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I plan to balance load between my app servers using a separate Nginx server as a software load balancer. But actually, it's because a load balancer in front of the app servers allows me to update an app server (OS, application, etc.) when I want without downtime.

Then I realized, how do I update the load balancer itself? i.e. my site is still going to go down when I need to update the OS and Nginx on the server that acts as the load balancer for my application.

As hard load balancing is out of question, I was wondering if it's possible to connect two Nginx webservers together so that they balance load across app servers together, while enabling me to update one at a time, when I need to, without causing downtime.

Is this realistic?

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There are a number of very good ways to do failover. One is to just switch which machine has what IP (this can be done automatically). But once you figure that out, the redundancy can keep going up the ladder until you've got a much more complex system than is absolutely necessary. How much downtime do you have by installing the update and then restarting the balancer, a couple seconds? Are the dozen page requests you might get during that blip worth setting up and maintaining an extra box to balance requests? Can you mitigate those page requests by properly scheduling the update? –  Andrew Domaszek Sep 20 '13 at 17:49
    
@AndrewDomaszek Updating would take 10-15 minutes (although I plan to do it only on slow days), which is why I am considering a load balancer. –  its_me Sep 20 '13 at 18:15

2 Answers 2

up vote 2 down vote accepted

You need common access point. This should be done with virtual addresses or routing. Best/easiest way to accomplish this without additional hardware and protocols is to use LVS.

LVS is kernel level IP balancer that can do the job and is very fast and effective, has impressive throughput. You can also configure filewall on the same node.

For your configuration, easiest way is to configure LVS-NAT with two nodes (active-backup), it supports heartbeat between nodes and uses virtual IP for both sides. If master node will fail, then slave one will push arp update to the switch and takeover IP address.

There are tons of configuration utilities, some distributions feature fancy utilities for configuration, ex. RHEL/Centos/Fedora has piranha web GUI.

For real servers you can configure timeouts, scheduling, ratios, monitoring, session persistence etc. Pretty flexible.

Also, it's good to share session info between nodes.

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Recommended docs:

LVS official: http://www.linuxvirtualserver.org/Documents.html

LVS wiki: http://kb.linuxvirtualserver.org/wiki/Main_Page

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this really depends on the way you host your machines. the better cheap hosters that rent hardware servers or virtual machines offer some kind of failover-ip, that might be routed onto different servers; usually switchable manually (or automated via some monitoring-tool, but this is not recommended). this is comfortable enough for simple setups.

if you use some kind of server-housing and run servers with multiple nics, thus are able to setup your own network between machines with bonded nics, local networks, direct links between servers etc you are ablte to use a solution like keepalived or pacemaker. but the setup and tuning should be done by people familiar with this (my experience). this solution, if setup right, works full automated and rock solid. ips and services might be switched manually without loss of packages; very usefull for updates, maintainace and reboots.

both solutions are still poor man's HA-solutions but are still better then nothing. the point is, to have a soltion, even if it is limited somehow.


btw, what you are asking for is not loadbalancing but failover; just for the records :)

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