Service worker in production
This tutorial is a reference for deploying and supporting production apps that use the Angular service worker. It explains how the Angular service worker fits into the larger production environment, the service worker's behavior under various conditions, and available resources and fail-safes.
Service worker and caching of app resources
Conceptually, you can imagine the Angular service worker as a forward cache or a CDN edge that is installed in the end user's web browser. The service worker's job is to satisfy requests made by the Angular app for resources or data from a local cache, without needing to wait for the network. Like any cache, it has rules for how content is expired and updated.
In the context of an Angular service worker, a "version" is a collection of resources that represent a specific build of the Angular app. Whenever a new build of the app is deployed, the service worker treats that build as a new version of the app. This is true even if only a single file is updated. At any given time, the service worker may have multiple versions of the app in its cache and it may be serving them simultaneously.
To preserve app integrity, the Angular service worker groups all files into a version together. The files grouped into a version usually include HTML, JS, and CSS files. Grouping of these files is essential for integrity because HTML, JS, and CSS files frequently refer to each other and depend on specific content. For example, an index.html file might have a <script> tag that references bundle.js and it might attempt to call a function startApp() from within that script. Any time this version of index.html is served, the corresponding bundle.js must be served with it. For example, assume that the startApp() function is renamed to runApp() in both files. In this scenario, it is not valid to serve the old index.html, which calls startApp(), along with the new bundle, which defines runApp().
This file integrity is especially important when lazy loading modules. A JS bundle may reference many lazy chunks, and the filenames of the lazy chunks are unique to the particular build of the app. If a running app at version Xattempts to load a lazy chunk, but the server has updated to version X + 1 already, the lazy loading operation will fail.
The version identifier of the app is determined by the contents of all resources, and it changes if any of them change. In practice, the version is determined by the contents of the ngsw.json file, which includes hashes for all known content. If any of the cached files change, the file's hash will change in ngsw.json, causing the Angular service worker to treat the active set of files as a new version.
With the versioning behavior of the Angular service worker, an application server can ensure that the Angular app always has a consistent set of files.
Every time the user opens or refreshes the application, the Angular service worker checks for updates to the app by looking for updates to the ngsw.json manifest. If an update is found, it is downloaded and cached automatically and will be served the next time the application is loaded.
One of the potential side effects of long caching is inadvertently caching an invalid resource. In a normal HTTP cache, a hard refresh or cache expiration limits the negative effects of caching an invalid file. A service worker ignores such constraints and effectively long caches the entire app. Consequently, it is essential that the service worker gets the correct content.
To ensure resource integrity, the Angular service worker validates the hashes of all resources for which it has a hash. Typically for an app created with the Angular CLI, this is everything in the dist directory covered by the user's src/ngsw-config.json configuration.
If a particular file fails validation, the Angular service worker attempts to re-fetch the content using a "cache-busting" URL parameter to eliminate the effects of browser or intermediate caching. If that content also fails validation, the service worker considers the entire version of the app to be invalid and it stops serving the app. If necessary, the service worker enters a safe mode where requests fall back on the network, opting not to use its cache if the risk of serving invalid, broken, or outdated content is high.
Hash mismatches can occur for a variety of reasons:
Caching layers in between the origin server and the end user could serve stale content.
A non-atomic deployment could result in the Angular service worker having visibility of partially updated content.
Errors during the build process could result in updated resources without ngsw.json being updated. The reverse could also happen resulting in an updated ngsw.json without updated resources.
The only resources that have hashes in the ngsw.json manifest are resources that were present in the dist directory at the time the manifest was built. Other resources, especially those loaded from CDNs, have content that is unknown at build time or are updated more frequently than the app is deployed.
If the Angular service worker does not have a hash to validate a given resource, it still caches its contents but it honors the HTTP caching headers by using a policy of "stale while revalidate." That is, when HTTP caching headers for a cached resource indicate that the resource has expired, the Angular service worker continues to serve the content and it attempts to refresh the resource in the background. This way, broken unhashed resources do not remain in the cache beyond their configured lifetimes.
It can be problematic for an app if the version of resources it's receiving changes suddenly or without warning.
The Angular service worker provides a guarantee: a running app will continue to run the same version of the app. If another instance of the app is opened in a new web browser tab, then the most current version of the app is served. As a result, that new tab can be running a different version of the app than the original tab.
It's important to note that this guarantee is stronger than that provided by the normal web deployment model. Without a service worker, there is no guarantee that code lazily loaded later in a running app is from the same version as the initial code for the app.
There are a few limited reasons why the Angular service worker might change the version of a running app. Some of them are error conditions:
The current version becomes invalid due to a failed hash.
An unrelated error causes the service worker to enter safe mode; that is, temporary deactivation.
The Angular service worker is aware of which versions are in use at any given moment and it cleans up versions when no tab is using them.
Other reasons the Angular service worker might change the version of a running app are normal events:
The page is reloaded/refreshed. The page requests an update be immediately activated via the SwUpdate service.
Service worker updates
The Angular service worker is a small script that runs in web browsers. From time to time, the service worker will be updated with bug fixes and feature improvements.
