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

[Michael Santos]

On Mon, Jan 05, 2015 at 08:55:06PM -0500, Christopher Phillips wrote:
>   Hmm, I've had so little experience with tuntap interfaces I didn't think
> of them. That sounds like the way to go to get things working on OSX,
> though at a much larger amount of effort (both the learning curve on my
> part, and re-implementing the degradation behaviors I get for free from the
> kernel). Thanks for suggesting it; once I get things to where it's usable
> on Linux in all the ways I want, some research and a rewrite may be in
> order to get it fully portable.

Sure, that is definitely reasonable.

Working with tuntap devices isn't too difficult. For comparison, here is
a very simple example that forwards ethernet frames over erlang
distribution and randomly drops or delays a percentage of the frames:

https://gist.github.com/msantos/f2823fcba40975003dc3

Set up 2 distributed nodes:

    erl -name n@REDACTED -setcookie CHOCOLATEBUTTERSCOTCH
    erl -name n@REDACTED -setcookie CHOCOLATEBUTTERSCOTCH

Then create the tunnel:

    % Drop 10% of the frames, randomly delay 40% up to 1 second
    wastrel:start('n@REDACTED', "10.1.1.1", "10.1.1.2", 0.1, 0.4).

Try to login using ssh (or whatever)!

    ssh 10.1.1.2

Meh, I've had to use worse ...
    
Notes:
* relies on this erlang tuntap driver

    https://github.com/msantos/tunctl

* beam needs to be running as root or have CAP_NET_ADMIN set:

    sudo setcap cap_net_admin=ep /usr/local/lib/erlang/erts-*/bin/beam*

> On Mon, Jan 5, 2015 at 8:11 PM, Michael Santos <michael.santos@REDACTED>
> wrote:
> 
> > On Mon, Jan 05, 2015 at 09:16:42AM -0500, Christopher Phillips wrote:
> > > For OSX ipfw was deprecated in Lion and removed in Yosemite. I've done a
> > > bit of looking at the replacement, pf, and it looks like dropping packets
> > > based on percentage is doable, as is bandwidth throttling (something I'd
> > > like to add, in general), but I don't see any way to induce a delay,
> > beyond
> > > an implicit one based on tos prioritization. If someone knows how and can
> > > point me in the right direction I'd appreciate it.
> >
> > The portable way is to use a tuntap device. Then you can arbitrarily
> > drop packets, throttle bandwidth, introduce latency, whatever, from your
> > code. Sort of like quickcheck for networks :)
> >
> > > On Mon, Jan 5, 2015 at 6:52 AM, Sergej Jurečko <sergej.jurecko@REDACTED
> > >
> > > wrote:
> > >
> > > > This looks like a great tool and something that could easily be added
> > to
> > > > unit tests.
> > > > Anyone with ipfw skills to add bsd/osx support?
> > > >
> > > > Sergej
> > > > On Jan 5, 2015 1:41 AM, "Christopher Phillips" <lostcolony@REDACTED>
> > > > wrote:
> > > >
> > > >> 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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> > > >>
> > > >>
> >
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> >