Frequently Asked Questions

Why do I need to use CDN Failover?

Simply because no network or system is infallible, including Content Delivery Networks (CDNs).

CDN Failover solves a common problem which causes downtime on websites, reacting to outages in an undeniably intelligent, sophisticated manner.

Most importantly it manages to achieve this without adding extra load to your Web servers, which is probably what you're trying to avoid by using a CDN to host your static assets in the first place.

Who was CDN failover written for?

CDN Failover is a software utility for developers, webmasters and sysadmins or anyone who runs a website and has access to the files on their website.

When should I use CDNs to speed up my website?

We're not here to convince you of the need to use CDNs to speed up your website. It's a long established fact that the majority of the busiest sites on the Internet advocate. Offloading static content to a CDN means much less work for your Web servers and also faster delivery to your browser because files are served from the closest geographical node to you.

You might be surprised by some of the reported information contained within these articles and infographics:

Amazon loses 1% of sales for every 100ms of latency

Results from speed improvement: Strangeloop infographic

Yahoo! - Methods to speed up your website

When you consider the minuscule fee that some pay-as-you-go CDNs charge in exchange for a noticeable website performance improvement the compelling evidence above is difficult to dismiss.

When do CDNs fail?

There is currently a great deal of discussion about relying on a Single CDN Strategy for busy websites and why both real and perceived downtime disrupts the income generated by affected websites.

If the Internet has taught us anything it's that no system is infallible. We wrote CDN Failover because our monitoring systems frequently pick up CDN outages, caused by a variety of issues, just when we least expect them. We're sure that you'll agree that removing another Single Point Of Failure from your website can only be a good thing.

What does CDN Failover do?

There are a large number of scenarios where a Content Delivery Network failing means that the website relying on that CDN to serve its static assets also fails; in part or completely. By specifying the location of the static assets on your server's local filesystem and/or multiple CDNs this downtime is absolutely avoidable.

CDN Failover is flexible: you control what happens and where. We know that you don't want your servers disrupted. You can specify individual tags on your site (all the common static asset tags such as script, video, href and img tags) as well as protect jQuery libraries amongst a myriad of other options. The software can be set to failover to your local filesystem or to other CDNs should your primary CDN fail to respond.

In an ideal world (to keep load off your Web servers) you would have two CDNs and then also failover to your local filesystem if they couldn't be reached for some reason. We realise that this might suit some websites but not all. For example, multiple CDNs (hosting all of the primary CDN's content in duplicate) might not suit websites where assets are continually updated by designers and automated systems can't be scripted to synchronise content with rsync/sFTP.

In some scenarios though a busy website might be signed-off, set live and static assets could only change once every few months, in which case spending five minutes to upload your assets to a secondary CDN every now and again isn't much work for the added peace of mind that your CDN infrastructure is always going to be intact and that Web server load won't spike erratically if a CDN fails.

Who are the people behind CDN Failover?

We are a team of highly experienced engineers who have been providing Internet infrastructure services to household names for fifteen years. Some members of our team have been responsible for building High Availability Linux systems in the Cloud and deploying Autonomous Systems on the Internet and other members have been coding for far too many years. We're trusted with high value, mission critical website infrastructure and respected internationally by our peers.

The creation of CDN Failover is the culmination of many years of systems, networks and programming experience; melded together into a clever combination of all three to solve a problem that keeps causing websites downtime.

Does CDN Failover work on Linux and Windows?

Yes, during the development of CDN Failover we looked at a number of ways that we could write the software so that it could be deployed on all of our servers. We opted for jQuery and PHP for a number of reasons; mostly performance and cross-platform compatibility so it would work on almost all PHP servers, such as Windows and Linux. Even if a visitor's browser doesn't have javascript enabled CDN Failover can provide automated failover. We provide much more insight in our lengthy and detailed readme file which is over 6000 words in total.

What are the system requirements for CDN Failover?

To use CDN Failover all your Web server needs is PHP 5.2+ installed and you need access to your website’s files.

If you don't use a commercial CDN directly (but use a free CDN for jQuery libraries for example) then you don't even need a CDN account, you can enable failover from the free CDN to your local filesystem.

If your budget is really limited then you could even use a free commercial CDN service to host your static assets and failover to your local filesystem if it has problems. A compelling proposition indeed: enterprise-level functionality for free.

Can I submit bug reports?

Yes, please use the e-mail address at the bottom of

Can I use the CDN Failover commercially?

Absolutely, a full commercial licence is only a one-off $19.95. This licence covers the current version which you can keep using forever without upgrading if you wish. If you have multiple Domain Names on your website then you can specify multiple Domain Names at checkout time but if possible simply set up your Web server to redirect multiple Domain Names to one primary Domain Name so you only need one licence. For example have your .net Domain Name redirect to your .com Domain Name and then serve all your content from the .com in order to just use a single Domain Name’s licence.

If you use multiple IP Addresses for your website’s WWW record (using Round Robin DNS) then just one licence will suffice too.

Please note that up to FIVE first-level subdomains for your Domain Name are included for free by one licence (for example: assets.domainname.tld). We also include the root domain domainname.tld and the www record for free.

