Precaching

Precaching is a caching technique used in web development, especially in Progressive Web Apps. It involves downloading and storing a set of resources in advance, so they can be served from a local cache rather than over the network. The primary goals are to improve startup performance, reduce latency, and enable offline functionality for the assets deemed critical for the app's initial or recurring use.

In practice, precaching is implemented in a service worker. During the installation event, the service worker opens a named cache and adds a predefined list of assets, such as HTML, CSS, JavaScript, fonts, and essential images. Each asset can be associated with a revision or fingerprint to detect updates. When the manifest changes, the cached entries are refreshed on the next installation, ensuring the app uses the latest versions.

Precaching is often contrasted with runtime caching. Precached assets are retrieved proactively at install time, providing deterministic availability, while runtime caching stores assets as they are requested during normal operation. Build tools and libraries, like Workbox, automate manifest generation and cache management.

Considerations include cache storage limits, invalidation strategies, and the risk of serving stale content if updates are not propagated promptly. Developers balance cache size with the importance of assets and may combine precaching with runtime caching for dynamic content. Precaching is a core technique in PWAs, contributing to reliability and perceived performance.