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Comment by mmcclimon | Hacker News Reader

SSEs are one of the standard push mechanisms in JMAP [1], and they're part of what make the Fastmail UI so fast. They're straightforward to implement, for both server and client, and the only thing I don't like about them is that Firefox dev tools make them totally impossible to debug.

1. https://jmap.io/spec-core.html#event-source

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It is, however, interesting to note that Fastmail’s webmail doesn’t use EventSource, but instead implements it atop fetch or XMLHttpRequest. An implementation atop XMLHttpRequest was required in the past because IE lacked EventSource, but had that been the only reason, it’d just have been done polyfill style; but it’s not. My foggy recollection from 4–5 years ago (in casual discussion while I worked for Fastmail) is that it had to do with getting (better?) control over timeout/disconnect/reconnect, probably handling Last-Event-ID, plus maybe skipping browser bugs in some older (now positively ancient and definitely unsupported) browsers. The source for that stuff is the three *EventSource.js files in https://github.com/fastmail/overture/tree/master/source/io.

> the only thing I don't like about them is that Firefox dev tools make them totally impossible to debug

You can't say that and not say more about it, haha. Please expand on this?

Also, I'm a Fastmail customer and appreciate the nimble UI, thanks!

I think their information could be outdated. Since Firefox 82, you can supposedly inspect the content of SSE streams: https://developer.mozilla.org/en-US/docs/Tools/Network_Monit...

Before that... yeah, the Firefox dev tools were not very helpful for SSE.

Hmm! You're right that I hadn't looked it a while, so I checked before making the comment above. I'm still seeing the same thing I always have, which is "No response data available for this request". Possibly something is slightly wrong somewhere (though Chrome dev tools seem fine on the same), but you've given me something to look into, thanks!

That is interesting. I just tested it myself, and at least for my setup (Firefox on Mac on ARM), the events only showed up in the dev tools if the server closed the SSE connection... so, maybe Firefox still hasn't fully fixed this problem.

Yeah, that seems to be the case (confirmed with their little example at https://github.com/mdn/dom-examples/tree/master/server-sent-...). Once the connection is closed you can see things, but that's not particularly useful for debugging!

Funny because I love fastmail but literally the only complaint I have is their android app takes too long to load

The Fastmail UI is indeed snappy, except when it suddenly decides it has to reload the page, which seems to be multiple times a day these days (and always when I need to search for a specific email). Can you make it do what one of my other favorite apps does: when there's a new version available, make a small pop up with a reload button, but don't force a reload (until maybe weeks later)?

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