The key to a successful site setup on Amazon EC2 is scalability and redundancy. RightScale makes
this easy by providing server templates and multi-server
deployments. To get started, let’s take the simplest case: a single
server set-up. We have a free “Rails all-in-one” server template
that is excellent not just to play around with, but also to use as
development server, staging server, or even as production server
for small sites that don’t need more horsepower or much
redundancy.
Our Rails all-in-one is described in more detail
elsewhere,
but you see on the right what’s involved: it runs Apache as a
reverse proxy in front of 4 Mongrel/Rails processes all backed by a
simple MySQL installation. Last, but not least, we set-up cron jobs
that run a mysqldump every 10 minutes to Amazon S3 so you have your
data safe in case the instance dies unexpectedly. Apache in the
front can be set-up to serve up static and cached pages, it can do
HTTP and/or HTTPS, it can canonicalize the hostname (e.g.
redirect http://mysite.com to http://www.mysite.com), and it can
serve-up a maintenance page while you’re updating your app. Oh, of
course Apache load balances across the 4 Mongrels too!
Redundant site
Ready for more? You’re almost ready to launch for real, you
expect some traffic soon and don’t want to be reliant on a single
server anymore. Time to upgrade to a fully redundant site
architecture using 4 servers! 
The set-up almost all our customers use consists
of two front-end servers and two back-end database servers giving
us full redundancy. We use this ourselves for the RightScale site
itself! Let’s walk through the set-up from beginning to end.
It all starts when a user types http://www.mysite.com into the
browser. The browser does a DNS lookup
and gets two IP addresses which are the public IPs of the two
front-end instances. The browser picks one and tries to connect. If
it fails, it rather quickly tries the other, this gives you the
fault tolerance you need in case one of the instances dies or has
other problems. Also, having multiple IP addresses for your site is
the only form of fail-over that browsers support, see this
page for additional details.
The first thing the request from your browser hits is Apache,
which has the same roles as in the all-in-one server: dealing with
SSL, canonicalizing the hostname, serving
up static files, putting up a maintenance page, and anything else
you might want a full-fledged web server for. For requests destined
to your application, Apache acts as a reverse proxy and forwards
the request to HAproxy on the same machine.
HAproxy is a very nice piece of software that proxies and load
balances requests to back-end servers. We use it for HTTP here, but it can also do plain UDP and TCP load balancing,
for example for DNS or mail servers. We
chose HAproxy because it has good support for health checks and the
ability to redirect requests to alternate servers if a back-end
fails mid-way. HAproxy is set-up to send a request to each back-end
process (Mongrel/Rails in our example) to ensure that it’s running
properly. It then only forwards requests to servers that respond.
While Apache can do load balancing across multiple back-end servers
as well using modbalanceproxy it does not include health
checks. What this means is that when a sever goes down it has to
send live customer requests to it every few seconds to see whether
it has come back up. This means that while any Mongrel process is
down on any server your customers are going to be impacted because
some of their requests are being sent into a black hole. Not
nice…
HA proxy forwards the request to one of the Mongrel/Rails
processes on either of the two servers. Load balancing across both
servers is nice because it means that you can shut the Mongrels on
one server down to update the code without impacting customers at
all.
Everything on the front-end servers is open source software
except for your application. So we need a way you can get you app
code onto the instance at boot time, and a way you can update the
code. Note that for major upgrades we always recommend to launch
fresh instances so you keep the old ones around for a day, just in
case you want to switch back. (Hey, that’s really cheap insurance
at only $2.40 per day per server!) We provide two different
RightScripts to do minor code updates: one pulls the code from a
tarball located on S3, the other does an svn export from your
subversion repository. We recommend the S3 route for production use
because else starting new servers depends on the availability of
your svn repository and often the svn export is the slowest portion
of the entire instance boot process. But sometimes the svn route is
just so much more convenient, specially if you’re playing with a
test set-up where you change the code frequently. In addition, for
Rails, we set-up the app code directory structure the same way
capistrano does, so you can point your capistrano config file at
your instance and do a “cap update”. Again, something we don’t
recommend for production servers but really handy for test and dev
boxes.
Behind the front-end servers we place two replicated MySQL
instances managed through our Manager for
MySQL with backups to Amazon S3. We use frequent backups from
the slave server where the load of the backup itself doesn’t affect
production and daily backups from the master as added
security.
Scalable redundant site
For a fully redundant and scalable site we recommend an
architecture that is a natural extension from the 4-server set-up
using more of the same components. We basically add a number of
Mongrel/Rails application servers and hook them into the load
balancing rotation on the two front-end servers. This array of app
servers can now be expanded and contracted as warranted by the load
on the web site: expand to handle surges in traffic when your PR
and marketing lands a success, contract at night when the load on
your site goes down and you’d rather hold on to your $$. The
wonderful thing is that with this set-up you are paying for the
average cost of your hosting needs, not for a once-a-month
peak!
If you look closely we’re running the app server on the two
front-end load balancing instances. We find that the load balancing
takes very few resources and that there’s room for some application
cycles. Using HA proxy it’s easy to have less traffic go to the
local app servers than to remote dedicated instances. The reason we
keep the app on the front-end instances (as opposed to switching to
pure load balancing instances) is that this way there are always
two app servers available even if the array is scaled back to zero
servers. Or put differently, when your site is under minimal load
at 4am it scales down to 4 instances as opposed to 6. If the
load-balancing or serving of static files becomes a significant
load, it is of course possible to switch of the app serving on the
front-end or, alternatively, to add 2 additional from-end load
balancing instances.
The way we currently handling the changes to the load balancer
config when servers come online is to automatically edit the config
file using operational RightScripts and do a seamless restart of
HAproxy which ensures that no connections are dropped in the
change.
If you are interested in using our site setups please don’t
hesitate to try out the free Rails all-in-one server template and
please contact us for more at sales@rightscale.com. The
multi-server set-ups are not available in pre-packaged form with
the free RightScale accounts.
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