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Load Balancing for web sites
Loadbalancing: Load balance techniques
SSC result publishing site overhit. Some suggestions.
On 26th July People were eagerly waiting to know SSC results from internet. Education board a month back announced that they would publish SSC result in their portal http://www.educationboard.org.bd. Although the site would start working in evenning at 4.00 pm, guardians welcomed that initiative. Everyone liked internet as that might save crowd, noise, schorching heat and transport cost. Another interesting reason why people liked it was they wanted to know result of candidates of neighbours and relatives. Well, may people did not had computers let alone internet in their house. They found rushing to nearby cybercafes.
At 3.50 am, I visited the educationboard site. It was a pleasant looking site designed by BUET Compute center. But I had doubt that such a heavy site would not be able to sustain in huge hits.
My afraid became true, traffic increased to half a lac in few minutes after its operation, site drastically slowed, eventually crashed and left dead whole night long . This is not the first case in Bangladesh. It happened previously many times. For example,during Parliament election, all election result publishing sites went dead although numbers of hits were not unexpected. To tell in better way all new paper portals: ittefaq, prothom-alo, jugantor etc went to comma.
Another example is Gonophone Quiz competition. This renowned ISP declared a online quiz competion with huge prize money. Such a big company, who had lots of technical experts, did not either consider how to handle good amount of hits, or ignored the pitfalls.
But Loadbalancing is not difficult and often could be done in affordable cost. If we implement some loadbalancing features in our next sites we could avoid shamefull notices like “site is temporarily unavialable due to more than 1 lac hit in last hour”.
All load balancing distributes a common workload among multiple machines, but the term is difficult to define more specifically in a Web context. The definition of load varies among load-balancing products for Web servers: One product defines load as the retrieval of static HTML pages, and another defines load as the back-end processing that dynamic Web content requires. Vendors don't use a common definition of balance, either. Web-based load-balancing products differ significantly in their methods for distributing servers' loads.
1. Load-Balancing Techniques
We can achieve load balancing on a Web server using the Domain Name System. DNS's round-robin function spreads incoming TCP/IP requests evenly among several servers. This load-balancing technique is easy to implement. But DNS doesn't keep track of which servers are active and which are down, so it sends TCP/IP requests to servers whether they are online or offline. To distribute your Web site's load without interrupting its performance, we need to implement a load-balancing solution that works seamlessly when a server fails.
2. Virtual IP addressing.
Virtual IP addressing lets multiple servers respond to requests for one IP address. For example, suppose you have three servers with the IP addresses 222.222.231.1, 222.222.231.2, and 222.222.231.3. You can use virtual IP addressing software to configure all three servers to use IP address 222.222.231.10. You designate one server as the routing, or scheduling, server. The scheduling server receives all inbound traffic and routes requests for Web content to other servers based on load-balancing parameters you set. If the scheduling server fails, virtual IP addressing software assigns routing responsibilities to another server.
Because the Web interface uses one IP address, virtual IP addressing achieves uninterrupted service unless all your Web servers fail. The disadvantage of virtual IP addressing is that you can't easily watch the flow of Web traffic from an external network monitor because all the servers use one IP address.
3. HTTP redirecting.
HTTP redirecting distributes a Web site's load among multiple servers by connecting users' browsers directly to the servers. When you select a Web site's URL, you usually connect directly to the computer servicing that URL. For example, type http://www.winntmag.com, and the server designated to respond to requests for that HTTP address will provide the desird Web site. However, if this site has a replica on a server with the URL http://www.linux.com, an HTTP-redirecting program can redirect users' browsers to http:// www.linux.com to balance the Web site's load. HTTP-redirecting software automatically directs browsers to a Web site replica if the primary URL's server fails. HTTP redirecting's main disadvantage is that it doesn't work with all Web browsers.
4. Application load balancing.
Application load-balancing software distributes a Web site's workload among servers according to the content browsers request. A primary Web server accepts all incoming Web traffic and performs tasks such as static HTML file transmissions. The primary server redirects back- end applications, such as Active Server Pages (ASP) and Common Gateway Interface (CGI) programs, to other computers. Servers process back-end applications more efficiently when those ap- plications aren't intermingled with HTML file processing, so application load balancing reduces response time for back-end applications.
5. Testing sites Load tolerance
There are few good tools availble to check how much load a site can handle. Windows 200 has a tool called WCAT. Linux has similar one tool called PagePoker available to download from http://node.to/hacks/
Some suggestion for SSC result processing portal
1. The SSC result portal will be implemented in numbers of servers each having a real IP. It may be a matter of cost.
I have an Innovativ idea. Education board may contact ISP Association and ask each ISPs to upload a copy of database. I talked to Mr. Sumon Ahmed Sabir. He told that ISP Association will consider such case, since this is a national interest.
2. DNS servers of current portal will have list of all servers Ips.
3. Site home page should be HTML files. Dynamic PHP/ASP sites are generally not stored in Cache since their size is unknown to proxy servers.
4. Site will be only with TEXT content. Images multiplied by numbers of hits may choke down the web server.
5. Instead of database, LDAP technology should be used. LDAP is much faster than database.
