I’ve recently been looking back on what we have written about our architecture in the past, and came to a stunning realization. That realization is that while we have many many different articles about what we have been doing there hasn’t been a good, solid overview of our architecture in a long time. In fact, the last really comprehensive write-up was done by Jeff before this blog even existed. And, boy I do have to say there has been quite a lot of change behind the scenes since then. So, my dear readers I’m going to take some time – and my next few blog posts – to give everyone an in depth look into how we have the Stack Exchange Network setup to serve between 12 and 14 Million page views per day.

How these posts will breakdown

Since we have obviously grown, and are offering more services to our users I’m going to break these posts out by each of the 4 major services we offer to our user base:

  • Core Q&A (this includes the API)
  • Careers
  • Chat
  • Community Blogs

Each one of these systems all work towards our goal of making the internet better, but they have different requirements and different challenges.

In this first post, I’ll be focusing on our core Q&A system, since that is after all our bread and butter.

Core Q&A

First, a high level overview of how everything is put together:

The Hardware

Our core hardware setup hasn’t changed all that much. Well, I should say the chassis haven’t changed that much. We’ve done a lot of work to upgrade the internals of the servers when needed to address performance issues as they came up, as well as handle issues that resulted from Stack Overflow being so big.

Web Tier

Of these 10 Servers, 3 are dedicated to Stack Overflow with an additional 3 servers serving Stack Overflow and the Stack Exchange Network. We have one server dedicated to Dev/QA – which also hosts meta.stackoverflow.com. Our Web Tier machines normally operate between 5 and 20% utilization. We have plenty of room to grow on these boxes.

  • 10 Dell R610 IIS web servers:
    • 2x Intel Xeon Processor E5640 @ 2.66 GHz Quad Core with 8 threads
    • 16 GB RAM
    • Windows Server 2008 R2
    • 2 drives
      • RAID 1
      • 2x Intel 320 300GB SSD (RAID 1)

DB Tier

We have two database server pairs. One pair is dedicated to running Stack Overflow, and the other runs the rest of the network. We run development against the secondary server of the non-stack overflow database pair. Both of our database pairs run at about 20% utilization, so once again we have room to grow here as well.

  • 2 Dell R710 database servers:
    • 2x Intel Xeon Processor X5680 @ 3.33 GHz
    • 96 GB RAM
    • 8 spindles
      • Mirrored Pair for OS
      • 6 disk RAID10 for databases
    • SQL Server 2008 R2 SP1
  • 2 Dell R710 database servers (Stack Overflow Dedicated):
    • 2x Intel Xeon Processor X5680 @ 3.33 GHz
    • 96 GB RAM
    • 8 drives
      • Mirrored Pair for OS
      • 6 drive RAID10 of Intel X25-E SSDs for Database
    • SQL Server 2008 R2 SP1

Caching Tier

We run redundant Redis servers for our caching tier.

  • 2 Dell R610 Redis servers:
    • 2x Intel Xeon Processor E5640 @ 2.66 GHz
    • 16 GB RAM
    • CentOS

Network Layer

We use HAProxy for our load balancing, and Cisco Switching.

  • 2 Dell R610 HAProxy servers:
    • 1x Intel Xeon Processor E5640 @ 2.66 GHz
    • 4 GB RAM
    • Ubuntu Server
  • 6 WS-C2960S-48TS-L Gigabit switches
    • FlexStack (two stacks, 4 switches and 2 switches)

Data Integrity

As with any system, making sure that your data is backed up and the backups are good is an integral part to your service offering. We backup our databases nightly and restore them to two different locations. One local to our NY data center for our devs to work against, and one remote in our OR data center.


Overall I believe that we are in a good place and have plenty of room to grow given our current setup. As always we will constantly be looking at our infrastructure and tweaking it to get the best performance possible and give our users the best experience possible.