[erlang-questions] load balancing

Vladimir Ralev <>
Fri Nov 14 02:03:34 CET 2014


Hi,

I do this a lot and here are two cents about common issues I see. It all
depends on the protocol (reconnects/retransmits/statefulness/session-based)
and if you have control over the clients. The best load balancing is always
on the client side. If you have control over the clients just code them to
search and pick their server with your custom timeout rules and recovery.
The next best thing is if the clients have SRV awareness or even a simple
multiple A DNS record awareness that would be a very reasonable solution
Proper SRV support from random apps is rare in my experience, even common
client for protocols like SIP, HTTP and XMPP and bad with SRV. Normal DNS A
dispenser is sufficient most of the time, but doesn't handle failover cases
very well. There is no advantage in using a load balancer unless your
protocol is stateful and requires app-level affinity or sticky sessions,
however there are a few exceptions. TCP itself is stateful however if your
protocol knows to reconnect and heartbeat independently you can ignore
that. In the context of Erlang, you may have to consider how are you going
to handle production maintenance operations such as moving nodes around and
detecting cluster in bad state.  Load balancers can be reliable and do
great for mid-session failover cases, but it's more expensive and
difficult. If you are using a load balancer it has to be able to maintain
existing TCP connections until they terminate but not send new connections
to the serviced nodes. Another production operation you need to consider is
IP-takeover to replace a node at some IP address, Erlang doesn't handle
this very well on a running system and it has to be done with virtual
adapters or redirection at the OS level. If your protocol stamps IP
addresses or names inside the payload like SIP/SDP you will need
application awareness of the load balancing and IP takeover operations.
Many solutions for SIP or HTTP are hybrid with active/passive DNS pointing
to N IP load balancers feeding M app-level load balancers to assure
protocol specific sticky-ness (HTTP session/Call-ID).

On Fri, Nov 14, 2014 at 1:09 AM, Roberto Ostinelli <>
wrote:

> Dear list,
> I have many servers in a cluster that handle long-lived TCP connections
> with multiple clients.
>
> I'd like to collect some opinions on what you'd believe to be the best
> options to balance the load on these servers:
>
>  - Implement a protocol that a client has to first ask to which server to
> connect to, and then go for it
>  - Use SRV records
>  - Have all traffic through some proxy like HAProxy (but then, doesn't
> this mean that a single box has to handle millions of connections?)
>
> I do realize that this is not necessarily related to Erlang, but as we
> often have to deal with backend servers with long-lived TCP connections I
> thought there might be the correct experience here.
>
> Hopefully some kind sould will provide me with some suggestions :)
>
> Best,
> r.
>
>
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> 
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>
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