In general when you are load balancing a cluster you can evenly spread the connections through the cluster and you get pretty consistent and even load balancing. However with some applications such as RDS (Microsoft Terminal Servers), you can get very high load from just a  few users doing heavy work. The solution to this is to use some kind of server load feedback agent. We’ve had one of these for a while in our product but now with a lot of help from Simon Horman we’ve managed to integrate the functionality into the main branch (well soon anyway) of HAproxy. We thought it would be a good idea to open source the previous work on Ldirectord/LVS, make it compatible with HAProxy, and release our Windows service code as GPL.

Until the work is merged and tested with an official release of HAProxy we’ve compiled a patched version of HAProxy dev19 ish here…. (http://downloads.loadbalancer.org/agent/haproxy-agent-check-20130813.tar.gz) Or you can get the patches from the mailing list archive…

Simply compile as usual and then modify your RDS cluster:

listen RDSTest	bind 192.168.69.22:3389	mode tcp	balance leastconn	persist rdp-cookie	server backup 127.0.0.1:9081 backup  non-stick	tcp-request inspect-delay 5s	tcp-request content accept if RDP_COOKIE	timeout client 12h	timeout server 12h	option tcpka	option redispatch	option abortonclose	maxconn 40000	server Win2008R2 192.168.64.50:3389  weight 100  check agent-port 3333 inter 2000  rise 2  fall 3 minconn 0  maxconn 0  on-marked-down shutdown-sessions

The important bit agent-port 3333 tells HAProxy to constantly monitor each backend server server in the cluster by doing a telnet to port 3333 and grabbing the response which will usually be a percentage idle value i.e.

80% – I am not very busy please increase my weight and send me more traffic
10% – I’m busy please decrease my weight and stop sending me so much traffic
drain – Set the weight to 0 and gradually drain the traffic from this server for maintenance
stop – Stop all traffic immediately, kill this backend server

If you have a Linux backend you could create a simple service calling the following script:

#!/bin/bashLOAD=(/usr/bin/vmstat 1 2| /usr/bin/tail -1| /usr/bin/awk 'print $15;' | /usr/bin/tee)echo "$LOAD%"#This outputs a 1 second average CPU idle

Call the script  /usr/bin/lb-feedback.sh
make sure that you make it executable:

chmod +x /usr/bin/lb-feedback.sh

Insert this line into /etc/services

lb-feedback 3333/tcp # loadbalancer.org feedback daemon

Now create the following file called /etc/xinetd.d/lb-feedback

# default: on# description: lb-feedback socket serverservice lb-feedback port = 3333 socket_type = stream flags = REUSE wait = no user = nobody server = /usr/bin/lb-feedback.sh log_on_success += USERID log_on_failure += USERID disable = no

Then change permissions and restart xinetd:

chmod 644 /etc/xinetd.d/lb-feedback/etc/init.d/xinetd restart

You can now test this service by using telnet:

telnet 127.0.0.1 3333Trying 127.0.0.1...Connected to 127.0.0.1.Escape character is '^]'.95%Connection closed by foreign host.

Now if you have a Windows server as your backend you can use the following our open source monitor service. You can download the Loadbalancer.org windows feedback agent here (http://downloads.loadbalancer.org/agent/CpuMonitor_4.2.3.zip)

Once you have installed Loadbalancer.org feedback service you should find the monitor.exe file in program files/LoadaBalancer.org

Simply hit the ‘start’ button and the agent should start responding to telnet on port 3333 (you may need to make an exception for that port in your Windows firewall).

You can change the ‘mode’ setting to drain then ‘apply settings and restart’ and HAproxy will then set the weight to 0 and status to drain (blue) i.e.:

Or you can set the ‘mode’ to maintenance then ‘apply settings and restart’ and HAproxy will then immediately set the status to DOWN (yellow) i.e.:

When the agent is running in normal mode it will report back the percentage idle of the system based on the settings in the feedback agent XML file:

Notice that you can control both the importance of CPU & RAM utilization and also a threshold, so the following logic is used:

If CPU importance = 0 then ignore
If RAM importance = 0 then ignore
If Threshold level is reached on any monitor then immediately go into DRAIN mode

Otherwise to calculate the percentage idle reported by the agent we
would be to divide the utilization by the number of factors involved i.e.

If you are using two services then:

utilization = utilization + cpuLoad * cpuImportance%;
utilization = utilization + ramOccupied * ramImportance%;
utilization = utilization / 2

So if importance was 1 for both cpu and ram you would only get 0% reported if both CPU and RAM were 100%

And if the importance is zero then ignore completely i.e.

utilization = utilization + cpuLoad * cpuImportance%;
//utilization = utilization + ramOccupied * 0 (importance is zero so ignore)
utilization = utilization (one service only so don’t divide)

Also the final section TCPService effictvley lets you load balance on number of established connections to your server, so you could balance based on the number of RDP connections to port 3389.

For this setting MaxConnections is important to specify as otherwise the agent will have no idea how to calculate the the load i.e.
utilization = MaxConnections / 100 * number of current connections * importance%

In the following screen shot from a Loadbalancer.org appliance you can see that the Win2008R2 server is healthy and 99% idle, whereas the Linux server was busy at 43% idle before the Linux agent was put into maintenance mode and the server taken out of the group.

Does that make sense? Have a play with the config file and let us know what you think….

You Can Learn More About the LoadBalancer.org’s Product Line By Going to www.LoadBalancerSolutions.com/LoadBalancer-org

The original article/video can be found at Open Source Windows service for reporting server load back to HAProxy (load balancer feedback agent).