between your read and write, Kubernetes can reject a stale update. Fetch the latest object, compare it with the desired state again, and retry the resulting change rather than blindly resubmitting an old representation.
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Make retries safe and expose outcomes
A reconcile can run again after an error or a new event. Avoid repeating external side effects simply because the controller retried; design actions so repeated processing can safely continue toward the desired state. Report progress and failures through the controller’s status conventions, and monitor repeated failures. Kubernetes operator projects do not share one universal status schema.
PUT and PATCH: choosing the right update
| Approach | What it changes | Concurrency considerations | Main risk to consider |
|---|---|---|---|
| PUT update | Replaces the current resource representation. | The client must provide the resourceVersion it read. If it is stale, the API server can return HTTP 409 Conflict; the client must handle the conflict. |
A client that decodes and rewrites a resource may drop fields it does not know about. |
| PATCH | Describes a partial change to a resource. | Patch formats can support conditions that test consistency; choose a strategy suited to whether the change depends on existing values and whether lost-update detection is required. | A partial update still needs an explicit concurrency strategy when correctness depends on the existing state. |
These behaviors are documented in the Kubernetes API concepts. Whichever method you choose, a conflict is a signal that the state changed since it was read—not a reason to overwrite newer state without reconsidering the desired change.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why idempotence matters during updates
Controllers may process the same resource repeatedly. The Operator SDK recommends idempotent reconciliation so repeated processing continues moving resources toward the desired state instead of becoming stuck or causing harmful repeated effects. In practice, compare before writing and distinguish setting a resource to a value from performing a one-time action such as sending a notification or charging for an operation. Any non-repeatable external action needs its own safeguards; the right mechanism depends on the application.
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Quick Recap
Common update failures and how to respond
- HTTP 409 Conflict: The update was based on a stale
resourceVersion. Read the current object, recompute the intended change, and retry with current state. - Unrelated fields disappear after an update: A full read-modify-write may have omitted fields unknown to the client. Prefer a suitable partial patch when appropriate, or ensure the full representation preserves fields the client does not own.
- The controller keeps issuing writes: Check whether reconciliation recognizes that the resource already matches the desired state. Avoid writing unchanged values, and verify the comparison reflects fields managed by the controller.
- Retries repeat an external effect: Separate convergence work from one-time side effects and make the latter safe to retry. Reconciliation itself should remain repeatable.
- Updates are not noticed: Verify that the primary resource and relevant dependent resources are watched, and that their events lead to reconciliation.
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