My first experience doing a computer system project was building a server using vanilla Java and then a client on an Android phone. Since then, I've found that there are a lot of frameworks to help manage scalability and remove the need to write boilerplate code.
I'm trying to understand what services like Tokio and Rayon enable.
I came across this paragraph on the Tokio tutorial page and I'm having a hard time understanding it
When you write your application in an asynchronous manner, you enable it to scale much better by reducing the cost of doing many things at the same time. However, asynchronous Rust code does not run on its own, so you must choose a runtime to execute it.
I first thought a "runtime" might refer to where the binary can run, but it looks like Tokio just provides functions that are already available in the Rust standard library while Rayon implements functions that aren't in the standard library.
Are the standard implementations for asynchronous functions written poorly in the standard library or am I not understanding what service Tokio is providing?
Rust currently does not provide an async runtime in the standard library. For full details, see Asynchronous Programming in Rust, and particularly the chapter on "The Async Ecosystem."
Rust currently provides only the bare essentials for writing async code. Importantly, executors, tasks, reactors, combinators, and low-level I/O futures and traits are not yet provided in the standard library. In the meantime, community-provided async ecosystems fill in these gaps.
Rust has very strict backward compatibility requirements, and they haven't chosen to lock-in a specific runtime. There are reasons to pick one over another (features versus size for example), and making it part of the standard library would impose certain choices that aren't clearly the right ones for all projects. This may change in the future as the community projects better explore this space and help determine the best mix of choices without the strong backward compatibility promises.
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I'm trying to write a library that will connect to remote servers and exchange data. I did this in C++ using Boost::Asio and am trying to do the same with Rust.
One of the problems I have is mapping concepts from Asio, like async_write/read to Tokio, starting with the fact that seemingly all Tokio examples demand that I replace my main() with an async main(), while I would like to encapsulate all my async code in structures and associated implementations.
Is it possible to use Tokio without replacing main()? Is mio perhaps the only way?
You can create a runtime manually using Runtime::new() which is what the tokio main macro is doing under the hood. It's just for an awful lot of apps, especially examples that's just boilerplate. So the macro automates the simple case.
However, depending on the context of your library, it may be more idiomatic to provide a future based API, and then leave the app consumer to set up the runtime.
I'm building a library to be used on a no_std platform which allows you to do some common network-related IO, such as making HTTP requests or reading from/writing to Websockets.
Now, I would like this library to be a well-behaved citizen so that it can be easily included in other no_std applications. I hence want to package the library by implementing reasonable traits etc. The library would allow me to not have to use alloc, so supporting non-alloc no_std would be ideal.
These are the options I have looked at:
embedded_hal and nb: These crates are really low level (no generic traits like Read and Write or anything higher level) and the async model doesn't seem to be compatible with async/await
genio/core_io/...: These don't support async IO at all.
embrio: Seems interesting but it seems like using it would tie me to one specific environment, making the library less portable.
tokio v0.2.x: I would love to use it but there is no no_std support at all.
futures::io v0.3.x: Again, would love to use it but there is no no_std support.
Which async IO abstraction should I use in a no_std environment? If there is no good option right now, which one should I bet on/help out with for the future?
Take a look a look at embassy-rs. There is a very active community. Currently, embassy-rs supports;
Hardware abstraction layers
Time
Networking
Bluetooth
Lora
USB
DFU
All built on rust async. There are also some really nice macros to generate static buffers for tasks, so you don't need alloc.
As far as I understand libgreen is not a part of Rust standard library anymore. Also I can't find a separate libgreen package. There are a few alternatives - coroutine, which does not provide actual green threads for now, and green-rs, which is broken. Do I right understand that for now there is no lightweight Go-like processes in Rust?
You are correct that there's no lightweight tasking library in std (or the rest of the main distribution), that green doesn't compile and that coroutine doesn't seem to fully handle the threading aspect yet. I do not know of any other library in this space.
As for what happened: the RFC linked to by that issue—RFC 230—is the canonical source of information. The summary is that it was found that the method by which green threading/IO was handled (std tried to abstract across both models, allowing them to be used interoperably automagically) was not worth the downsides. Now, std aims to just provide a minimum base-line of useful support: for IO/threading, that means "thin", safe wrappers for operating system functionality.
