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CHAPTER 15 BA CK UP AND FAULT TOLER ANC E
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The second type of fault-tolerant solution is called active-active. In an active-active topology, multiple active servers are used. In this scenario, you will often also engage in load balancing those servers. This gives you the ability to provide increased bandwidth to mission-critical services. The complexity introduced in this scenario is that the data store must be either synchronized with the primary server or shared between the servers, as is the case with using multiple servers as bridgehead servers using an Xsan to reshare data. Data in an active-passive topology must be synchronized between the servers, but not as frequently as in active-active environments.
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While active-passive topology traffic reroutes data using the failover daemon (failoverd), in an active-active topology traffic must be routed between multiple servers on a constant basis. There are two predominant methods for doing this. The first is using round-robin DNS. Roundrobin DNS controls the order in which a load-balanced service forwards traffic across multiple servers. The administrator fills the domain records with multiple address records using the same host name but points to one or more IP addresses that serve the same service. As the DNS request is looking for your DNS server, the web server responds to the client with the next address from the list. The most basic round-robin DNS setup shapes traffic based on the assumption that each server can handle the same load, which is considered an equal cost path. A typical list of A addresses in your zone file might look like the following if you have three servers: afp afp afp IN IN IN A A A 10.0.0.7 10.0.0.8 10.0.0.9
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You can tell one server to handle more traffic than the others, as is the case with unequal cost paths, or paths that send more traffic to one server than the others. You can do this by listing the same server multiple times in the listing of A records for a given address. For example, if you have one server at 10.0.0.4 that is much faster than the others in a six-node cluster of web servers, you might direct traffic using the following entries in a zone file: afp afp afp afp afp afp IN IN IN IN IN IN A A A A A A 10.0.0.4 10.0.0.4 10.0.0.5 10.0.0.6 10.0.0.7 10.0.0.8
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You can use the rrset-order command to define how round-robin requests are forwarded. The default rrset order is cyclic. There are three different options for record handling: Cyclic records are returned in numerical order. Fixed records are returned in the same order that they are listed in the file. Random records are returned in a random order. When using round-robin DNS, the traffic is shaped in a static manner and cannot adapt to changes in the servers, such as performance counters and current user load.
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CHAPTER 15 BAC KUP A ND FAULT TOLERA NCE
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A load balancer can provide many features. Among them is the ability to shape traffic based on multiple factors (IP addresses, port, amount of traffic), automatically failover to another server when one server becomes unresponsive, and provide a single address for systems to use when requesting a service. The load balancer will probably not handle synchronizing data but will perform almost everything else that will be required when architecting this type of solution. Load balancers include Coyote Point Equalizers, F5s/Dell BigIPs, Barracuda Load Balancers, and Cisco Load Directors. The cost of deploying a solution using round-robin DNS is typically very low compared to the cost of purchasing a load balancer. However, the load balancer will become a more scalable solution over the long run. In cluster environments, the load balancer can become a single point of failure. To build a completely fault-tolerant environment, implement a backup load balancer to automatically go live if there is a problem with the main load balancer. It may seem redundant, but even load balancers can and will fail.
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