Tag: simple

Content Delivery Network (CDN) using Linode VPS

This month one of the neat things I’ve done was to set up a small content delivery network (CDN) for speedy downloading of files across the globe. For one reason and another (mostly the difficulty in doing this purely with DNS and the desire not to use AWS), I opted to do this using my favourite VPS provider, Linode. All in all (and give or take DNS propagation time) I reckon it’s possible to deploy a multi-site CDN in under 30 minutes given a bit of practice. Not too shabby!

For this recipe you will need:

  1. Linode account
  2. A domain name and DNS management

What you’ll end up with:

  1. 3x Ubuntu 12.04 LTS VPS, one each in London, Tokyo and California
  2. 3x NodeBalancers, one each in London, Tokyo and California
  3. 1x user-facing general web address
  4. 3x continent-facing web addresses

I’m going to use “mycdn.com” wherever I refer to my DNS / domain. You should substitute your domain name wherever you see it.

So, firstly log in to Linode.

Create three new Linode 1024 small VPSes (or whatever size you think you’ll need). I set mine up as Ubuntu 12.04 LTS with 512MB swap but otherwise nothing special. Set one each to be in London, Tokyo and Fremont. Set the root password on each. Under “Settings”, give each VPS a label. I called mine vps-<city>-01. Under “Remote Settings”, give each a private IP and note them down together with the VPS/data centre they’re in.

At this point it’s also useful (but not strictly necessary) to give each node a DNS CNAME for its external IP address, just so you can log in to them easily by name later.

Boot all three machines and check you can login to them. I find it useful here to do an

apt-get update ; apt-get dist-upgrade.

You can also now install Apache and mod_geoip on each node:

apt-get install apache2 libapache2-mod-geoip
a2enmod include
a2enmod rewrite

You should now be able to bring up a web browser on each VPS (public IP or CNAME) in turn and see the default Apache “It works!” page.

Ok.. still with me? Next we’ll ask Linode to fire up three NodeBalancers, again one in each of the data centres, for each VPS. I labelled mine cdn-lb-<city>-01. Each one can be configured with a Port, 80 with, for now, the default settings. Add a host to each NodeBalancer with the private IP of each VPS, and the port, e.g. 192.168.128.123:80 . Note that each VPS hasn’t yet been configured to listen on those interfaces so each NodeBalancer won’t recognise its host as being up.

Ok. Let’s fix those private interfaces. SSH into each VPS using the root account and the password you set earlier. Edit /etc/network/interfaces and add:

auto eth0:1
iface eth0:1 inet static
	address <VPS private address here>
	netmask <VPS private netmask here>

Note that your private netmask is very unlikely to be 255.255.255.0 (probably) like your home network and yes, this does make a difference. Once that configuration is in, you can:

ifup eth0:1

Now we can add DNS CNAMEs for each NodeBalancer. Take the public IP for each NodeBalancer over to your DNS manager and add a meaningful CNAME for each one. I used continental regions americas, apac, europe, but you might prefer to be more specific than that (e.g. us-west, eu-west, …). Once the DNS propagates you should be able to see each of your Apache “It works!” pages again in your browser, but this time the traffic is running through the NodeBalancer (you might need to wait a few seconds before the NodeBalancer notices the VPS is now up).

Ok so let’s take stock. We have three VPS, each with a NodeBalancer and each running a web server. We could stop here and just present a homepage to each user telling them to manually select their local mirror – and some sites do that, but we can do a bit better.

Earlier we installed libapache2-mod-geoip. This includes a (free) database from MaxMind which maps IP address blocks to the continents they’re allocated to (via the ISP who’s bought them). The Apache module takes the database and sets a series of environment variables for each and every visitor IP. We can use this to have a good guess at roughly where a visitor is and bounce them out to the nearest of our NodeBalancers – magic!

