That would make sense that whatsapp backend is in Erlang. Erlang was developed for telecommunication systems where you need performance and the ability to hot swap code.

Erlang's hotswap and Python's hotswap are of comparable power, ie, both are crappy. Being able to do something isn't the same as doing it well.

You can hot swap all languages.

If you're claiming that Erlang is no better at hot-swapping code than "all languages", you haven't seen Erlang's hot-swapping abilities.

"All languages" provide you the ability to run arbitrary code, and thus theoretically support hot-swapping, but if you try to put it into practice you'll soon run into real obstacles you hadn't considered that either bite you or at the least make your job a lot harder. Erlang provides those practical solutions that make hot-swapping easy (in other words, practically possible).

TL;DR: On the question of buy vs. build when it comes to hot-swapping abilities, Erlang falls on the left while "all languages" start at the right.

I have seen it, it is no better than Python's, exactly as I said.

Hot swapping Erlang in production is rarely done, and if it is done, it mostly done during debugging, not serving production traffic.

I invite you to do a straw-poll on the erlang mailing list.

> I have seen it, it is no better than Python's, exactly as I said.

I'm going to have to call your bluff and ask you to substantiate your claim. Please show where Python provides, as a part of the language and/or the standard library, the equivalents of these Erlang built-ins:

1. Module versioning (via vsn)

2. The ability to run multiple versions of the same module at the same time, transparently and for as long as needed.

3. The versioning (via Module vsn), tracking (via handle_call/3 and derivatives), and migrating (via code_change/3) of State.

4. The dependency tracking and ordering between code updates of multiple modules (via release_handler)

These are the native features that make hot-swapping in production not only feasible, but also powerful and easy. Of course, these are built on top of foundational primitives that are possible in any language, sure, but not all languages (including Python) have these natively available.

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> Hot swapping Erlang in production is rarely done, and if it is done, it mostly done during debugging, not serving production traffic.

> I invite you to do a straw-poll on the erlang mailing list.

I'm sorry, but this point you're introducing now is unrelated to the discussion we're having, so I'll refuse to respond to this in any way except to remind you of the point we _are_ discussing:

1. adrr's claim: " Erlang was developed for telecommunication systems where you need performance and the ability to hot swap code." https://news.ycombinator.com/item?id=23286646

2. To which you responded: "Erlang's hotswap and Python's hotswap are of comparable power, ie, both are crappy. Being able to do something isn't the same as doing it well.

You can hot swap all languages." https://news.ycombinator.com/item?id=23287624

Hot swapping support in Erlang is first class, but even with that, it is not an easy feat to accomplish in large systems. Which is why, in my experience, people tend to avoid it if they can do without it. Systems i have worked with either had scheduled downtime periods or one could easily take down a node for upgrades without impacting the general availability of the system. In such cases, there is no real incentive to invest time and energy in managing hot swapping. But if you absolutely need all parts of your system be up 24/7, Erlang has the best story I've heard so far.

Basically anything that is protocol-oriented like WhatsApp. A lot of people will say fault tolerant, low latency, highly available and massively concurrent systems, and they’ll be right, but I think that’s only half the story. You also get a battle-tested blueprint for building these kinds of distributed systems. You will find a pattern here and there in other languages and VMs, but the cohesiveness of the package (language + BEAM + OTP + observability + patterns) is something really unique. Compare this to the Java ecosystem, where there are dozens of concurrency primitives and an even greater number of distributed systems libraries and frameworks like Akka, Mesos, Zookeeper, Storm, Flink, Samza, Heron, Helix...

If the problem you're solving can be easily split into many independent, low volume, maybe long running, communication oriented tasks, you owe it to yourself to look into the Erlang ecosystem. Think handling signaling for calls, chat connections, inputs from millions of IoT endpoints.

>IIRC, Whatsapp backend used to be on Erlang

Used to be? I know WhatsApp switched from FreeBSD to Linux and swapped hundreds bare metal servers to thousands of VMs. I dont think WhatsApp ever swapped / rewrote their system in another language under Facebook.

There was some confusion about that a little while ago: https://news.ycombinator.com/item?id=21111662

Afaik it still is.