[erlang-questions] Load balancing/multiplexing rpc calls amongst Erlang Nodes

Geoff Cant <>
Wed Oct 24 01:41:37 CEST 2012


So one cluster load balancing scheme that worked reasonably well for me at ngmoco was based on gossip with estimation.

Each node has some kind of metric for how loaded it is (in my case number of a certain kind of process running, in your case could be RPCs being evaluated), and a reasonable estimation for how much a remote job request will affect that metric (in my case spawning a new process remotely would increase the process count by one :).

Each node maintains a table of
{node(), LoadMetric, BroadcastTimestamp}.

* It updates its own entry continually (the local metric should be pretty accurate), and broadcasts its metric to other nodes every broadcast period (5s).
* Each node receiving a timely load broadcast (they contain timestamps, discard any that are more than 1 gossip period old) overwrites its load table entry with the load metric and broadcast timestamp (i.e. every 5s, the running estimate of remote load will be brought up to date)
* Every broadcast period (every time you're about to broadcast), scan the load table and delete entries with a timestamp older than 3*broadcast period. -- Ignore nodes that are probably faulty/down
* Each time a node routes/transfers load/sends an RPC to a remote node, it bumps the local load table entry for the remote node -- this is the estimation step.

Now you have a local ets table of size M rows (cluster cardinality) which you can read and make your load balancing decision on. I generally went for lowest loaded node if there is more than  one entry and the local node otherwise.

The local estimation step is important if you have a deterministic load balancing function.


You can also extend/modify this implementation to add a graceful-cluster-exit scheme for a node by adding an administrative mechanism to stop a node broadcasting its own load. The other nodes will stop transferring load to it and load on it will eventually finish up.

I'm sorry I don't have an implementation for you to use as I don't have permission to release the code. It's not super-hard to write however.

Cheers,
-Geoff

On 2012-10-23, at 02:52 , Joshua Muzaaya <> wrote:

