Serving Angular app as a static content in Express Server

To give a better perspective, I'll address a third cache option as well, to clarify the differences.

Types of caching

Basically we have 3 possible layers of caching, based on the priority they are checked from the client:

1. Service Worker cache (client-side)
2. Browser Cache, also known as HTTP cache (client-side)
3. Server side cache (CDN)

PS: Some browser like Chrome have an extra memory cache layer in front of the service worker cache.

Characteristics / differences

The service worker is the most reliable from the client-side ones, since it defines its own rules over how to manage the caching, and provide extra capabilities and fine-grained control over exactly what is cached and how caching is done.

The Browser caching is defined based on some HTTP headers from the assets response (Cache-Control and Expires), but the main issue is that there are many conditions in which those are ignored. For instance, I've heard that for files bigger than 25Mb, normally they are not cached, specially on mobile, where the memory is limited (I believe it's getting even more strict lately, due to the increase in mobile usage).

So between those 2 options, I'd always chose the Service Worker cache for more reliability.

Now, talking to the 3rd option, the CDN checks the HTTP headers to look for ETag for busting the cache. The idea of the Server-side caching is to only call the origin server in case the asset is not found on the CDN.

Now, between 1st and 3rd, the main difference is that Service Workers works best for Slow / failing network connections and offline, since the cache is done client-side, so if the network is off, then the service worker retrieves the last cached information, allowing for a smooth user experience. Server-side, on the other hand, only works when we are able to reach the server, but at the same time, the caching happens out of user's device, saving local space, and reducing the application memory consumption.

So as you see, there's no right / wrong answers, just what works best for your use case.

Some Sources

• MDN Cache
• MDN HTTP caching
• Great article from web.dev
• Facebook study on caching duration and efficiency

Let's answer your questions:

what is the main difference between these two approaches (caching with ETag and caching with Service Workers)

Both solutions cache files, the main difference is the need to reach the server or stay locally:

• For the ETag, the browser hits the server asking for a file with a hash (the etag), depending on the file stored in the server, the server will answer with a "the file was not modified, use your local copy" with a 300 HTTP response or "here is a new version of that file" with a 200 HTTP response and a new file. In both cases the server always decides. and the user will wait for a round trip.
• With the Service worker approach you can decide locally what to do. You can write some logic to control what/when to use a local copy (cached) or when go to the server. This is very useful for offline capabilities since the logic is happening in the client, and there is no need to hit the server.

when we should use one over the other?

You can use both together. You can define some logic in the service worker, if there is no connection return the local copies, otherwise go to the server.

What would be the most efficient when it comes to performance:

• Server side caching and serving Angular app static files
• Implementing Angular Service Worker for caching
• Do both 1 and 2

My recommended approach is use both approaches. Although treat your files differently, the 'index.html' file can change, in this case use the service worker (in case there is no internet access) and if there is internet access let the web server answer with the etag. All the other static files (CSS and JS) should be immutable files, this is you can be sure the local copy is valid, in this case add a hash to the files' name (so they are always unique files) and cache them. When you have a new version of your app, you will modify the 'index.html' pointing to new immutable files.