Clustering Solutions and Zero Downtime Hosting Pitfalls

Written by Godfrey Heron

Continued from page 1

One ofrepparttar inherent difficulties with failover for Web Hosting companies operating on different networks isrepparttar 134338 limitations imposed byrepparttar 134339 DNS caching system.

As DNS records are passed fromrepparttar 134340 original DNS servers (i.e., ns1/, they are cached or stored at several different ISP’s alongrepparttar 134341 way. Which is why it takes a while for a newly registered domain name to resolve to its IP address.

Each DNS record has a TTL (time to live) setting assigned. By manipulating this value, it is possible to alter how long that particular IP address/ DNS record combo is stored. If your site is on 2 different servers with 2 different IP addresses, you could setrepparttar 134342 ‘time to live’ with a value of, say, 2 minutes.

The failover software would check server availability by “pinging”repparttar 134343 web server every few minutes to determine whether it’s IP address is responding appropriately. (perhaps by looking for a particular text string in a web page).

If a failure is detected, thenrepparttar 134344 software would pull repparttar 134345 non-working web server IP address out ofrepparttar 134346 list of IP addresses assigned torepparttar 134347 your web site’s domain name. If/when your web server IP comes back online it would be restored torepparttar 134348 list.

With a TTL setting of 2 minutes, theoretically, your web site should be down for just 2 minutes, while switching DNS information torepparttar 134349 other web server.

The problem with this scenario, is that, while some ISP’s caching might respond to such low figures, other ISP’s may decide to ignore,(to save on bandwidth utilization), any TTL’s below a certain value, say, 60 minutes. So it is entirely possible that some of your visitors would see your websites and for others, your site would be down for 1 hour or more, even though one of your servers was operating perfectly.

Static non interactive web sites are great candidates for server clustering, butrepparttar 134350 wicket becomes a bit sticky for dynamically generated sites. Most database application software in general, although having some replication capabilities, are not happy with multiple server master/slave relationships and real time updating between servers. The issue can become very problematic if your site requires frequent updates.

Then there isrepparttar 134351 problem of how to keep your websites synchronized. Unix/Linux servers have a built in synchronizing software tool called rsync. You can also automaterepparttar 134352 synchronizing process by setting up a cron job to run periodically. DNS caching and synchronizing issues can be so problematic so as to nullifyrepparttar 134353 advantages of server clustering. For example, a cron job to synchronize your servers every few minutes might very well use up your server capacity.

Your customers will also have to contend with their desktop email client software having dual email addresses for each email account on each web server. e.g.,

It is important to realize that DNS operates by default in a round robin manner, so that, if you have repparttar 134354 same web site on 2 separate servers, it is very likely that server 1 will get 50% of allrepparttar 134355 web traffic.

Now, this is important for a number of reasons, but one ofrepparttar 134356 principal reasons to keep this in mind, is that, you will not be able to effectively keep a ”back-up” site (as some providers would have you believe) which will only be used whenrepparttar 134357 primary server goes down. For e.g. a site saying” we’re sorry our main server is down but you may contact us at:

On a final note, hardware based load balancing solutions tend to be quite expensive and also introduce a potential single point of failure intorepparttar 134358 system,repparttar 134359 load balancer itself. There is a very prominent Data Center that began offering load balanced hosting solutions, whererepparttar 134360 load balancer itself failed on several occasions, althoughrepparttar 134361 web servers were operating perfectly. The net effect torepparttar 134362 public however, was thatrepparttar 134363 sites were unavailable.

Reasonable cost effective software based solutions may be obtained as a service model or by purchasing repparttar 134364 software yourself. Zoneedit is an example of a service model, and Simplefailover is an example of a software based model which maybe purchased on a server license basis.

In conclusion, at this point in time, there are several limiting factors to successfully implementing a “true” high availability multiple server web hosting system. Depending on your clientele andrepparttar 134365 nature of their web sites,this may indeed be a very viable alternative. For others, simply setting up a server with high quality components, redundant RAID hard drives and a good supply of server spare parts may berepparttar 134366 best way to ensure high availability.

Godfrey Heron is the Website Manager of the Irieisle Multiple Domain Hosting Services company. Signup for your free trial, and host multi domains and web sites on one account:

Web Hosting: 4 Factors in Choosing a Hosting Company

Written by Richard Martin

Continued from page 1

4)Fees. I'd rather pay a little more to get allrepparttar features than fight it out with a sub par web hosting company who I can't reach if my site goes down. I've found decent web hosting for as cheap as $4 per month but generally pay around $15 per month if I want allrepparttar 134337 bells and whistles that I'll need to host a larger site. A lot of people worry about hosting fees, but I've hadrepparttar 134338 experience that I generally get what I pay for. Finally, I'd look carefully atrepparttar 134339 web hosting company's refund policy. Most offer a full refund if you aren't happy with your service. If you are paying for a full year of web hosting up front, you might want to examinerepparttar 134340 company's refund policy in detail.

You may reproduce this article on your website. We would appreciate a link back to our site which has Vioxx articles and personal injury resources. This article may not be altered and links should be kept live. Thanks.

Richard Martin is a contributing writer at has injury lawyer articles.

    <Back to Page 1 © 2005
Terms of Use