add section why cache limitation is necessary with examples vuejs-tem…

…plates#70

README.md

@@ -22,6 +22,14 @@ $ npm run dev
 
 If port 8080 is already in use on your machine you must change the port number in `/config/index.js`. Otherwise `npm run dev` will fail.
 
+## What's not Included
+
+* You should configure your web server to add HTTP headers to prevent caching of critical service worker files.
+If you don't do this, [browsers might cache the content for up to 24 hours](https://stackoverflow.com/questions/38843970/service-worker-javascript-update-frequency-every-24-hours/38854905#38854905).
+In addition, you should add HTTP headers to prevent the contents of the static folder to be cached unintentionally long.
+
+See ["Configuring your Web server to prevent caching"](docs/prevent_caching.md) in the docs for more information.
+
 ## What's Included
 
 * Service Worker precaching of application shell + static assets (prod)
@@ -62,4 +70,4 @@ You can fork this repo to create your own boilerplate, and use it with `vue-cli`
 
 ``` bash
 vue init username/repo my-project
-```
+```

docs/prevent_caching.md

@@ -0,0 +1,63 @@
+# Configuring your Web server to prevent caching
+
+## Why you want to do this
+
+You should configure your web server to add HTTP headers to prevent caching of critical service worker files.
+If you don't do this, [browsers might cache the content for up to 24 hours](https://stackoverflow.com/questions/38843970/service-worker-javascript-update-frequency-every-24-hours/38854905#38854905).
+
+In addition, you should add HTTP headers to prevent the contents of the static folder to be cached unintentionally long.
+
+## Mechanisms at work
+
+### Service worker
+
+When you run the production build and deploy the application to a web server, the browser will call the service worker on the first visit and will download and cache all resources. 
+When the browser accesses the site a second time, it will use the cached resources.
+To notice changes in your app it is crucial that the browser downloads the latest version `service-worker.js` from your server.
+The browser will only do this when the [Cache-Control](https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/http-caching) header associated with `service-worker.js` indicates that a previous download has expired or [24 hours have passed](https://stackoverflow.com/questions/38843970/service-worker-javascript-update-frequency-every-24-hours/38854905#38854905).
+If a new service worker is available, it will download the new contents in the background.
+Once the download is complete, the default behavior is that the new content will be shown on the next visit of the site. 
+You can change the template's [`service-worker-prod.js`](../template/build/service-worker-prod.js) to display the message to the user to reload the page, or issue a `window.location.reload()` automatically to trigger an update depending on your application's needs.
+
+Depending on your setup, you might want to have a short caching time for your service worker or no caching at all.
+At minimum, you should add caching headers to disable or minimize caching for `service-worker.js` and `index.html`.
+
+### The static folder
+
+The folder [`static`](../template/static) contains the manifest and the favicons of your app.
+
+You should limit or disable caching for all files in this folder to ensure users have receive the latest version of these files.
+
+### Additional information
+
+More information about caching headers can be found in the blog post ["Caching best practices & max-age gotchas" by Jake Archibald](https://jakearchibald.com/2016/caching-best-practices/). 
+
+## How to add caching headers for your server
+
+Here are some examples that worked for other users of this template.
+See [Issue #70](https://github.com/vuejs-templates/pwa/issues/70) for a discussion.
+
+Apache: Disable all caching for the two essential files
+
+    <Files index.html|service-worker.js>
+    FileETag None
+    Header unset ETag
+    Header set Cache-Control "max-age=0, no-cache, no-store, must-revalidate"
+    Header set Pragma "no-cache"
+    Header set Expires "Wed, 11 Jan 1984 05:00:00 GMT"
+    </Files>
+
+Apache: Limit caching to five minutes to all files to prevent unlimited caching also for files in static
+
+    Header set Cache-Control "max-age=300, must-revalidate"
+
+Nginx: Limit caching for the two essential files
+
+    location ~ (index.html|service-worker.js)$ {
+      # ...
+      add_header Last-Modified $date_gmt;
+      add_header Cache-Control 'no-store, no-cache, must-revalidate, proxy-revalidate, max-age=0';
+      if_modified_since off;
+      expires off;
+      etag off;
+    }