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The Erlang Shell

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Re: The Erlang Shell

#11

Post by rdtsc »

> When systems have faults due to concurrency and distribution, debuggers will not work.

Those are very tricky indeed, mix in threads with pointers and a system becomes haunted. "This one customer noticed a crash, on this hardware, after running for 10 weeks, but ... we couldn't reproduce it". "Oh I suspect there is a memory overwrite or use after free that corrupted the data". People start doubting their sanity -- "Oh I swear I saw it crash, that one time, or ...did I, maybe I was tired...".

Someone (could have even been Jesper Andersen, the author) said that the biggest performance gain an application gets is when it goes from not working or being crashed to working. And the biggest slowdown is also when it goes from working to crashing unexpectedly.

There was talk of 60 hour weeks here before, one of the things that happens at 8pm at night is people huddled over a keyboard debugging some of these hard to track bugs. Managers and some programmers see it as great heroism, pats on the back for everyone, but, others see it as reaping the fruits of previous bad decisions and it is a rather sad sight for some. It all depends on the perspective.

I guess the point is one of the main qualities of Erlang is not concurrency but fault tolerance. Many systems copy the actor concurrency patterns and claim "we have Erlang now but it is also faster!", and that is a good thing, but I haven't heard too many claim "We copied Erlang's fault tolerance, and it is even better than Erlang's!". [+]

[+] you can do the same pattern up to a point using OS processes, LXC containers, separate servers, having watchdog processes restart workers, for example.

Re: The Erlang Shell

#12

Post by agentultra »

bitwalker wrote: post #8
rurban wrote: post #4

Oh nice. They have now almost a Common Lisp environment. Getting closer to Greenspun's tenth rule.

Is it actually possible to do hot code reloading while preserving the state of the running system in Common Lisp? I'm genuinely curious, I kind of had this impression that it was a feature fairly unique to Erlang.

> Is it actually possible to do hot code reloading while preserving the state of the running system in Common Lisp?

Yes.

> I'm genuinely curious, I kind of had this impression that it was a feature fairly unique to Erlang.

It's not. iIRC there's even a version of the JVM with code reloading.

What's interesting about Common Lisp is that not only can code be dynamically compiled into the running image but class definitions can be changed and live instances will be updated without stopping the program or reloading anything. There are a tonne of features in Common Lisp like this that are geared towards robust, maintainable software.

Re: The Erlang Shell

#13

Post by phamilton »

I'm another recent Erlang convert (though I'm still trying to get used to the paradigm and ecosystem). There are a few concepts that haven't quite clicked for me.

I come from a Rails background. While Erlang's hot code reloading and the ability to attach to running nodes is cool, I am not quite able to connect the dots on how this is useful. In Rails, a process is short-lived by design (the lifecycle of a request). Attaching to a process is useless, but the Rails console essentially gives you an isolated process to inspect production code and data. I don't see how extending that to an existing process would be useful.

So, challenging my assumptions, perhaps short-lived processes aren't the best approach for a web service. Perhaps that's the paradigm in Rails because Ruby lacks the tooling to make a long-lived process easy to maintain. If a long-lived process is as easy to maintain as short-lived processes, then suddenly new architectures open up. How are these architectures different? What are the benefits? What are some examples?

Re: The Erlang Shell

#14

Post by tel »

lostcolony wrote: post #10
tel wrote: post #7

I find it funny that the author willingly gave up static typing to have Erlang process control—I made the opposite switch as I grew tired of the overhead and attrition of modeling a complex domain in Erlang. I think the Erlang VM is easily my favorite place to live as a programmer, but I do with Erlang-the-language gave more.

What were your thoughts re: Dialyzer?

It's nice and once I learned how to use it I tried to be very consistent with it. The success types provide good documentation, but I found that the types still felt very flat. The fundamental component of Erlang abstraction is the process and the process tree, so your types reflect that and don't end up composing well.

Re: The Erlang Shell

#15

Post by simula67 »

agentultra wrote: post #12
bitwalker wrote: post #8

Earlier quoted context omitted.

Is it actually possible to do hot code reloading while preserving the state of the running system in Common Lisp? I'm genuinely curious, I kind of had this impression that it was a feature fairly unique to Erlang.

