riak_core nodes on Heroku?
Ciprian Dorin Craciun
ciprian.craciun at gmail.com
Sun Jun 17 07:47:05 EDT 2012
On Thu, Jun 14, 2012 at 4:37 PM, Samuel Elliott <sam at lenary.co.uk> wrote:
> It's actually very much not possible.
> Erlang nodes need to listen on one port for the EPMD, and then another
> for HTTP (and maybe more for other coordination). Heroku only lets you
> listen on a single node per process, and you can't guarantee scaled
> processes occur on the same dyno or even machine to another, so you
> can't explicitly run epmd separately.
Although not strictly related to Heroku's dyno's (for which I
don't have too much "in-depth" knowledge), in a project I'm currently
working, that provides a similar environment with Heroku, a major
problem in running distributed Erlang nodes (epecially Riak), is the
EPMD process. (From my point of view this is quite a serious
limitation for Erlang and the OTP...)
The problem is as follows (I make one assertion presented below,
but which I'm certain is FALSE, thus this paragraph is solely for
* all the Erlang nodes in a distributed application must use the
same EPMD port (as specified by using `ERL_EPMD_PORT`);
* now if Heroku provides you with an unique IP per dyno then this
is no problem, as each Erlang node dyno could run his own `epmd`
* if Heroku doesn't provide an uinque IP per dyno then "game
over"; (although it could seem it works, but if two dynos of your own
run on the same machine, then when one dies, the other remains
"invisible" from further discoveries;)
The assertion I've made is that Heroku doesn't firewall
communication on private IP's between arbitrary dyno's. WHICH I DON'T
THINK IS THE CASE, as I would be an important security issue...
> Yes, distribution via other means seems sensible.
As Sam said, you could however solve these issues by:
* providing your own implementation of an `erl_epmd` solver;
* and your own implementation of an `inet_tcp_dist`;
* both using as communication an HTTP module (like `mochiweb` or
`misultin`) multiplexing between the two;
But this road is very hard... If interested you could read one of
my posts to the Erlang list exactly about this topic, but which
(One ideea: use ZeroMQ for communication, but again all this goes
if the previous assertion is true.)
(Another one: use RabbitMQ, to provide transport for such modules.
You could use direct exchanges for `inet_amqp_dist` and you wouldn't
need anymore the `erl_epmd` resolver.)
> As for the routing mesh issues, they seem to have support for
> websockets etc in the proxy, so presumably it is possible somehow to
> route TCP messages through.
I have the feeling that Heroku (on their Cedar) stack use the
`mochiweb` Erlang web-server to provide the routing... As such you
could try to find "undocumented" ways to fool mochiweb into routing
requests to the respective dyno.
Hope this helps,
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