So, let’s poke the Apache configuration a bit. rm /etc/apache2/sites-enabled/000-default. Create a new file /etc/apache2/sites-available/mirror.mycdn.com and give it the following contents:

<VirtualHost>
	ServerName mirror.mycdn.com
	ServerAlias *.mycdn.com
	ServerAdmin webmaster@mycdn.com

	DocumentRoot /mirror/htdocs

	DirectoryIndex index.shtml index.html

	GeoIPEnable     On
	GeoIPScanProxyHeaders     On

	RewriteEngine     On

	RewriteCond %{HTTP_HOST} !americas.mycdn.com
	RewriteCond %{ENV:GEOIP_CONTINENT_CODE} NA|SA
	RewriteRule (.*) http://americas.mycdn.com$1 [R=permanent,L]

	RewriteCond %{HTTP_HOST} !apac.mycdn.com
	RewriteCond %{ENV:GEOIP_CONTINENT_CODE} AS|OC
	RewriteRule (.*) http://apac.mycdn.com$1 [R=permanent,L]

	RewriteCond %{HTTP_HOST} !europe.mycdn.com
	RewriteCond %{ENV:GEOIP_CONTINENT_CODE} EU|AF
	RewriteRule (.*) http://europe.mycdn.com$1 [R=permanent,L]

	<Directory />
		Order deny,allow
		Deny from all
		Options None
	</Directory>

	<Directory /mirror/htdocs>
		Order allow,deny
		Allow from all
		Options IncludesNoExec
	</Directory>
</VirtualHost>

Now ln -s /etc/apache2/sites-available/mirror.mycdn.com /etc/apache2/sites-enabled/ .

mkdir -p /mirror/htdocs to make your new document root and add a file called index.shtml there. The contents should look something like:

<html>
 <body>
  <h1>MyCDN Test Page</h1>
  <h2><!--#echo var="HTTP_HOST" --></h2>
<!--#set var="mirror_eu"       value="http://europe.mycdn.com/" -->
<!--#set var="mirror_apac"     value="http://apac.mycdn.com/" -->
<!--#set var="mirror_americas" value="http://americas.mycdn.com/" -->

<!--#if expr="${GEOIP_CONTINENT_CODE} == AF"-->
 <!--#set var="continent" value="Africa"-->
 <!--#set var="mirror" value="${mirror_eu}"-->

<!--#elif expr="${GEOIP_CONTINENT_CODE} == AS"-->
 <!--#set var="continent" value="Asia"-->
 <!--#set var="mirror" value="${mirror_apac}"-->

<!--#elif expr="${GEOIP_CONTINENT_CODE} == EU"-->
 <!--#set var="continent" value="Europe"-->
 <!--#set var="mirror" value="${mirror_eu}"-->

<!--#elif expr="${GEOIP_CONTINENT_CODE} == NA"-->
 <!--#set var="continent" value="North America"-->
 <!--#set var="mirror" value="${mirror_americas}"-->

<!--#elif expr="${GEOIP_CONTINENT_CODE} == OC"-->
 <!--#set var="continent" value="Oceania"-->
 <!--#set var="mirror" value="${mirror_apac}"-->

<!--#elif expr="${GEOIP_CONTINENT_CODE} == SA"-->
 <!--#set var="continent" value="South America"-->
 <!--#set var="mirror" value="${mirror_americas}"-->
<!--#endif -->
<!--#if expr="${GEOIP_CONTINENT_CODE}"-->
 <p>
  You appear to be in <!--#echo var="continent"-->.
  Your nearest mirror is <a href="<!--#echo var="mirror" -->"><!--#echo var="mirror" --></a>.
 </p>
 <p>
  Or choose from one of the following:
 </p>
<!--#else -->
 <p>
  Please choose your nearest mirror:
 </p>
<!--#endif -->

<ul>
 <li><a href="<!--#echo var="mirror_eu"       -->"><!--#echo var="mirror_eu"        --></a> Europe (London)</a></li>
 <li><a href="<!--#echo var="mirror_apac"     -->"><!--#echo var="mirror_apac"      --></a> Asia/Pacific (Tokyo)</a></li>
 <li><a href="<!--#echo var="mirror_americas" -->"><!--#echo var="mirror_americas"  --></a> USA (Fremont, CA)</a></li>
</ul>

<pre style="color:#ccc;font-size:smaller">
http-x-forwarded-for=<!--#echo var="HTTP_X_FORWARDED_FOR" -->
GEOIP_CONTINENT_CODE=<!--#echo var="GEOIP_CONTINENT_CODE" -->
</pre>
 </body>
</html>

Then apachectl restart to pick up the new virtualhost and visit each one of your NodeBalancer CNAMEs in turn. The ones which aren’t local to you should redirect you out to your nearest server.