The Angular service worker is downloaded when the app is first opened and when the app is accessed after a period of inactivity. If the service worker has changed, the service worker will be updated in the background.
Most updates to the Angular service worker are transparent to the app—the old caches are still valid and content is still served normally. However, occasionally a bugfix or feature in the Angular service worker requires the invalidation of old caches. In this case, the app will be refreshed transparently from the network.
Debugging the Angular service worker
Occasionally, it may be necessary to examine the Angular service worker in a running state to investigate issues or to ensure that it is operating as designed. Browsers provide built-in tools for debugging service workers and the Angular service worker itself includes useful debugging features.
Locating and analyzing debugging information
The Angular service worker exposes debugging information under the ngsw/ virtual directory. Currently, the single exposed URL is ngsw/ state. Here is an example of this debug page's contents:
NGSW Debug Info:
Driver state: NORMAL ((nominal))
Latest manifest hash: eea7f5f464f90789b621170af5a569d6be077e5c
Last update check: never
=== Version eea7f5f464f90789b621170af5a569d6be077e5c ===
Clients: 7b79a015-69af-4d3d-9ae6-95ba90c79486, 5bc08295-aaf2-42f3-a4cc-9e4ef9100f65
=== Idle Task Queue ===
Last update tick: 1s496u
Last update run: never
* init post-load (update, cleanup)
It is just a code snippet explaining a particular concept and may not have any output
The first line indicates the driver state:
Driver state: NORMAL ((nominal))
NORMAL indicates that the service worker is operating normally and is not in a degraded state.
There are two possible degraded states:
EXISTING_CLIENTS_ONLY: the service worker does not have a clean copy of the latest known version of the app. Older cached versions are safe to use, so existing tabs continue to run from the cache, but new loads of the app will be served from the network.
SAFE_MODE: the service worker cannot guarantee the safety of using cached data. Either an unexpected error occurred or all cached versions are invalid. All traffic will be served from the network, running as little service worker code as possible.
In both cases, the parenthetical annotation provides the error that caused the service worker to enter the degraded state.
Latest manifest hashLatest manifest hash: eea7f5f464f90789b621170af5a569d6be077e5c
This is the SHA1 hash of the most up-to-date version of the app that the service worker knows about.
Last update check
Last update check: never
This indicates the last time the service worker checked for a new version, or update, of the app. never indicates that the service worker has never checked for an update.
In this example debug file, the update check is currently scheduled, as explained the next section.
=== Version eea7f5f464f90789b621170af5a569d6be077e5c === Clients: 7b79a015-69af-4d3d-9ae6-95ba90c79486, 5bc08295-aaf2-42f3-a4cc-9e4ef9100f65
In this example, the service worker has one version of the app cached and being used to serve two different tabs. Note that this version hash is the "latest manifest hash" listed above. Both clients are on the latest version. Each client is listed by its ID from the Clients API in the browser.
Idle task queue
=== Idle Task Queue === Last update tick: 1s496u Last update run: never Task queue: * init post-load (update, cleanup)
The Idle Task Queue is the queue of all pending tasks that happen in the background in the service worker. If there are any tasks in the queue, they are listed with a description. In this example, the service worker has one such task scheduled, a post-initialization operation involving an update check and cleanup of stale caches.
The last update tick/run counters give the time since specific events happened related to the idle queue. The "Last update run" counter shows the last time idle tasks were actually executed. "Last update tick" shows the time since the last event after which the queue might be processed.
Errors that occur within the service worker will be logged here.
Browsers such as Chrome provide developer tools for interacting with service workers. Such tools can be powerful when used properly, but there are a few things to keep in mind.
When using developer tools, the service worker is kept running in the background and never restarts. This can cause behavior with Dev Tools open to differ from behavior a user might experience.
If you look in the Cache Storage viewer, the cache is frequently out of date. Right-click the Cache Storage title and refresh the caches.
Stopping and starting the service worker in the Service Worker pane triggers a check for updates.
Service Worker Safety
Like any complex system, bugs or broken configurations can cause the Angular service worker to act in unforeseen ways. While its design attempts to minimize the impact of such problems, the Angular service worker contains several failsafe mechanisms in case an administrator ever needs to deactivate the service worker quickly.
To deactivate the service worker, remove or rename the ngsw.json file. When the service worker's request for ngsw.json returns a 404, then the service worker removes all of its caches and de-registers itself, essentially self-destructing.
Also included in the @angular/service-worker NPM package is a small script safety-worker.js, which when loaded will unregister itself from the browser. This script can be used as a last resort to get rid of unwanted service workers already installed on client pages.
It's important to note that you cannot register this worker directly, as old clients with cached state may not see a new index.html which installs the different worker script. Instead, you must serve the contents of safety-worker.js at the URL of the Service Worker script you are trying to unregister and must continue to do so until you are certain all users have successfully unregistered the old worker. For most sites, this means that you should serve the safety worker at the old Service Worker URL forever.
This script can be used both to deactivate @angular/service-worker as well as any other Service Workers which might have been served in the past on your site.
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