When to Use watch.Interface, cache.NewInformer, cache.NewSharedInformer, and cache.NewSharedIndexInformer?

watch.Interface vs. cache.NewInformer vs. cache.NewSharedIndexInformer: A Guide to Abstraction Levels

Introduction

When monitoring Kubernetes resources and reacting to changes, developers can choose from various options in the Kubernetes client-go package. This article aims to clarify the differences between watch.Interface, cache.NewInformer, cache.NewSharedInformer, and cache.NewSharedIndexInformer.

watch.Interface

watch.Interface is a low-level abstraction that allows you to monitor changes to Kubernetes resources through a ResultChan(). It provides Added/Modified/Deleted events, giving you visibility into resource changes. However, it only includes the "after" state of the resource.

cache.NewInformer

cache.NewInformer introduces a higher level of abstraction. It includes a watcher, lister, and in-memory cache. By implementing a cache.ResourceEventHandler, you can receive OnAdd()/OnUpdate()/OnDelete() calls. This provides you with both the "before" and "after" states of the resource, making it more convenient for change handling.

cache.NewSharedInformer

cache.NewSharedInformer shares the connection with the API server and other resources between your informers. This optimizes resource usage and improves performance. It is recommended over cache.NewInformer unless you have specific requirements for isolation.

cache.NewSharedIndexInformer

cache.NewSharedIndexInformer adds an index to the data cache. This is particularly useful if you are working with a large dataset and need efficient indexing for quick lookup and filtering. It is the most feature-rich option but also the most complex to implement.

Recommendation

In most use cases, it is recommended to use SharedInformers instead of the lower level abstractions. SharedInformers provide performance benefits and simplify resource management. Instantiate new SharedInformers from the same SharedInformerFactory for optimal resource use.

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