Pretty neat! The last step is to add a user-facing A record, I used mirror.mycdn.com, and set it up to DNS-RR (Round-Robin) the addresses of the three NodeBalancers. Now Set up a cron job to rsync your content to the three target VPSes, or a script to push content on-demand. Job done!

For extra points:

  1. Clone another VPS behind each NodeBalancer so that each continent is fault tolerant, meaning you can reboot one VPS in each pair without losing continental service.
  2. Explore whether it’s safe to add the public IP of one Nodebalancer to the Host configuration of a NodeBalancer on another continent, effectively making a resilient loop.

An Interview Question

I’d like to share a basic interview question I’ve used in the past. I’ve used this in a number of different guises over the years, both at Sanger and at ONT but the (very small!) core remains the same. It still seems to be able to trip up a lot of people who sell themselves as senior developers on their CVs and demand £35k+ salaries.

You have a list of characters.

  1. Remove duplicates

The time taken for the interviewee to scratch their head determines whether they’re a Perl programmer, or at least think like one – this is an idomatic question in Perl. It’s a fairly standard solution to anyone who uses hashes, maps or associative arrays in any language. It’s certainly a lot harder without them.

The answer I would expect to see would run something like this:

#########
# pass in an array ref of characters, e.g.
# remove_dupes([qw(a e r o i g n o s e w f e r g e r i g e o n k)]);
#
sub remove_dupes {
  my $chars_in  = shift;
  my $seen      = {};
  my $chars_out = [];

  for my $char (@{$chars_in}) {
    if(!$seen->{$char}++) {
      push @{$chars_out}, $char;
    }
  }

  return $chars_out;
}

Or for the more adventurous, using a string rather than an array:

#########
# pass in a string of characters, e.g.
# remove_dupes(q[uyavubnopwemgnisudhjopwenfbuihrpgbwogpnskbjugisjb]);
#
sub remove_dupes {
  my $str  = shift;
  my $seen = {};
  $str     =~ s/(.)/( !$seen->{$1}++ ) ? $1 : q[]/smegx;
  return $str;
}

The natural progression from Q1 then follows. It should be immediately obvious to the interviewee if they answered Q1 inappropriately.

  1. List duplicates
#########
# pass in an array ref of characters, e.g.
# list_dupes([qw(a e r o i g n o s e w f e r g e r i g e o n k)]);
#
sub list_dupes {
  my $chars_in  = shift;
  my $seen      = {};
  my $chars_out = [];

  for my $char (@{$chars_in}) {
    $seen->{$char}++;
  }

  return [ grep { $seen->{$_} > 1 } keys %{$seen} ];
}

and with a string

#########
# pass in a string of characters, e.g.
# list_dupes(q[uyavubnopwemgnisudhjopwenfbuihrpgbwogpnskbjugisjb]);
#
sub list_dupes {
  my $str  = shift;
  my $seen = {};
  $str     =~ s/(.)/( $seen->{$1}++ > 1) ? $1 : q[]/smegx;
  return $str;
}

The standard follow-up is then “Given more time, what would you do to improve this?”. Well? What would you do? I know what I would do before I even started – WRITE SOME TESTS!

It’s pretty safe to assume that any communicative, personable candidate who starts off writing a test on the board will probably be head and shoulders above any other.

If I’m interviewing you tomorrow and you’re reading this now, it’s also safe to mention it. Interest in the subject and a working knowledge of the intertubes generally comes in handy for a web developer. I’m hiring you as an independent thinker!