Today part of Amazon Web Services failed, taking down with it a slew of startups that all run on Amazon’s Cloud infrastructure. AirBNB was one of the biggest, but also Heroku, Reddit, Minecraft, Flipboard & Coursera down with it. Its not the first time. What the heck happened, and why should we care?
1. Root Cause
The AWS service allows companies like AirBNB to build web applications, and host them on servers owned and managed by Amazon. The so-called raw iron of this army of compute power sits in datacenters. Each datacenter is a zone, and there are many in each of their service regions including US East (Northern Virginia), US West (Oregon), US West (Northern California), EU (Ireland), Asia Pacific (Singapore), Asia Pacific (Tokyo), South America (Sao Paulo), and AWS GovCloud.
Today one of those datacenters in the Northern Virginia region had a failure. What does this mean? Essentially firms like AirBNB that hosted their applications ONLY in Northern Virginia experienced outages.
As it turns out, Amazon has a service level agreement of 99.95% availability. We’ve long since said goodbye to the five nines. HA is overrated.
2. Use Redundancy
Although there are lots of pieces and components to a web infrastructure, two big ones are webservers and database servers. Turns out AirBNB could make both of these tiers redundant. How do we do it?
On the database side, you can use Amazon’s multi-az or alternately read-replicas. Each have different service characteristics so you’ll have to evaluate your application to figure out what will work for you.
Then there is the option to host mysql or Percona directly on Amazon servers yourself and use replication.
[quote]Using redundant components like placing webservers and databases in multiple regions, AirBNB could avoid an Amazon outage like Monday’s that affected only Northern Virginia.[/quote]
When do I want RDS versus mysql? Here are some use cases for RDS versus roll your own MySQL.
Now that you’re using multiple zones and regions for your database the hard work is completed. Webservers can be hosted in different regions easily, and don’t require complicated replication to do it.
3. Have a browsing only mode
Another step AirBNB can take to be resilient is to build a browsing only mode into their application. Often we hear about this option for performing maintenance without downtime. But it’s even more valuable during a situation like this. In a real outage you don’t have control over how long it lasts or WHEN it happens. So a browsing only mode can provide real insurance.
For a site like AirBNB this would mean the entire website was up and operating. Customers could browse and view listings, only when they went to book a room would the encounter an error. This would be a very small segment of their customers, and a much less painful PR problem.
Facebook has experience intermittent outages of it’s service. People hardly notice because they’ll often only see a message when they are trying to comment on someone’s wall post, send a message or upload a photo. The site is still operating, but not allowing changes. That’s what a browsing only mode affords you.
[quote]A browsing only mode can make a big difference, keeping most of the site up even when transactions or publish are blocked.
Drupal, an open source CMS system that powers sites like Adweek.com, TheHollywoodReporter.com, and Economist.com uses this technology. It supports a browsing only mode out of the box. An amazon outage like this one would only stop editors from publishing new stories temporarily. A huge win to sites that get 50 to 100 million with-an-m pageviews per month.
4. Web Applications need Feature Flags
Feature flags give you an on/off switch. Build them into heavy duty parts of your site, and you can disable those in an emergency. Host components multiple availability zones for extra peace of mind.
One of our all time most popular posts 5 Things Toxic to Scalability included some indepth discussion of feature flags.
5. Consider Netflix’s Simian Army
Netflix takes a very progressive approach to availability. They bake redundancy and automation right into all of their infrastructure. Then they run an app called the Chaos Monkey which essentially causes outages, randomly. If resilience from constantly falling and getting back up can’t make you stronger, I don’t know what can!
Take a look at the Netflix blog for details on intentional load & stress testing.
6. Use multiple cloud providers
If all of the above isn’t enough for you, taking it further you’d do as George Reese of enstratus recommends and use multiple cloud providers. Not being beholden to one company could help in more situations than just these type of service disruptions too.
Basic EC2 Best Practices mean building redundancy into your infrastructure. Multiple cloud providers simply take that one step further.
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