When Monitoring Kubernetes Resources: watch.Interface, cache.NewInformer, or cache.NewSharedInformer?

watch.Interface vs. cache.NewInformer vs. cache.NewSharedIndexInformer

When monitoring resources in a Kubernetes cluster, there are several options available in the Kubernetes client-go package. This article aims to clarify the differences between watch.Interface, cache.NewInformer, cache.NewSharedInformer, and cache.NewSharedIndexInformer.

Level of Abstraction

The primary difference between these APIs lies in their level of abstraction:

• watch.Interface provides the lowest level of abstraction, giving direct access to event streams from the API server.
• cache.NewInformer adds a cache to the watch interface, making it easier to manage resources in memory.
• cache.NewSharedInformer and cache.NewSharedIndexInformer are higher-level abstractions that share resources across multiple informers and add indexing for efficient data retrieval, respectively.

Functionality

watch.Interface

• Obtains a watch channel that emits Added, Modified, and Deleted events for a specific resource.
• Provides raw access to events, but requires manual event handling.

cache.NewInformer

• Similar to watch.Interface, but automatically manages the cache and provides OnAdd(), OnUpdate(), and OnDelete() handlers for event handling.
• Includes access to the cached resource state, making it ideal for tracking changes over time.

cache.NewSharedInformer

• Shares the API server connection and watch channel with other informers.
• Efficiently manages resources by avoiding duplicate connections and reducing memory overhead.

cache.NewSharedIndexInformer

• Adds an index to the cached data, allowing for faster retrieval of resources by specific attributes.
• Useful when working with large datasets or needing to query resources based on specific criteria.

Recommendation

In most cases, it is recommended to use cache.NewSharedInformer or cache.NewSharedIndexInformer for resource monitoring. These higher-level abstractions provide a balanced approach between performance and ease of use, while handling the complexities of watch management and caching.

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