service worker: Add storage section to README.

Thanks to Joshua Bell for explaining Quota Manager to me.

Bug: 795989
Change-Id: I9a26a93f52687761419dc387e37d1baccf04671e
Reviewed-on: https://chromium-review.googlesource.com/1194805
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587361}

content/browser/service_worker/README.md

@@ -3,6 +3,7 @@
 [content/renderer/service_worker]: /content/renderer/service_worker
 [content/renderer/service_worker]: /content/renderer/service_worker
 [content/common/service_worker]: /content/common/service_worker
+[disk_cache]: /net/disk_cache/README.md
 [embedded_worker.mojom]: https://codesearch.chromium.org/chromium/src/content/common/service_worker/embedded_worker.mojom
 [service_worker_container.mojom]: https://codesearch.chromium.org/chromium/src/content/common/service_worker/service_worker_container.mojom
 [service_worker_database.h]: https://codesearch.chromium.org/chromium/src/content/browser/service_worker/service_worker_database.h
@@ -12,12 +13,18 @@
 [third_party/blink/public/platform/modules/service_worker]: /third_party/blink/public/platform/modules/service_worker
 [third_party/blink/public/web/modules/service_worker]: /third_party/blink/public/web/modules/service_worker
 [third_party/blink/renderer/modules/service_worker]: /third_party/blink/renderer/modules/service_worker
-[Blink Public API]: /third_party/blink/public
+[Blink Public API]: /third_party/blink/public/README.md
+[Cache Storage API]: /content/browser/cache_storage/README.md
+[LevelDB]: /third_party/leveldatabase/README.chromium
 [Onion Soup]: https://docs.google.com/document/d/1K1nO8G9dO9kNSmtVz2gJ2GG9gQOTgm65sJlV3Fga4jE/edit?usp=sharing
+[Quota Manager]: /storage/browser/quota
+[ServiceWorkerDatabase]: https://codesearch.chromium.org/chromium/src/content/browser/service_worker/service_worker_database.h
+[ServiceWorkerStorage]: https://codesearch.chromium.org/chromium/src/content/browser/service_worker/service_worker_storage.h
+[Service Worker specification]: https://w3c.github.io/ServiceWorker/
 
 This is Chromium's implementation of [service
-workers](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API). See the
-[Service Worker specification](https://w3c.github.io/ServiceWorker/).
+workers](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API).
+See the [Service Worker specification].
 
 ## Directory structure
 
@@ -42,6 +49,47 @@ workers](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API). S
 - [third_party/blink/renderer/modules/service_worker]: Renderer process code in
   Blink. This is the closest code to the web-exposed Service Worker API.
 
+## Storage
+
+Service worker storage consists of the following.
+- **Service worker registration metadata** is stored in a [LevelDB] instance
+  located at ${DIR_USER_DATA}/Service Worker/Database.
+- **Service worker scripts** are stored in a [disk_cache] instance using the
+  "simple" implementation, located at ${DIR_USER_DATA}/Service
+  Worker/ScriptCache. Registration metadata points to these scripts.
+
+Code pointers include [ServiceWorkerDatabase] and [ServiceWorkerStorage].
+
+The related [Cache Storage API] uses a [disk_cache] instance using the "simple"
+implementation, located at ${DIR_USER_DATA}/Service Worker/CacheStorage. This
+location was chosen because the [Cache Storage API] is currently defined in the
+[Service Worker specification], but it can be used independently of service
+workers.
+
+For incognito windows, everything is in-memory.
+
+### Eviction
+
+Service workers storage lasts indefinitely, i.e, there is no periodic deletion
+of old but still installed service workers. Installed service workers are only
+evicted by the [Quota Manager] (or user action). The Quota Manager controls
+several web platform APIs, including sandboxed filesystem, WebSQL, appcache,
+IndexedDB, cache storage, service worker (registration and scripts), and
+background fetch.
+
+The Quota Manager starts eviction when one of the following conditions is true
+(as of August 2018):
+- **The global pool is full**: Chrome is using > 1/3 of the disk (>2/3 on CrOS).
+- **The system is critically low on space**: the disk has < min(1GB,1%) free
+  (regardless of how much Chrome is contributing!)
+
+When eviction starts, origins are purged on an LRU basis until the triggering
+condition no longer applies. Purging an origin deletes its storage completely.
+
+Note that Quota Manager eviction is independent of HTTP cache eviction. The
+HTTP cache is typically much smaller than the storage under the control of the
+Quota Manager, and it likely uses a simple non-origin-based LRU algorithm.
+
 ## UseCounter integration
 
 Blink has a UseCounter mechanism intended to measure the percentage of page
@@ -50,10 +98,10 @@ measurement because a feature use in a service worker potentially affects many
 page loads, including ones in the future.
 
 Therefore, service workers integrate with the UseCounter mechanism as follows:
-- If a feature use occurs before the service worker finished installing, it
+- **If a feature use occurs before the service worker finished installing**, it
 is recorded in storage along with the service worker. Any page thereafter that
 the service worker controls is counted as using the feature.
-- If a feature use occurs after the service worker finished installing, all
+- **If a feature use occurs after the service worker finished installing**, all
 currently controlled pages are counted as using the feature.
 
 For more details and rationale, see [Design of UseCounter for