Linux System Administration

Load Balancing and High Availability

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High availability (HA) provides reliability: keeping services running. It relies on

redundant servers, a heartbeat exchange to say “I’m still alive,” and a failover proce-

dure to quickly substitute a healthy server for an ailing one.

In this chapter, we’re mainly concerned with LB, which administrators will generally

encounter first and need more often. As sites become more critical to an organiza-

tion, HA may also become necessary. Toward the end of this chapter, we’ll provide

some useful links for information on setting up combined LB/HA systems.

The example LB configuration we’ll use in this chapter is a simple one consisting of

three public addresses and one virtual address, all listed in Table 7-1.

The VIP is the address exposed to external clients by the load balancer, which will

relay requests to the web servers.

Load-Balancing Software

The simplest form of load balancing is round-robin DNS, where multiple A records

are defined for the same name; this results in the servers taking turns responding to

any incoming requests. This doesn’t work well if a server fails, though, and it doesn’t

take into account any special needs the service may have. With HTTP, for example,

we might need to maintain session data such as authentication or cookies and ensure

that the same client always connects to the same server. To meet these needs, we’ll

get a little more ...