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Subject: Re: [boost] [asio] RFC on new reliable UDP library
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-09-10 11:20:58

On 9 Sep 2014 at 15:03, Rodrigo Madera wrote:

> Looks very interesting and I would love to see that lib happen.
> I believe in building up on knowledge recursively. So the first thing I
> would do is read on the enormous effort ZeroMQ [1] has put into analysing
> this problem domain. They have a (very) large set of material debating and
> explaining design decisions made on the library. After that, there is
> nanomsg [2], which is a derivative work from the lessons learned from
> ZeroMQ itself. Have you read through those in search of insights? It would
> be a very good source of debate material for this brand new library. There
> are problems they have encountered already, though you may decide to solve
> them differently.

Yes, we did. And therein lies the debate: ASIO's design was a product
of its time, and while its design was close to optimal for its time,
it now makes life harder than it should be in a world with 40Gbit
ethernet, mobile devices where power consumption is important, and
where non-Linux operating system kernels can DMA straight to and from
user space if and only if you are very careful with how you use ASIO.

The debate lies in to what extent does one break with ASIO's
established norms and practices to achieve the features I listed.
Even deciding on how much C++ 14 to use, or how much to embrace C++
17 resumable functions is controversial.

> Of course, that is if you haven't read that already. It just reminded me of
> how many insights these guys had in all the years they spent using and
> creating network middleware code. And basing a new network library off of
> this knowledge could be good.

I would strongly emphasise that no one is proposing we try doing
ZeroMQ or anything like it "better". In our reliable messaging
library we would provide the absolute minimum abstraction with which
one could build *any* reliable messaging solution of their choice.
The contract does involve the replication of at least one reliable
UDP implementation (actually one of Chris' early implementations) as
the original has heavily bitrotted, but apart from that we were
thinking of UDT as a second backend, and leaving it at that. We chose
UDT mainly because we believe we can reuse some of it for duplicating
Chris' RUDP implementation.

> About the language, I would totally look forward into C++11 and up design.

Be aware that C++ 11, and especially C++ 17 as it currently is
thought to become, consider future-promise as the sole way for i/o
completion handlers to be invoked via the new
future<T>.then(callable) function. In particular, 17's resumable
function support *requires* that the function must return a future,
so if you want language based coroutine support, you must use

As much as Chris and many ASIO users are opposed to that design
choice, we must play with the hand we have been dealt by WG21. I will
say that future-promise can be made constexpr and no-alloc, and that
makes an awful lot of the inefficiency go away, though an implicit
synchronisation is unavoidable as .then() must check if the operation
has already completed.

On 9 Sep 2014 at 21:31, Andrew Marlow wrote:

> Is it going to include reliable multicast? If so then a good place to
> start is the RFCs and various attempts at reliable multicast in the past.
> The two that spring to mind are NORM (
> for which there is a reference implementation, and PGM (Pragmatic General
> Multicast, Don't forget Scalable
> Reliable Multicast, SRM,
> There are
> whole frameworks based just on reliable multicast, e.g. spread,
> Personally, I think this is far too big an area
> for boost. $0.02.

We are very much thinking of an abstraction framework with which one
could implement things like the above. The hope would be that our new
library would take care of platform-specific hassle like making whole
system zero-copy networking right to and from the NIC disappear,
stuff like that.

We are also considering a monadic continuations framework allowing
code just above ASIO to use ASIO generically. In other words, the
standard C++ i/o monad equals ASIO. This ought to very significantly
ease the writing of highly generic code avoiding much of the
boilerplate you often need to join up ASIO async handlers, and
therefore should make much easier implementing ZeroMQ or reliable
multicast or any other messaging protocol.

Another debate is indeed where the bottom level wire protocol should
end and the monadic high level part should begin. For example some
feel this approach would make impossible the reuse of third party
implementation libraries.


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