Reconciler

A reconciler is a software component responsible for bringing two representations of state into alignment. In declarative systems, the reconciler observes the current state of a system and a desired state expressed by a user or policy, then issues updates to converge toward the desired state. Reconciliation emphasizes idempotence and repeatability; running the reconciler multiple times should not cause unintended changes if the state is already aligned.

In Kubernetes, the reconciler is central to the controller pattern. A controller watches the cluster’s current state via the API server, compares it with the desired state defined by a resource specification, and issues API calls to create, update, or delete resources. The reconciliation loop handles retries, error handling, and conflict resolution to achieve eventual consistency.

In user interface frameworks like React, the reconciler refers to the subsystem that computes differences between the previous render tree and the new one and applies the minimal set of changes to the host environment. Modern implementations use incremental work scheduling and can render asynchronously, improving responsiveness while maintaining correctness of the UI.

In data management and integration, a reconciler can compare data across systems, detect discrepancies, and apply resolutions or flag conflicts for review. This is common in ETL pipelines, data synchronization services, and financial reconciliation processes.

Key considerations for a reconciler include ensuring idempotence, handling partial failures gracefully, determinism in update order, and providing observability through metrics and logs to diagnose divergence and resolution performance.

The term is used across domains and often refers to the same underlying goal: compute an intended state and systematically adjust the current state to realize it.