[erlang-questions] [ANN]: Damocles, a library for testing distribution scenarios on a single machine

[Christopher Phillips]

https://github.com/lostcolony/damocles

I asked a while back on this mailing list if anyone had any useful
libraries or similar for testing distribution scenarios. I only got back a
few responses (maybe co-op riak_test? Maybe make use of the underlying
Linux traffic control and network emulation apps?), and my own searches,
while finding a few libraries, didn't find anything I could easily co-op
for my purposes.

To that end, I went ahead and spent part of my break on this, and it just
got sufficiently feature complete to throw out there. I haven't had a
chance to really start using it heavily, and I've only been testing it on
my dev box, but a basic run through of the functionality as I typed up the
readme worked (so any issues being pointed out would be appreciated).
Essentially, it allows you to create and manipulate local interfaces on a
Linux machine to emulate packet delay and loss (using the underlying
traffic control and network emulation mechanisms), with a number of
convenience methods to (hopefully) easily describe fairly intricate
distribution scenarios.

Things like "create these 5 interfaces, (now from my test code, launch a
copy of my app on each one, or even a different app on one of them, to see
what happens when that resource is flaky); now make it so 1 and 2 can't
talk to 3 and 4, and vice versa, but everyone can still talk to 5, but
replies have a 50% chance of being dropped from 5 when responding to 1 and
2, and there's a 300ms delay between 3 and 4; (now, let's run more of our
test code to assert that trying to write to any node still succeeds); okay,
now let's restore the network back to normal (and have our test code make
sure the write was retained)", or whatever, can be set up in a
straightforward, automated manner as part of a common test run, and not be
reliant on certain VMs being up, nor the tests being run on a specific
network. The tradeoff, obviously, being that you can't really load test
things with it. Still, it fits my basic needs, and I figured it might be of
use to others.

I'll be adding some simple examples when I next get free time (I ran out of
it from the holiday break without getting to them; dunno when I will), and
will try and get to any bugs or simple suggestions in a timely manner, but
hopefully it's fairly straightforward and useful as is.
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