A Conversation with Guido about Callbacks
oubiwann.blogspot.com
oubiwann.blogspot.com
For Python, you can get the benefits of asynchronous IO without callback hell if you use the Stackless fork.
A similar idea for Node is node-fibers.
Both let you build exception-safe coroutines.
On top of that, gevent outperforms both twisted and tornado because the eventloop is libev, meaning it's written in C instead of pure python.
To be clear: gevent is significantly faster than eventlet and offers the same functionality.
- It can't use multiple cores, even if Python got the ability in the future, gevent code simply wouldn't work as intended.
- While gevent is not preemptive, context switches are implicit, that means you cannot write code without locks unless you want to do some handwaving. You have to know the entire system to know that a multistep operation that calls functions is actually safe.
On the net, gevent may be better than Twisted or Tornado, but I think that is saying more about how much those suck than how good gevent is.
It is intended to create an environment similar to preemptive threads, yet due to it being a networking library it is unlikely gevent users will have to use the semaphore and locking primitives provided with gevent.
Regarding the weaknesses of callbacks, I disagree and would like to see evidence supporting your case.
> - It can't use multiple cores, even if Python got the ability in the future, gevent code simply wouldn't work as intended.
This is pure speculation. If Python's multicore handling was better (existed?), it's likely gevent would be implemented similar to other systems where you have threads allocated to each core and the concurrency primitives can then be executed on cores according to some algorithm.
> - While gevent is not preemptive, context switches are implicit, that means you cannot write code without locks unless you want to do some handwaving. You have to know the entire system to know that a multistep operation that calls functions is actually safe.
While this is true, I don't think it is likely to occur. Gevent's creator agrees: http://www.gevent.org/intro.html#cooperative-multitasking
> On the net, gevent may be better than Twisted or Tornado, but I think that is saying more about how much those suck than how good gevent is.
You're an Erlang programmer. You have to have that opinion.
(sausagefeet and I are friends IRL so I know his biases)
The weakness of callbacks is the lack of multicore support. It is not pure speculation that gevent simply won't work in a preemptive environment if it wants to utilize multiple cores. The sentence right about this makes my point:
> yet due to it being a networking library it is unlikely gevent users will have to use the semaphore and locking primitives provided with gevent.
Why is that? It's because gevent context switches on I/O events, not preemptively, so you have some control over when context switches happen. You can depend on 'my_dict[foo] += 1' to happen atomically. If you want to utilize multiple cores with gevent, as code using it is written now, you can't because all of the code out there depend on operations between I/O events happening atomically. This isn't a secret in the callback/gevent world. On the contrary, it's presented as a selling point. No longer worry about those locking primitives those real threading libraries give you because we are single threaded. (Ocaml/Lwt has the same problem).
> While this is true, I don't think it is likely to occur
I'll concede it's far less likely. I'm curious how large gevent code bases are in the wild, a large application that one cannot fit in their entire head could very well have a lot of the same race conditions that a preemptively threaded application has.
> You're an Erlang programmer. You have to have that opinion
Really? I didn't realize when I started using Erlang I signed a contract saying that. I also write Ocaml, so I guess any criticism I levy against Java is a product of that too, not rational thought.
You suggest that gevent won't work in preemptive environment if it wants to support multiple cores, alas I'm still not convinced as to why.
The greenlets are cooperatively scheduled coroutines, as we know, but with proper multicore support I don't see why they couldn't adapted to be preemptively scheduled. If we're imagining that Python now has multicore support, we can also imagine greenlets being updated to work with it. Assuming gevent would receive no modifications to work with such an important update to Python is why I accused you of speculating.
> It's because gevent context switches on I/O events, not preemptively, so you have some control over when context switches happen.
That's correct. They are cooperatively scheduled coroutines. If someone wants to context switch, they can write code that does it or wait for I/O events. The lack of an explicit yield statement does not prevent coroutines from switching. In fact, greenlets are quite powerful here, more powerful than what's offered by Twisted and Tornado, because they don't have to yield directly to the caller. You could imitate CPS which is impossible with the explicit context switch model.
I am not disagreeing with you on this point, btw, just trying to be more explicit for other readers about the cooperatively scheduled nature.
Interested readers might want to check this out: http://en.wikipedia.org/wiki/Computer_multitasking#Cooperati...
> I'm curious how large gevent code bases are in the wild
Well, eventlet itself was written to support Second Life and sits behind multiple big video games.
Gevent is in production use for many large companies, most notably Spotify. They are moving to gevent from twisted for performance and cleaner code.
> I didn't realize when I started using Erlang I signed a contract saying that. I also write Ocaml, so I guess any criticism I levy against Java is a product of that too, not rational thought.
My statement is facetious, but the reality is that Erlang got concurrency right and Python is trying to glue concurrency ideas into the language as an afterthought. If Python didn't have it's lousy GIL we wouldn't have to pull such tricks to work concurrency into a single threaded system. Alas, it has the GIL.
Now, to conclude some of my thoughts, I believe gevent is an excellent way to build web systems. The coroutines are short-lived, exist to fetch some data with an easy nonblocking interface and then concat some strings together to produce the web page / API output.
If folks were building CPU bound systems it would straight-up be a mistake to use Python for that. Ocaml could be a better choice, alas, as you say, it has the same issue with a GIL though the computations would execute a LOT faster.
Edit: Solve the problems you actually have.
I hope this conversation is informative for readers.
UI programming is callback heavy, but I doubt that any language will support "better" ways for that.
So basically UI programmers are used to callbacks, others not that much, and now that event-loop servers are becoming popular, we see confusion from people not familiar with events.