Read this https://aturon.github.io/blog/2016/08/11/futures/ and also:
Steve Klabnik's response in the comments:
In the beginning, Rust had only green threads. Eventually, it was
decided that a systems language without systems threads is... strange.
So we needed to add them. Why not add choice? Since the interfaces
could be the same, why not abstract over them, and you could just
choose which one you wanted?
At the same time, the problems with green threads by default were
becoming issues. Segmented stacks cause slow C interop. You need a
runtime to manage them, etc. Furthermore, the overall abstraction was
causing an unacceptable cost. The green threads weren't very green.
Plus, with the need to actually release someday looming, decisions
needed to be made regarding tradeoffs. And since Rust is supposed to
be a systems language, having 1:1 threads and basically no runtime
makes more sense than N:M threads and a runtime. . So libgreen was
removed, the interface was re-done to be 1:1 thread centric.
The 'release someday looming' is a big part of it. We want to be
really stable with Rust, and with all the things to do to actually
ship a 1.0, we didn't want to crystallize an interface we weren't
happy with. Heck, we pulled out a lot of libraries that are even less
important for similar reasons, like rand. Engineering is all about
tradeoffs, and we decided to choose minimalism.
mio is a non starter for us, as are most of the other async i/o frameworks for Rust, because we need Windows and besides we don't want
to get locked into an expensive to replace library which may get
orphaned.
Totally understood here, especially in the general case. In the
specific case, mio is going to either have Windows support, or a
windows-specific version of mio is going to be released, with a
higher-level package providing the features for all platforms. And in
this case, it's maintained by one of the people who's currently using
Rust heavily in production, so it's not likely to go away anytime
soon. But, unless you're actively involved, it's hard to know things
like that, which is, of itself an issue.
One of the reasons we were comfortable removing libgreen is that you
can write your own libraries to do different kinds of IO. 1.0 is a
strong core that we feel good about stabilizing forever, not the final
bit. Libraries like https://github.com/carllerche/mio can test out
different ways of handling things like async IO, and, when they're
mature enough, we can always pull them back in the standard library if
need be. But in the meantime, it's just one line to your Cargo.toml to
add them in.
And such text from reddit:
Unfortunately they ended up canning the greenlet support because
theirs were slower than kernel threads which in turn demonstrates
someone didn’t understand how to get a language compiler to generate
stackless coroutines effectively (not surprising, the number of
engineers wired the right way is not many in this world, but see
http://www.reddit.com/r/rust/comments/2l0a4b/do_rust_web_servers_use_libuv_through_libgreen_or/
for more detail). And they canned the async i/o because libuv is
“slow” (which it is only because it is single threaded only, plus
forces a malloc + free per async operation as the buffers must last
until completion occurs, plus it enforces a penalty over synchronous
i/o see
http://blog.kazuhooku.com/2014/09/the-reasons-why-i-stopped-using-libuv.html),
which was a real shame - they should have taken the opportunity to
replace libuv with something better (hint: ASIO + AFIO, and yes I know
they are both C++, but Rust could do with much better C++ interop than
the presently none it currently has) instead of canning
always-async-everything in what could have been an amazing step up
from C++ with most of the benefits of Erlang without the disadvantages
of Erlang.
For newcomers, there is now may, a crate that implements green threads similar to goroutines.
I'm looking for a way to run an arbitrary Haskell code safely (or refuse to run unsafe code).
Must have:
module/function whitelist
timeout on execution
memory usage restriction
Capabilities I would like to see:
ability to kill thread
compiling the modules to native code
caching of compiled code
running several interpreters concurrently
complex datatype for compiler errors (insted of simple message in String)
With that sort of functionality it would be possible to implement a browser plugin capable of running arbitrary Haskell code, which is the idea I have in mind.
EDIT: I've got two answers, both great. Thanks! The sad part is that there doesn't seem to be ready-to-go library, just a similar program. It's a useful resource though. Anyway I think I'll wait for 7.2.1 to be released and try to use SafeHaskell in my own program.