You should specify all five subdomains at checkout time even if you don't need them now so that you don't need to purchase another licence.

For further information please see the Parallelize section within the software's documentation which explains why this is great value and with some thought can increase your website's performance dramatically.

How do I purchase CDN Failover?

Simply click the Buy Now button on our website at Then enter your Domain Name(s) in the first column of boxes and any associated subdomains in the second corresponding column of boxes, accept the software licence and checkout. If you're a member of PayPal already just login and click once to pay. You will receive two e-mails shortly afterwards on how to retrieve your licensed software.

You are then commercially licensed for that version and have unlimited use of it on the Domain Names and subdomains that you specified at checkout. If we discover any serious issues for that version then you may receive an e-mail from us with a free upgrade licence otherwise check the website occasionally for new versions and if there are features that appeal to you then simply purchase the latest version.

We thought long and hard about how to support our time involved with CDN Failover and decided that this was the best approach: really affordable licences for individual versions instead of expensive ongoing support releases that cost the user a small fortune. We hope you agree this is the best way to do it.

Can I test the software on Development Servers before setting it live?

Yes, of course. We understand that testing is really important and we've tried our best to make it easy to do so without breaking a sweat. You should be able to test the software using the following addresses if they remain the base URL of the pages that you're browsing (as shown in your browser's Address Bar).
  • http://localhost
  • http://{specified}.domainname.tld
  • The list shown above means that you can test CDN Failover using or if for example your Domain Name was domainname.tld then you could use any of the first level subdomains/hostnames that you specified at checkout such as the following examples: http://dev.domainname.tld or http://assets.domainname.tld.

    To test the software using a private IP Address from the /16 range shown simply add a ServerAlias entry in the Virtual Host that points to the right place on your filesystem and then view that IP Address in your browser (such as

    These are in addition to any first-level hostname/subdomain of your licensed Domain Name that you declared at checkout so that you’re satisfied with how reliable the software is and how it would work in production.

    As with all critical software we recommend testing CDN Failover on a Dev server before deploying it on a live environment.

    You might find some comfort in the knowledge that the software is designed so that in most cases disabling the software across your whole site is just a case of commenting out one line of PHP code.

    How did you make the software fast, reliable, compatible and secure?

    Contained within our in-depth documentation we look closer at all of these points. Included with the software we describe a number of ways of further securing your installation.

    From a security perspective you can rename and change the paths of the files to hide it from the Web entirely; some call it security through obscurity. You can rename and view the Status Page through a Web browser and also add password protection with .htaccess or something similar if you wish. Visitors should be unaware that you are using CDN Failover however so renaming files should be adequate in most instances.

    To make the software as reliable as possible we opted for simplicity wherever possible with both a systems and a coding perspective in mind. We have always found this to be the best approach for production systems. Albert Einstein coined our development mantra: "Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction."

    One example of how we made the software fast and highly compatible is that we catered for three popular methods built into PHP in order to periodically check a CDN’s availability. Although we recommend one method for efficiency we don’t necessarily expect any of these methods to manually added at installation time (we don't want to disrupt your server's configuration) and as a result the software will still work without two of them being enabled on most default PHP installations.

    We have also made it possible to continue to serve private files hosted on a CDN protected by ephemeral Security Tokens. This means that you can serve both public and private content and still failover if your CDN fails for any reason.

    After installation what maintenance do I have to do?

    You'll be glad to know that there's no maintenance at all for the software. You need to make sure that you don't allow someone to randomly delete whole directories of images or move your assets to different filesystem paths without telling you but otherwise barring cancelling an account with a CDN you can just keep using CDN Failover in perpetuity.

    Do you offer any other software?

    Currently we’re busy with clients but there are plans to include some exciting functionality in a future release of CDN Failover so please check the website periodically.

    Do you offer refunds?

    We aren’t able to offer refunds unfortunately because of our lack of administrative staff. Hopefully you will be able to use your commercial licence for CDN Failover on a future website if you decide not to use it immediately however. A quick reminder that your licence covers up to five free subdomains/hostnames for your Domain Name so a single licence is really versatile and only costs $19.95 if you can't make use of it right now. As long as you still have access to or own that Domain Name then you should be able to use it at a later date without any headaches.

    What's included in your documentation?

    We like to think that we've gone the extra mile with CDN Failover's documentation (the readme file is over 6000 words alone). Having been around website infrastructure for so long and the newly-emerging Front End Optimisation becoming so important to Search Engine rankings, website sales and user experience we're very familiar with what makes a website work properly.

    Our in-depth docs provide some invaluable PHP performance improvement techniques which we've used on really busy websites, security advice gleaned from hard-won experience as well as when certain options are recommended and under which scenarios. As you would expect we've kept the most valuable advice for the documentation included with the software.

    We've also spent time making things clear and simple, especially for non-English speakers, so that you can be up and running in a matter of minutes and not left scratching your head for hours on end.

    We hope that you find CDN Failover as beneficial to your uptime as we do.

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    CDN Failover © 2014 Scalable Coding Limited.
    Registered in Scotland at 8 Albany Street, Edinburgh EH1 3QB United Kingdom. Company No SC438151.