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Please correct me, but my understanding is that with software load balancing a service must be run on each server while there is one DS that notifies the other servers that a server has gone down and that they should consume that servers load.

With hardware load balancing what happens in a fail-over? Could someone explain? Is there advantages with using hardware load balancing when it comes to fail-over, or is there advantages with software? Or do they both have their pros and cons?

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up vote 3 down vote accepted

There are a couple ways failover is done. (probably more, but these are the main ones and the ones I've used):

  1. Clustered system with a floating IP; this works fine for static content as there is no session ID; in theory the must under-utilized server will respond to a request first, and they all share an IP.

  2. A proxy load balancer node. This can be a software one such as apache sending users to a specific set of servers; it can be set up to have session based rules to always send a specific session/ip/etc to the same server so that session based websites will work

  3. A hardware load balancer (such as the ones f5 make). You can make a node enter and exit a pool and specify a port for which all traffic will be sent to a specific pool of servers. This can work with any type of IP traffic and with both this and a more software based load balancer, you can have redundant nodes and a floating IP between the load balancers, or an active-standby kind of set up.

Depending on how it is configured, the difference between a software and hardware load balancer will essentially be how it is configured and what it runs on. i.e. comparing a physical router like a home one or an enterprise grade Cisco router, or using a regular x86 machine with multiple NICs and IPtables/DNSMasq under linux.

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