Server-side people just need to figure out how to use callbacks properly and everything is gonna be fine.
All people need to understand is to nest callbacks properly, nested callbacks are the things that confuse most people.
> For Python, you can get the benefits of asynchronous IO without callback hell if you use the Stackless fork.
You can't just abstract away every programing feature that seems hard and expect everything to be fine, it just doesn't work that way, Java developers are still suffering because Java tries real hard to abstract memory management.
I wonder why you mean by using callbacks properly ? The critical point mentioned by Guido is error/corner-case handling: the idiomatic callback-based code I have seen becomes difficult very quickly. I have not seen a case where e.g. deferred-based code ala twisted has been simpler than gevent.
I don't think that's right. There is a huge difference. UI callbacks ("onClick" and the like) execute synchronously. But the callbacks in event servers execute at completely different times. Indeed, they're typically nested inside code that has long since terminated. This is what causes the learning curve, not grokking events or functions-as-values, which any half-decent programmer can easily do in the unlikely event that they didn't a long time ago. Indeed, the way we write asynchronous callbacks is cognitively dissonant with everything we've been trained to understand about scoping. The idea of a nested scope that doesn't mean "contained within" or "covered by", but rather "at some unknown time later", is inconsistent enough with how programming languages normally work that it can easily trip you up even after you get good at it.
Asynchronous programming is a very different thing than e.g. browser UIs in Javascript. I've programmed those for years. It still took me a long time to get my head around async servers.
Rob Pike designed one of the spiritual ancestors of Go, Squeak ( http://doc.cat-v.org/bell_labs/squeak/ ) using the CSP model of concurrency specially to build (G)UIs.
Nobody particularly likes reading callbacky code, but it has a lot of power. What we don't have enough of is "structured alternatives" for our "goto."
The fact that Erlang has a richer set of concurrency semantics — that, among other things, let you quite easily manage a graph of dependencies so that your callback logic becomes much simpler — doesn't change the fact that it relies on callbacks (something "calls back" a function in order to supply information asynchronously) as a fundamental unit of programming.
I don't know gevent in detail, but I suspect that it's possible to create an Erlang-style process/supervisor-style framework on top of it, albeit with less elegance and more syntax than Erlang. That would change the underlying concurrency mechanism, but the interface — callbacks — would remain very similar.
Incidentally, while figuring out how to implement something inherently asynchronous in Go I looked at some IRC client implementations, and noticed how all of those used callback-style programming one way or another. That doesn't inspire much confidence. :(
All in all, I am not sure yet whether this is purely due to my inexperience with concurrent programming or whether I am just inherently incompatible with it. I'd love to be able to play with alternatives to callbacks in the future, but for now it's just way over my head for the things I want to do.
> and noticed how all of those used callback-style programming one way or another.
What do you mean precisely here, because I think you are conflating two concepts. There is a distinction between using a callback as a unit of concurrency (do this async task and call THIS function when you're done), and genuinely being an event handler (call THIS function when you get receive a particular IRC event). I would expect to see the latter in Go for IRC code, not the former.
But there is another aspect of this too, beyond code looking prettier. Go/Erlang/Haskell scale to multiple cores as the runtime evolves to support this. Callback codes doesn't, it can't. This doesn't seem to be because callback developers don't see the value in multiple cores, Node has built-in support for spawning a VM per core. This is quite restrictive. Callback developers have to either limit themselves to solving problems that don't need to communicate with each other or use some other means to communicate between VMs. That doesn't sound like progress to me. But shrug, plenty of people don't neat things in Node so maybe it doesn't matter.
You're probably right about the IRC clients using the latter style of callbacks. But then it turns pretty much into the following style:
irc_connect();
register_event_x(callback_x);
register_event_y(callback_y);
while(read())
dispatch_event();
On the scaling on multiple cores: Sure it's a nice advantage when you're already used to concurrent programming, but for most things I've written so far it doesn't really matter. In the few cases that I had a component that required heavy disk I/O or computational resources, it was only a small and easily-identifyable component that could be implemented in a separate OS thread. I'm not familiar with Node, but both glib2 and libev allow spawning a separate event loop in each thread and provide mechanisms to communicate between each other (idle functions in glib, ev_async in libev). Those are for C, however, and I have to admit that callback-style programming isn't too convenient there due to the lack of closures and automatic garbage collection.
> But in my mind that only complicates things as I can't tell anymore whether I can receive a particular event at some time because I have no idea whether there are any blocking handlers going on
Don't really get what you mean here. The select operator seems like it would solve the problem your implying.
> then suddenly I can't easily argue anymore about the order of message arrival and what effect that has on shared state
You can barely, sometimes not even, reason about the order of message arrival and it's effect on state in callbacks, though. At every point in a callback you have the entire state of the program to deal with, at least in Erlang or Go you only have the context of your local process/goroutine to deal with. I have seen plenty of callback code explode because, what seemed like, an obvious sequence of events got reorderd or sent before the previous work was expected to be done, or any variation, and the code didn't handle it properly. You might be thinking "well, duh, that is easy to fix and a silly mistake to make", but it isn't. In a shared memory, especially where mutability is the default mode of action, an event-driven framework can be represented as a function that takes a 2 dimensional matrix: 1 dimension is every variable in my program, the other is every event that can happen in my program. At every point in the program any variation of this matrix must be a valid state. In a language like Go, or Erlang, I can cut this matrix down significantly so that I only have to worry about the matrix involving some subset of events and some subset of variables. In short, that's a massive win IMO.
Here is my attempt at it - https://gist.github.com/2192413
Thoughts?