We've been doing this for about 8 years now in lambdabot, which supports:
a controlled namespace
OS-enforced timeouts
native code modules
caching
concurrent interactive top-levels
custom error message returns.
This series of rules is documented, see:
Safely running untrusted Haskell code
mueval, an alternative implementation based on ghc-api
The approach to safety taken in lambdabot inspired the Safe Haskell language extension work.
For approaches to dynamic extension of compiled Haskell applications, in Haskell, see the two papers:
Dynamic Extension of Typed Functional Languages, and
Dynamic applications from the ground up.
GHC 7.2.1 will likely have a new facility called SafeHaskell which covers some of what you want. SafeHaskell ensures type-safety (so things like unsafePerformIO are outlawed), and establishes a trust mechanism, so that a library with a safe API but implemented using unsafe features can be trusted. It is designed exactly for running untrusted code.
For the other practical aspects (timeouts and so on), lambdabot as Don says would be a great place to look.
In the future, will managed runtimes provide additional protections against subtle data corruption issues?
Managed runtimes such as Java and the .NET CLR reduce or eliminate the possibility of many memory corruption bugs common in native languages like C#. Nonetheless, they are surprisingly not immune from all memory corruption problems. One intuitively expects that a method that validates its input, has no bugs, and robustly handles exceptions will always transform its object from one valid state to another, but this is not the case. (It is more accurate to say that it is not the case using prevailing programming conventions--object implementors need to go out of their way to avoid the problems I describe.)
Consider the following scenarios:
Threading. The caller might share the object with other threads and make concurrent calls on it. If the object does not implement locking, the fields might be corrupted. (Perhaps--unless notified that the object is thread-safe--runtimes should use an interlock on every method call to throw an exception if any method on the same object executing concurrently on another thread. This would be a protection feature and, just like other well-accepted safety features of managed runtimes, it has some cost.)
Re-entrancy. The method makes a callout to an arbitrary function (such as an event handler) that ultimately calls methods on the object that are not designed to be called at that point. This is even trickier than thread safety and many class libraries do not get this right. (Worse yet, class libraries are known to poorly document what re-entrancy is allowed.)
For all of these cases, it can be argued that thorough documentation is a solution. However, documentation also can prescribe how to allocate and deallocate memory in unmanaged languages. We know from experience (e.g., with memory allocation) that the difference between documentation and language/runtime enforcement is night and day.
What can we expect from languages and runtimes in the future to protect us from these problems and other subtle problems like them?
I think languages and runtimes will keep moving forward, keep abstracting away issues from the developer, and keep making our lives easier and more productive.
Take your example - threading. There are some great new features on the horizon in the .NET world to simplify the threading model we use daily. STM.NET may eventually make shared state much, much safer to handle, for example. The parallel extensions in .NET 4 make life very easy for threading compared to current technologies.
I think that transactional memory is promising for addressing some of these issues. I'm not sure if this answers your question in some way but this is an interesting topic in any event:
http://en.wikipedia.org/wiki/Software_transactional_memory
There was an episode of Software Engineering Radio on the topic a year or so ago maybe.
First of all, "managed" is a bit of a misnomer: languages like OCaml, Haskell, and SML achieve such protections and safety while being fully compiled. All relevant "management" occurs at compile time through static analysis, which aids optimization and speed.
Anyway, to answer your question: if you look at languages like Erlang and Haskell, state is isolated and immutable by default. With kind of system, threading and reentrancy is safe by default, and because you have to go out of your way to break these rules, it is obvious to see where unsafe code can arise.
By starting with safe defaults but leaving room for advanced unsafe usage, you get the best of both worlds. It seems reasonable that future systems that are safe by your definition may follow some of these practices as well.
What can we expect in the future?
Nothing. Thread-state and re-entrancy are not problems I see tools/runtimes solving. Instead I think in the future people will move to styles that avoid programming with mutable state to bypass these issues. Languages and libraries can help make these styles of programming more attractive, but the tools are not the solution - changing the way we write code is the solution.