The Erlang Shell

by strmpnk • 12 years ago

127 60 12 years ago

brickcap2• 12 years ago

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.

banachtarski• 12 years ago

Well you're relatively new :). It takes time to absorb an entire language's ecosystem. As long as you take it upon yourself to acquaint yourself with those features, things will be fine.

The biggest thing to get on your initial survey should be minimal awareness so you know that that feature exists when you encounter the problem it was originally built to solve.

Supervision though, is fundamental to OTP and Erlang. It's certainly easy to learn so I would consider that a low hanging fruit for you.

Hot code reloading is a consequence of the continuation passing style that is prevalent in every Erlang process and easy to understand too (although employing it in production can be a bit trickier).

lostcolony2• 12 years ago

Hot code reloading I'd personally grant you as an exception; it's a maintenance tool, not a development tool, and you can largely ignore it until you want it, if you're adhering to OTP principles.

Ignoring supervision, however, is...not wise, if you have -any- sort of reliability considerations (even ones as weak as "I'd prefer it to remain up"; if it's just a one off thing that you'll manually execute again if it fails, meh, fine).

Supervision is really, really easy to use, at least for the base use case (creating a good hierarchy, so that processes restart the other processes that should be restarted, is an architectural consideration that can be a bit hairy), and it's truly heartening to grep a log for something you've been running a while, find crashes that indicate issues you want to fix (even simple stuff, like sanitizing inputs), but the system is still running as though nothing happened.

brickcap2• 12 years ago

@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.

rurban• 12 years ago

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

bitwalker4• 12 years ago

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.

devgutt• 12 years ago

An interesting example of Erlang hot code reloading in embedded system: http://www.youtube.com/watch?v=96UzSHyp0F8

agentultra2• 12 years ago

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.

lispm• 12 years ago

One loads and replaces incrementally in Lisp.

tel3• 12 years ago

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.

lostcolony2• 12 years ago

What were your thoughts re: Dialyzer?

tel3• 12 years ago

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.

rdtsc3• 12 years ago

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.

chubot4• 12 years ago

Nitpick: mixing threads with pointers isn't really the problem; it's mixing threads with shared mutable state.

A concurrency idiom I've gotten a lot of mileage out of is to use threads, without shared state, but with pointers. Instead of shared state, you use a message passing style where you pass pointers over queues. Threads only mutate data that they are explicitly handed via a queue. After putting an object on a queue, they NULL out their local reference.

It's basically like Go's goroutine idiom, but with C/C++/Python, real OS threads, and a thread safe queue (which is all a channel is). There is also some relation to the SEDA architecture.

The logical structure of such a program is not really that different than an Erlang program, I imagine. It just works a little better with existing C/C++ codebases, and is potentially faster in some cases.

It has a completely different character than spaghetti written using threads and shared state, and can be made extremely reliable. Especially because the explicit channels provide a hook for testing black box behavior.

rdtsc3• 12 years ago

That is true. One can build an almost shared nothing architecture in any of the languages supporting thread safe queues and threads.

It is a continuum. One could write it with shared mutable state too, using locks. The problem is it is easy to mess up.

C, a large program could load modules, and some of them are thread safe some are not. One could bring in a new module that calls some initialization routine (say curl_global_init()) and then another part of the system ends up calling it as well. But it is not thread safe and should only be called once. It is not intentional sharing, but it just happens as it is running in the same memory space.

Someone likened that to running your code in Windows 3.1 when you Command & Conquer game would crash and take down the word processor with it. They didn't intend on doing so but it happened by accident as a bug. Sometimes running your application as a Windows 3.1 system is acceptable, sometimes it isn't. It depends on the problem.

tbrownaw• 12 years ago

Instead of shared state, you use a message passing style where you pass pointers over queues.

If you don't have a queue library and don't want to risk getting the locking wrong, you can get one from pipe(2). While simultaneous writes/reads are in theory permitted to be interleaved, the implementation would have to be actively insane to interleave anywhere other than page boundaries.

...I actually did this when I was at university, for the first class that covered locking and concurrency. Amazingly enough, I didn't fail that assignment for delegating what we were supposed to be learning to the OS.

It has a completely different character than spaghetti written using threads and shared state, and can be made extremely reliable.

Maybe this is why I don't get all the C++-hate? The things that make code unreliable or thread-unsafe tend to also make it hard to read and think about (even without threads), so I just find some other way that doesn't give me a headache.

Interestingly code that doesn't give me a headache tends to have a strong resemblance to functional-style code, even tho actual pure functional code also tends to be mildly headache-inducing.

phamilton7• 12 years ago

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?

diminish• 12 years ago

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

phamilton7• 12 years ago

That's the question I can't answer. I have some problems I want to solve. If I build them in Erlang the way I would on Rails, then there's not much gain. I'm trying to figure out how Erlang will allow me to solve those problems differently. One problem is how to handle authentication across our private and public APIs. With Rails, I've got a db with credentials and tons of memcached running to minimize latency (since every endpoint needs to check auth). How would a long running application in Erlang handle that differently? How is that better than what we currently have?

biokoda• 12 years ago

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.

phamilton7• 12 years ago

I get that. For persistent connections, it's a life saver. But for a synchronous web service, every connection comes and goes quickly. What do you gain from attaching to the console in that situation that you wouldn't get from the rails console?

shanselman• 12 years ago

Why should you not do it often?

e_proxus3• 12 years ago

If you want to develop web pages, don't do Erlang. If you want to do web services, do Erlang. I've used both Ruby (Rails) and Python (trying to do concurrency with tornado, greenlets, gevent) and they just fell flat. You can do it, but it is incredibly messy and unstable (because there's so much work involved).

