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Comment by sb8244 | Hacker News Reader

Hmm I'm not quite sure I follow the process examine. If your started process crashes (let's say supervisor fails to start due to too many failures), k8s would restart the process (your app) to try again. If a managed process in elixir dies, you could make that bring down your whole app, but generally you would want to handle it yourself.

This is similar to an app server in Ruby. A single worker might die, but it's still being managed by a central coordinator. If the coordinator dies, everything is down and the app restarts

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Not trying to be flippant, but by extending your analogy: if the Kubernetes supervisor dies (which is conceivable, no technology is infallible), then you are in the same scenario as something bringing your whole Elixir VM down. You can always run your Elixir VM using something like supervisord if you need another watchdog.

BTW, it's very hard to bring BEAM down. Most exceptions will bring kill the faulty process, but leave the supervisor tree untouched. Only "unmanaged" code (for example, calling a C library using a NIF) might crash the whole VM.

Then again, no technology is infallible.

True true, maybe there's just a single part that I was hung up on in parent comment

> (a process within the VM can die and K8s will have no idea)

I suppose that I see nothing wrong with this and would say that it's not weird or unusual to be the case. They're separate things, although they both achieve visually similar goals (keep things running as much as possible).

My point is less about K8s vs Elixir but rather a statement that this seems acceptable and shouldn't be a reason to rule out something like running Elixir on K8s.

> You can always run your Elixir VM using something like supervisord if you need another watchdog.

Sounds like heart: http://erlang.org/doc/man/heart.html

>BTW, it's very hard to bring BEAM down. Most exceptions will bring kill the faulty process, but leave the supervisor tree untouched. Only "unmanaged" code (for example, calling a C library using a NIF) might crash the whole VM.

Not completely true. Try to allocate more memory than available and then see what happens.

I'm actually curious. Do you have some first-hand experience with that kind of issue? I've only played around with Elixir and my only experience deploying BEAM-based applications on production has been RabbitMQ and CouchDB and both of those were very solid.

Also, isn't ENOMEM a standard error that you can handle using the standard error handling techniques?

I have and it wasn't good. It is impossible to handle out of memory exceptions inside the BEAM. At best, you can hope for a crash of the VM. At worse, it just hangs. And one will most likely not find out about this until they hit this scenario.

Interesting, definitely worth knowing beforehand (I was thinking of implementing a small service that could be pretty memory-intensive using Elixir.) I wonder if there's a way to try and manage memory to avoid ENOMEM.

If BEAM is allocating memory it must either be copying a message from one process to another, allocating space in a process for its stack, or for some bookkeeping like to spawn a new process. In the first two cases why doesn't it just kill the process and send the usual link and monitor messages to let other Erlang processes handle the failure?

How would it send a message if it can't allocate memory anymore?

Haha, yeah, that's the problem with ENOMEM handling in general: unless you have a very clear path to free memory immediately and then try to do some reasonable cleanup, you are screwed. Back in the bad old days of running Windows machines with 8mb of memory, I'd religiously handle that kind of error code... but I can't recall the last time I actually bothered. Commoditized hardware makes us lazy.

yeah I have to make liberal use of :hibernate to avoid just this.

The scenario I saw with this was the following:

BEAM VM comes up and tries to connect to the database. The database is temporarily unreachable from that machine, but only the Ecto process dies and BEAM VM doesn't die. Since the last process (BEAM) in the Dockerfile is running, Kubernetes thinks it's healthy and will try sending traffic to it.

Compare this to something like node, where the node process will die if it can't connect to the database, and Kubernetes won't try to direct traffic to it and will restart it for you. You can do this with Elixir supervisors as well - that's why I said Kubernetes replicates some of Elixir's functionality.

It's definitely possible to work around all this which is why we ended up using Kubernetes for our Elixir deployment (and I'd recommend it). It's just these little things that make it clear that Kubernetes was designed with something like node in mind and not BEAM.

The behaviour you are attributing to Node is really just the behaviour of poorly constructed software. There are at least some Node-based software products which correctly come up and wait for dependencies to become available -- and gracefully handle their subsequent transient unavailability.

There are 2 types of probes: readiness and liveness [1]. Define your readiness probe so it passes when you are ready to take the traffic (eg. connected to the db).

[1] https://kubernetes.io/docs/tasks/configure-pod-container/con...

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