Dynamic Load-Balancing DNS: dlbDNS
To get started, we downloaded BIND 8.1.2 from the Internet Software Consortium (www.isc.org/bind.html). Initially, time was spent installing and understanding DNS. DNS was installed on odie.cs.twsu.edu, a stand-alone Linux workstation.
During configuration, a new attribute called DNAME was added to distinguish the hosts taking part in dynamic load-balancing. Listing 1 is a snapshot from named.hosts.wsu, containing information on all hosts in a particular zone. In this listing, the set of hosts kira.cs.twsu.edu, sisko.cs.twsu.edu and q.cs.twsu.edu take part in dynamic load-balancing for http://www1.cs.twsu.edu/. The set of hosts kira.cs.twsu.edu, mccoy.cs.twsu.edu and emcity.cs.twsu.edu take part in dynamic load-balancing for http://www2.cs.twsu.edu/. The set of hosts kira.cs.twsu.edu, sisko.cs.twsu.edu and deanna.cs.twsu.edu take part in dynamic load-balancing for http://www3.cs.twsu.edu/. Hosts kira.cs.twsu.edu and sisko.cs.twsu.edu belong to multiple groups.
Here is the algorithm we added to the pre-existing DNS feature. If the service requested is of type DNAME, do the following:
Determine the set of participating servers for this service.
Request ratings from all participating servers by establishing a concurrent connectionless (UDP) connection with each server.
Using the ratings returned, determine the best server.
Handle error conditions such as “server is too busy to return the rating within the time frame”, “the rating returned by the server gets lost on its way back to the dlbDNS”, “all servers have same rating” and “a server is down”.
A rating daemon runs on each server taking part in dynamic load balancing. Here is the algorithm:
Receive request for rating from dlbDNS and respond by returning the host rating.
Calculate the host rating once every minute rather than calculating it at the time of request, as quick response time is a most important feature.
Ensure the host rating is updated every minute, independent of the dlbDNS request.
Handle error conditions such as dlbDNS closing the UDP sockets without waiting for host response.
Figure 1 shows the functionality of dlbDNS. The path traced by C indicates the process of updating the server rating by the rating daemons. The path traced by B indicates the communication between dlbDNS and the rating daemons to determine the best server. The path traced by A indicates the path traced by the user request. HOST 1 has a better rating than the other two hosts, so the user request gets directed to HOST 1.
Implementing dlbDNS provides efficient utilization of system resources and ensures that facilities newly added to the existing network will be utilized. Since DNS is used, applications such as FTP and TELNET will also utilize dlbDNS.
Uneven distribution of load across servers has been a major problem in the Computer Science department of Wichita State University. bugs.cs.twsu.edu, kira.cs.twsu.edu, roger.cs.twsu.edu and sisko.cs.twsu.edu are four Linux servers available for students in the department. These servers vary in potential and configuration.
dlbDNS was installed in December 1998 to effectively utilize the servers. lion.cs.twsu.edu, the actual DNS server, was made to direct DNAME requests toward odie.cs.twsu.edu where dlbDNS was installed. The lines added to the configuration file were:
; bestlinux IN DNAME bugs.cs.twsu.edu. bestlinux IN DNAME kira.cs.twsu.edu. bestlinux IN DNAME roger.cs.twsu.edu. bestlinux IN DNAME sisko.cs.twsu.edu. ;
Here, the bestlinux attribute was added to handle non-web requests from applications such as TELNET and FTP.
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