In Erlang, you would typically answer a web request in a new actor every time. Just as in Rails, running for example N number of threads serving one request each. Of course, in Erlang that number of actors can vary, which means your system will be more responsive and you'll have lower average latency. However, what differs mostly, is that in Erlang all those actors live inside one VM and you can have central actors managing things, collecting statistics, keeping global state, talking to databases / other services. This, I think, is the core strength of Erlang compared conventional single threaded / GIL systems.

A more concrete example would be to dynamically show the current request rate on a web page. You can just create an actor that gets a message from any other actor once they handle a request, and keeps track of how many such messages per second arrive. Then when rendering that web page, you just ask that actor for it's current value. Simple, beautiful, dynamic. No need for a database or any other central storage.

klibertp2• 12 years ago

If you want to develop web pages, don't do Erlang.

I'm not sure I agree. There's https://github.com/ChicagoBoss/ChicagoBoss and quite a few other frameworks (https://github.com/ChicagoBoss/ChicagoBoss/wiki/Comparison-o...) and there's also Elixir and some frameworks for it (https://github.com/dynamo/dynamo).

It's of course nowhere near the ease of development of simple web pages in Django or Rails, but it's not like you need to reinvent everything from scratch either. And you gain ability to scale almost without any effort. It's a tradeoff, of course, and using Erlang for every web page you build from now on would be probably an overkill. But there are web sites which are not services yet, which could still benefit from Erlang greatly. And I think (and hope) that the initial overhead of Erlang will get smaller and smaller over time as new frameworks emerge and that someday deploying your blog on top of Erlang will become viable choice :)

davidw3• 12 years ago

In Rails, a process is short-lived by design (the lifecycle of a request)

Yes and no. Rails processes are so huge that they are loaded up and kept around between requests. Erlang processes (which are not unix processes!) are much smaller and lighter. One Erlang program can handle tons of concurrent, always open requests, whereas Rails just isn't cut out for that.

As to what Erlang is good for, I think its sweet spot with web stuff is where you need a lot of always open connections, such as with web sockets.

I'm a fan of Erlang, but for most SaaS kinds of startuppy things, am not sure it's a good fit: you get so many more tools packaged up and easy to use for you with Rails that unless you know Rails does not work for you, I would almost always choose Rails.

Jacqued• 12 years ago

Since you recently started out, may I ask you what good resources you found to get started with Erlang ?

davidw3• 12 years ago

This is a good book to start with:

http://learnyousomeerlang.com/

Free on the web, but buy a copy - it's worth it.

Veratyr• 12 years ago

http://learnyousomeerlang.com/ seems to be the standard reference. I find it most amusing in both style and name (Learn You Some Erlang -> LYSE -> Lies) as well as informative.

Luc• 12 years ago

Besides 'Learn You Some Erlang', Joe Armstrong's new 'Programming Erlang' is very good, too: www.amazon.com/dp/B00I9GR4TW/

pessimizer• 12 years ago

In Rails, a process is short-lived by design (the lifecycle of a request).

I'm not sure if this is by design, or a limitation of Ruby.

short-lived processes aren't the best approach for a web service.

Not for me - I'm building all of my web services in Erlang these days, and there's little to no relationship between the life of most processes in the service and the life of requests. Although each request is usually serviced by its own process, that process may interact with other processes that may have been alive for as long as the application has been running. For example, my RAM caching strategies are usually written as part of the application, not put in front of them. Makes pseudo-ESI easy to hand-roll.

Also, short lived processes may have been spawned by long lived processes, so changing how they're created can be done without taking anything down.

I find this style so much more performant than the one thread per request thing from Ruby, Perl, PHP et al. It's very easy and natural to minimize the amount of time spent in the db (which is where I'm usually bound.)

phamilton7• 12 years ago

This is exactly the type of response I was looking for.

A few questions and comments:

1. RAM caching - I can see a big benefit to keeping cache in Erlang. No marshaling of data to and from memcached, for example. How do you handle eviction?

2. Avoiding the db - I'm going to guess this ties in with RAM caching. Are you doing a write through cache or something like that?

Thanks for your response, it's helping it all to come together for me.

_pmf_• 12 years ago

And all this without service interruption. And you get all this for free, just because you picked Erlang as the implementation language.

GDB or even VS's debugger can do a lot of magic even if you use C. Until absolutely necessary, leave debugging information reasonably high and optimization reasonably low or off and you'll get an amazing, dynamic environment to inspect a live C system.

skywhopper• 12 years ago

Reading about "correct software" as if it were an realistic goal makes me itch. Sure, we should be striving for as few bugs as possible, but any non-trivial application will never be made "correct", if only because it'd be impossible to write the full specification to compare it to.

The Erlang Shell

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