Several scenarios for Web Server load balancing: DNS Polling

This article is mainly about the simplest solution--dns polling.

DNS Polling

Most domain name registrars support the resolution of multiple A records, in fact, this is the DNS polling, the DNS server will resolve the request in accordance with the order of a records, one by one assigned to a different IP, so that the simple load balancing is completed.


Advantages

• Basically no cost, because often domain name Registrar of this analysis is free;
• Easy to deploy, in addition to the simple expansion of network topology, the new Web server as long as the addition of a public network IP.

Disadvantages

• Health Check, if a server goes down, the DNS server is not known, and the access is still assigned to this server. Modify DNS records to take effect at least 3-4 hours, or even longer;   the
• is unevenly distributed, and if the configuration is different between several Web servers, the stress can be different, but the access to DNS resolution allocations is evenly distributed. In fact, DNS is also a distribution algorithm, according to the current connection less allocation, you can set the rate weight allocation, and so on, but most of the current DNS server is not supported;  
• session hold, if it is a Web site that requires authentication, without modifying the software architecture, This is fatal because DNS resolution cannot permanently allocate access to the same server for authenticated users. Although there is a certain local DNS cache, but it is difficult to ensure that during user access, the local DNS does not period, and re-query the server and point to the new server, the original server saved user information can not be brought to the new server, and may require re-authentication identity, Switch back and forth for a long time each server has a different user information, the server resources is also a waste.

Deployment method

This is the Domain name Control Panel of WAN, the same domain name is resolved to 2 A records.



This is a free intelligent DNS Resolution service, Www.dns pod.com, not only can be more than a record, but also can use Chinese characteristics, according to telecommunications, Netcom, education Network user access, resolution to different IP.



Experiment

First resolve www.alcoholwang.cn A records to 61.172.244.75 and 61.172.244.91, and then use the Nslookup tool to view them.





Summarize

Before you see the article discussed, the problem of the pressure on the DNS server, in fact, do not worry too much, because the parsing records are usually an hour of cache, if the user's most recent level of DNS server resolution record has not expired, is not to query the primary server, usually DNS has an hour of cache, So this would be tantamount to allocating the pressure to the most recent DNS servers at each user level.

Several scenarios for Web Server load balancing: DNS Polling