Service workers augment the traditional web deployment model and empower applications to deliver a user experience with the reliability and performance on par with natively-installed code.
At its simplest, a service worker is a script that runs in the web browser and manages caching for an application.
Service workers function as a network proxy. They intercept all outgoing HTTP requests made by the application and can choose how to respond to them. For example, they can query a local cache and deliver a cached response if one is available. Proxying isn't limited to requests made through programmatic APIs, such as `fetch`; it also includes resources referenced in HTML and even the initial request to `index.html`. Service worker-based caching is thus completely programmable and doesn't rely on server-specified caching headers.
Unlike the other scripts that make up an application, such as the Angular app bundle, the service worker is preserved after the user closes the tab. The next time that browser loads the application, the service worker loads first, and can intercept every request for resources to load the application. If the service worker is designed to do so, it can *completely satisfy the loading of the application, without the need for the network*.
Even across a fast reliable network, round-trip delays can introduce significant latency when loading the application. Using a service worker to reduce dependency on the network can significantly improve the user experience.
## Service workers in Angular
Angular applications, as single-page applications, are in a prime position to benefit from the advantages of service workers. Starting with version 5.0.0, Angular ships with a service worker implementation. Angular developers can take advantage of this service worker and benefit from the increased reliability and performance it provides, without needing to code against low-level APIs.
Angular's service worker is designed to optimize the end user experience of using an application over a slow or unreliable network connection, while also minimizing the risks of serving outdated content.
The Angular service worker's behavior follows that design goal:
* Caching an application is like installing a native application. The application is cached as one unit, and all files update together.
* A running application continues to run with the same version of all files. It does not suddenly start receiving cached files from a newer version, which are likely incompatible.
* When users refresh the application, they see the latest fully cached version. New tabs load the latest cached code.
* Updates happen in the background, relatively quickly after changes are published. The previous version of the application is served until an update is installed and ready.
* The service worker conserves bandwidth when possible. Resources are only downloaded if they've changed.
To support these behaviors, the Angular service worker loads a *manifest* file from the server. The manifest describes the resources to cache and includes hashes of every file's contents. When an update to the application is deployed, the contents of the manifest change, informing the service worker that a new version of the application should be downloaded and cached. This manifest is generated from a user-provided configuration file called `ngsw-config.json`, by using a build tool such as the Angular CLI.
Installing the Angular service worker is as simple as including an `NgModule`. In addition to registering the Angular service worker with the browser, this also makes a few services available for injection which interact with the service worker and can be used to control it. For example, an application can ask to be notified when a new update becomes available, or an application can ask the service worker to check the server for available updates.
## Prerequisites
To use Angular service workers, you must have the following Angular and CLI versions:
* Angular 5.0.0 or later.
* Angular CLI 1.6.0 or later.
Your application must run in a web browser that supports service workers. Currently, the latest versions of Chrome and Firefox are supported. To learn about other browsers that are service worker ready, see the [Can I Use](http://caniuse.com/#feat=serviceworkers) page.
## Related resources
For more information about service workers in general, see [Service Workers: an Introduction](https://developers.google.com/web/fundamentals/primers/service-workers/).
For more information about browser support, see the [browser support](https://developers.google.com/web/fundamentals/primers/service-workers/#browser_support) section of [Service Workers: an Introduction](https://developers.google.com/web/fundamentals/primers/service-workers/), Jake Archibald's [Is Serviceworker ready?](https://jakearchibald.github.io/isserviceworkerready/), and
[Can I Use](http://caniuse.com/#feat=serviceworkers).
The remainder of this Angular documentation specifically addresses the Angular implementation of service workers.
