1. Head-of-line blocking caused by congestion on the single TCP connection used for message transmission between any two Erlang nodes. This can cause major problems for apps control vs. data plane design, such as Riak and Lasp. Work on Partisan (https://github.com/lasp-lang/partisan) appears to be a substantial improvement.
2. If the single remote distribution TCP connection between two nodes is broken, then the first Erlang process to send a message to the remote node is, hrrm, well, borrowed/co-opted by the BEAM VM to connect the new TCP session. IIRC that process is marked unrunnable by the scheduler until the connection is set up or perhaps there's an error. If that process is really important to the functioning of your app, for example, an important system manager in the control plane of the app, then you have a very difficult-to-diagnose latency Heisenbug to cope with.
-Scott
More generally, it doesn’t work when you have multiple schedulers, which nowadays is true for nearly all Erlang systems.
I’ve heard speculation that it really only helped scheduler efficiency and this didn’t apply all that well on SMP schedulers and has finally been removed since non-SMP builds are no longer supported.
As for Pony, their actor model doesn’t support selectively receiving messages so their queue handling may have some different trades to make there, as well as interaction with primitives like promises (which are used where Erlang would prefer a ref tagged receive clause). Spooneybarger has done work on backpressure in Pony so maybe he or other Wallaroo labs people can elaborate.
I never speak of either being better than the other. Rather, they are different. I'm an expert on backpressure in Pony, not so much in Erlang, so I'll leave it at that.
- The actor model for high-level coordination and communication
- CSP and/or FBP at the low-level, for high performance processing.
The reason for this is mainly that CSP/FBP inherently supports blocking channels between processes when processes are busy (or channels are full for FBP), thus providing implicit backpressure. This can simplify low-level programming a lot, since you don't need to worry about over-filled mailboxes or overbooked CPUs. OTOH, for communication e.g. across a cluster, this becomes problematic since you can't rely on the same level of integrity of the network connections ... meaning that the very communication-intensive nature of CSP/FBP might be less of a good fit.
Thus in my view, the actor model would fit better for communicating between services which are internally implemented e.g. with CSP/FBP.
I wrote a post about this, some time ago:
http://bionics.it/posts/flowbased-vs-erlang-message-passing
It will be interesting to see whether Pony manages to fill more than just the coordination role. Given its much higher raw-crunching performance, perhaps that is the case. I'm a little worried about the lack of implicit back-pressure (due to the actor model), but let's see.
Perhaps because of its high-performance, back-pressure won't be too costly to implement anyway? One can hope. Would be nice with a language that can encompass both programming paradigms, and be suited for the full spectrum between low-level crunching and high-level coordination and communication.