> Is it actually possible to do hot code reloading while preserving the state of the running system in Common Lisp? Yes. > I'm genuinely curious, I kind of had this impression that it was a feature fairly unique to Erlang. It's not. iIRC there's even a version of the JVM with code reloading. What's interesting about Common Lisp is that not only can code be dynamically compiled into the running image but class definit…

> there's even a version of the JVM with code reloading.

Yes, because of Java's extensible classloading mechanism, all you have to do is drop an war/ear to the 'deploy' folder of your application server. New files are deployed, rewritten files are redeployed and deleted files are undeployed.

Re: The Erlang Shell

#16

Post by bitwalker »

agentultra wrote: post #12
bitwalker wrote: post #8

Earlier quoted context omitted.

Is it actually possible to do hot code reloading while preserving the state of the running system in Common Lisp? I'm genuinely curious, I kind of had this impression that it was a feature fairly unique to Erlang.

> Is it actually possible to do hot code reloading while preserving the state of the running system in Common Lisp? Yes. > I'm genuinely curious, I kind of had this impression that it was a feature fairly unique to Erlang. It's not. iIRC there's even a version of the JVM with code reloading. What's interesting about Common Lisp is that not only can code be dynamically compiled into the running image but class definit…

> It's not. iIRC there's even a version of the JVM with code reloading.

I've seen some info about the JVM implementation, but I could've sworn it wasn't able to preserve state while hot swapping. I haven't dug in to it though, so it's totally possible it handles that.

> There are a tonne of features in Common Lisp like this that are geared towards robust, maintainable software.

I'm a fan of Lisp in general, though my experience is limited to a tiny bit of Scheme and experimenting with Clojure. I'm glad to see that stuff like this is available in at least one of the Lisp variants! Question for you (if you program in Common Lisp): What are the main selling points? I spend a lot of time already with a couple of languages, Elixir/Erlang (for fun), C#/F# (for work) - what is it that would make me want to switch from one of those to Common Lisp?

Thanks!

Re: The Erlang Shell

#17

Post by phamilton »

simula67 wrote: post #15
agentultra wrote: post #12

Earlier quoted context omitted.

> Is it actually possible to do hot code reloading while preserving the state of the running system in Common Lisp? Yes. > I'm genuinely curious, I kind of had this impression that it was a feature fairly unique to Erlang. It's not. iIRC there's even a version of the JVM with code reloading. What's interesting about Common Lisp is that not only can code be dynamically compiled into the running image but class definit…

> there's even a version of the JVM with code reloading. Yes, because of Java's extensible classloading mechanism, all you have to do is drop an war/ear to the 'deploy' folder of your application server. New files are deployed, rewritten files are redeployed and deleted files are undeployed.

Is there a mechanism to migrate existing data to the new code?

Re: The Erlang Shell

#18

Post by brickcap »

brickcap wrote: post #2

> As an Erlang programmer I often claim that “You can’t pick parts of Erlang and then claim you have the same functionality. It all comes together as a whole”. I am guilty of this. I am relatively new to erlang programming and I have skipped on some of the features that make the language great. For instance I have neglected learning both supervision and hot code reloading.

@lostcolony @banachtarski thanks guys. I will start with supervision first then. See if I can make use of it in some of my existing code.

Re: The Erlang Shell

#19

Post by diminish »

phamilton wrote: post #13

I'm another recent Erlang convert (though I'm still trying to get used to the paradigm and ecosystem). There are a few concepts that haven't quite clicked for me. I come from a Rails background. While Erlang's hot code reloading and the ability to attach to running nodes is cool, I am not quite able to connect the dots on how this is useful. In Rails, a process is short-lived by design (the lifecycle of a request). A…

curious, which type of projects do you plan to develop in Erlang? especially after rails.

Re: The Erlang Shell

#20

Post by biokoda »

phamilton wrote: post #13

I'm another recent Erlang convert (though I'm still trying to get used to the paradigm and ecosystem). There are a few concepts that haven't quite clicked for me. I come from a Rails background. While Erlang's hot code reloading and the ability to attach to running nodes is cool, I am not quite able to connect the dots on how this is useful. In Rails, a process is short-lived by design (the lifecycle of a request). A…

When you are running in production and something is not right, you can attach to the console and see what is going on. This is of course not something that you should do often, but can be a real life saver.

Simulating something that is designed to work with thousands or even millions of users at the same time is pretty much impossible. Hot code loading and attaching to console gives you the option to see what is going on in real time and even fix it if you have to.

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