[erlang-questions] Erlang suitability

Ladislav Lenart lenartlad@REDACTED
Fri May 18 16:05:22 CEST 2012


I am by no means an Erlang expert, but...

Netsplits are an inherent part of a distributed solution. Erlang or not, you
will have to deal with them one way or the other. And even if some 3rd party
framework takes care of them for you, they are still there. Erlang shines here,
because it gives you the tools you need to build a solution tailored to your
specific problem. I don't know of any other language that has builtin language
constructs and libraries that deal with beasts such as netsplits and HW / SW
failures. So for me, Erlang would fit nicely to your problem description.

Regarding 75 nodes in a cluster...

One objection was against native erlang distribution protocol (fully connected
mesh of nodes), not against Erlang language + platform itself. You can fairly
easily build your own comunication layer on top of TCP/IP, term_to_binary/1 and
binary_to_term/1. For example you can divide 75 nodes to fully connected islands
with one node responsible for communication with outside world. I believe
"hidden nodes" should be of value here.

I know that not a long ago, someone posted on this very list that their app runs
on ~700 nodes (sorry I dont' remember more) so 75 nodes is certainly doable.

Another problem is mnesia, which as some pointed to you, was not built to run on
this number of nodes. Nevertheless there are other possibilities, for example
riak. See basho.com for more.


Ladislav Lenart

On 18.5.2012 14:29, Ovid wrote:
> Netsplits. Damn. I forgot to put my thinking CAP on. In this case, a netsplit
> would be disastrous unless we fell back to a central data store such as Redis.
> At that point, Erlang doesn't look like a solution at all.
> On top of that, you're the second person to point out that a complete graph with
> 75 nodes is problematic. Now that I think about it, I guess I can see why. It
> now sounds like an Erlang solution is not the quick win we thought it might be
> (quelle surprise!). 
> Thanks to everyone for all of your answers.
> Cheers,
> Curtis
> --
> Live and work overseas - http://www.overseas-exile.com/
> Buy the book - http://www.oreilly.com/catalog/perlhks/
> Tech blog - http://blogs.perl.org/users/ovid/
> Twitter - http://twitter.com/OvidPerl/
>     --------------------------------------------------------------------------------
>     *From:* Fred Hebert <mononcqc@REDACTED>
>     *To:* Ovid <curtis_ovid_poe@REDACTED>
>     *Cc:* "erlang-questions@REDACTED" <erlang-questions@REDACTED>
>     *Sent:* Friday, 18 May 2012, 14:02
>     *Subject:* Re: [erlang-questions] Erlang suitability
>     Answers inline.
>     On 12-05-18 5:00 AM, Ovid wrote:
>>     Hi there,
>>     We've a system that run across 75 servers and needs to be highly
>>     performant, fault-tolerant, scalable and shares persistent data across all
>>     75 servers. We're investigating Erlang/Mnesia (which we don't know)
>>     because it sounds tailor-made for our situation.
>     As mentioned earlier in this thread, 75 servers is a bit much, but people
>     have done it before.
>>     We are not using Erlang for our first implementation, but are instead
>>     hacking together a solution from known technologies including Perl, MySQL
>>     and Redis. We're considering Erlang for our future work.
>>     We have two primary needs: Each box can bid on an auction and potentially
>>     spend a tiny amount of money and each of the 75 boxes will receive
>>     notifications of a small amount of money spent if they win the auction
>>     (the auction notification will probably not be sent to the box bidding in
>>     the auction).
>>     Use case 1: If the *total* of all of those small amounts exceeds a daily
>>     cap or an all-time cap, all 75 boxes must immediately stop spending
>>     bidding in auctions. It seems that each box can run a separate Erlang
>>     process and write out "winning bid" information to an Mnesia database and
>>     all boxes can read the total amount spent from that to determine if it
>>     should stop bidding.
>>     This seems trivial to set up.
>     It isn't trivial. You have think about what happens when a box is seen as
>     crashing. How strongly consistent do you want things to be? There is always
>     a risk that a box didn't crash, but was cut off in a netsplit. You might get
>     divergences in budget that will be hard to explain.
>     There is also a definite timing issue depending on how your data is being
>     observed. For example, you ask permission to bid on an item, but you do not
>     get instant feedback; by the time you sent maybe 5-10 bids, the cap is
>     finally reached and broken at once because the delay to the other network
>     made you keep on bidding without a final result. How much tolerance do you
>     have for this?
>     You mentioned in another post that "We need to ensure that were all 75 boxes
>     to mysteriously crash, we could bring them back up and not worry about data
>     integrity.", Possibly, but what about 1 node only? What about 5? What about
>     30 or 35? What if they crash and you missed winning bids because you went
>     out after bidding but before getting your notifications back (if that is
>     possible by the bidding rules of whatever exchange you're dealing with).
>     The most solid synchronous database setup might not give you the guarantees
>     you expect in the first place.
>>     Use case 2: we periodically need to reauthenticate to the auction system.
>>     We MUST NOT have all 75 boxes trying to reauthenticate at the same time
>>     because we will be locked out of the system if we do this. Having a
>>     central box handling reauthentication is a single point of failure that we
>>     would like to avoid, but we don't know what design pattern Erlang would
>>     use to ensure that only one of the 75 Erlang instances would attempt to
>>     reauthenticate at any one time (all 75 boxes can share the same
>>     authentication token).
>     That depends on: 1. how many times you can try to re-authenticate before
>     being blocked, 2. how close together they have to be.
>     Central points of failures are definitely something to avoid. Leader
>     election across 75 boxes might not be the funnest thing in the world either.
>     I could see a scheme where you use some distributed cached value that can
>     say "I am currently being logged" that can time out at some point, visible
>     to all readers. When you read that timeout value from each box (possibly
>     from an OTP Application that only handles auth), each reading of that value
>     adds or subtracts a random number to the timeout. This is to try and avoid a
>     cluster-wide synchronization on the timeout value, and instead have them
>     happen at different times. You could add an "I'm updating" flag related to
>     that value and that could give you good probabilities that only a fraction
>     of all the nodes attempt an authentication at any point in time close to the
>     timeout value.
>     Again, this would depend on how often your authentication needs to be done,
>     and to what frequency you're allowed to do it.
>     If it's too tight, you might need a central server or node that takes care
>     of it, with one or two fail-overs to add some reliability.
>     Note you will still have to care about netsplits ruining your day with this
>     whole scheme.
>     -- I had nothing to add on the rest of the mail, so cut if off.
>     Hope this helps,
>     Fred.
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