> Thank you so much. let me try that. But is it not possible to have a
> non-random method ? one that is intelligent and fair on all the nodes
> 
>  <http://www.linkedin.com/pub/muzaaya-joshua/39/2ba/202>
> Designed with WiseStamp -
> <http://r1.wisestamp.com/r/landing?u=cf16262215eb8784&v=3.11.21&t=1350985854695&promo=10&dest=http%3A%2F%2Fwww.wisestamp.com%2Femail-install%3Futm_source%3Dextension%26utm_medium%3Demail%26utm_campaign%3Dpromo_10>Get
> yours<http://r1.wisestamp.com/r/landing?u=cf16262215eb8784&v=3.11.21&t=1350985854695&promo=10&dest=http%3A%2F%2Fwww.wisestamp.com%2Femail-install%3Futm_source%3Dextension%26utm_medium%3Demail%26utm_campaign%3Dpromo_10>
> 
> 
> 
> On Tue, Oct 23, 2012 at 10:45 AM, Paul Peregud <>wrote:
> 
>>> find the erlang random generator skewing results making around 50/60
>> requests hitting one Node while others are waiting
>> 
>> This is unusual. I've seen PRNG from random module skewing the results,
>> but never to such extent. Please check if it is properly seeded
>> (random:seed/0 seeds with predefined constant).
>> 
>> If seeding is done properly, then you may want to consider switching to
>> crypto:rand_uniform/2. It's a bit slower, but it produces random numbers of
>> quality better then enough for purpose of load balancing.
>> 
>> 
>> On Tue, Oct 23, 2012 at 7:22 AM, Joshua Muzaaya <>wrote:
>> 
>>> Yes,i tries using the random method, but because requests are so
>>> frequently many, you find the erlang random generator skewing results
>>> making around 50/60 requests hitting one Node while others are waiting.
>>> Another thing is that, i am not using gen_servers at the Web Server layer.
>>> I am using yaws web server and for each connection, yaws spawns a process,
>>> this process communicates with Mnesia Nodes to query for data. But the
>>> connections are so many and i wanted to scale the application horizontally,
>>> adding more web servers and more mnesia Nodes. I came to think of a load
>>> balancing middle ware, abstracting my processes from knowing where the call
>>> has hit ( i.e on what mnesia node the call has hit). This middle ware
>>> ensures that requests are load balanced across my Mnesia DBs.
>>> 
>>> That is the background of the problem. Its a real-time Web Notification
>>> system, plugged into a major intranet Management System. However, clients
>>> are many, and yaws is sustaining 30,000 concurrent connections at low
>>> peaks. I am a software engineer in one of the telecommunications companies
>>> in Africa. I keep running into a few memory problems on single node yaws
>>> server, so i need ti add more web servers to assist. Also, mnesia sometime
>>> will get *** Too many DB Tables ** when requests are too many and too
>>> frequent. I changed everything to use dirty operations and when i by-passed
>>> the transaction manager, things improved a bit.
>>> 
>>> I need fellow erlangers to think of a load balancing algorithm in such a
>>> situation. Do you think a process dictionary like GPROC would be so useful
>>> ? i was kinda thinking about it last night, but i wonder how i would apply
>>> it in this case.
>>> 
>>> Having one gen_server to decide where the request may go, might alos slow
>>> down the application as all requests will have to go through that
>>> gen_server.
>>> 
>>>  <http://www.linkedin.com/pub/muzaaya-joshua/39/2ba/202>
>>> Designed with WiseStamp -
>>> <http://r1.wisestamp.com/r/landing?u=cf16262215eb8784&v=3.11.21&t=1350969121119&promo=10&dest=http%3A%2F%2Fwww.wisestamp.com%2Femail-install%3Futm_source%3Dextension%26utm_medium%3Demail%26utm_campaign%3Dpromo_10>Get
>>> yours<http://r1.wisestamp.com/r/landing?u=cf16262215eb8784&v=3.11.21&t=1350969121119&promo=10&dest=http%3A%2F%2Fwww.wisestamp.com%2Femail-install%3Futm_source%3Dextension%26utm_medium%3Demail%26utm_campaign%3Dpromo_10>
>>> 
>>> 
>>> 
>>> On Tue, Oct 23, 2012 at 2:53 AM, Yogish Baliga <> wrote:
>>> 
>>>> One option is to run proxy gen_server on each Mnesia box and register
>>>> these gen_server pids with pg2. Now you can do load balancing on pg2
>>>> processes based on message queue length as described here
>>>> 
>>>> http://dev.lethain.com/load-balancing-across-erlang-process-groups/
>>>> 
>>>> When I used this method in my last project in Erlang, it gave better
>>>> result than normal round robin. Under very low load, all requests were
>>>> redirected to single Mnesia instance.
>>>> 
>>>> http://dev.lethain.com/load-balancing-across-erlang-process-groups/
>>>> 
>>>> -- baliga
>>>> 
>>>> 
>>>> On Mon, Oct 22, 2012 at 2:22 PM, Paul Peregud <>wrote:
>>>> 
>>>>> May you specify why load balancing should be based on time and can not
>>>>> be random? Have you implemented random load balancing? Has it proved
>>>>> to be insufficient?
>>>>> 
>>>>> On Mon, Oct 22, 2012 at 9:51 AM, muzaaya_joshua <>
>>>>> wrote:
>>>>>> Building from this question (
>>>>> http://stackoverflow.com/q/5339329/431620 ),
>>>>>> imagine an application with N Erlang Web Servers, and N/2 Mnesia
>>>>> Database
>>>>>> Nodes. The set up is such that the Web Servers, each, runs on its own
>>>>>> hardware server (say HP DL385), and each Mnesia Instance, runs on its
>>>>> own
>>>>>> hardware Server as well.
>>>>>> 
>>>>>> Web Servers make rpc:call/4 calls to the back end (the Mnesia DB
>>>>> Servers).
>>>>>> The Data is all replicated across all the Mnesia instances. Now, you
>>>>> want to
>>>>>> have the calls being made to the Database servers, MULTIPLEXED, more
>>>>>> precisely ( by TIME), on each Web Server, so that some kind of LOAD
>>>>>> BALANCING is attained.
>>>>>> 
>>>>>> If Web Server A makes a connection to Mnesia Instance 3, it cannot
>>>>> make the
>>>>>> next connection to the same Instance. All Database Nodes need to be
>>>>> kept
>>>>>> busy and not having any one of them idle while the others are
>>>>> working. The
>>>>>> Load balancing Algorithm should not be random, but should be aimed at
>>>>>> balancing the load on the Database Servers.
>>>>>> 
>>>>>> Qn 1: Come up with your load balancing strategy, in such a situation.
>>>>> Also,
>>>>>> please show with some sample illustrative code, how you would
>>>>> implement this
>>>>>> strategy.
>>>>>> 
>>>>>> Qn 2: If a Mnesia Instance goes down, how would your load balance
>>>>> Algorithm
>>>>>> adapt to the changes in the cluster ?
>>>>>> 
>>>>>> Qn 3: Is there any Erlang library aimed at load balancing of Erlang
>>>>> Servers
>>>>>> working within the same system, and calling each other via rpc:call/4
>>>>> ?




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