Surviving the Digg Effect: High-Availability Load Balancing with HAProxy

It starts with a trickle of traffic. Then, a link hits the front page of Digg or Slashdot. Suddenly, your Apache processes max out, swap usage spikes, and your server stops responding. By the time you SSH in to restart httpd, the traffic is gone—and so is your reputation.

I’ve seen this scenario play out a dozen times this year. The problem isn't usually the code; it's the architecture. Relying on a single server to handle application logic, database queries, and static assets is a recipe for disaster in 2009.

If you are running mission-critical applications targeting the Norwegian market, you need redundancy. Today, we are going to look at the industry standard for software load balancing: HAProxy. We will look at how to deploy it on CentOS 5.3 to split traffic across multiple nodes, ensuring your site stays up even when one backend server goes dark.

The C10k Problem and Why Apache Struggles

Apache 2.2 is a workhorse, but its process-based model (Prefork) is heavy. Each connection consumes significant RAM. When you have thousands of concurrent keep-alive connections (the C10k problem), your server runs out of memory long before it runs out of CPU.

HAProxy (High Availability Proxy) solves this by acting as a reverse proxy. It handles thousands of concurrent connections with an event-driven model, forwarding only valid requests to your backend web servers. It is lightweight, incredibly stable, and free.

Setting Up HAProxy 1.3 on CentOS 5

Let's assume you have a setup with one load balancer and two web servers. In a CoolVDS environment, I recommend using our Xen-based VDS instances for this. Unlike OpenVZ, Xen guarantees RAM and CPU allocation, meaning your load balancer won't choke because a neighbor is compiling a kernel.

First, install HAProxy (you may need the EPEL repository):

Now, let's configure /etc/haproxy/haproxy.cfg. We will set up a standard Round Robin configuration.

global
    log 127.0.0.1   local0
    maxconn 4096
    user haproxy
    group haproxy
    daemon

defaults
    log     global
    mode    http
    option  httplog
    option  dontlognull
    retries 3
    option  redispatch
    maxconn 2000
    contimeout 5000
    clitimeout 50000
    srvtimeout 50000

listen webfarm 0.0.0.0:80
    mode http
    stats enable
    stats auth admin:password
    balance roundrobin
    option httpclose
    option forwardfor
    server web01 192.168.0.101:80 check
    server web02 192.168.0.102:80 check

Configuration Breakdown

• balance roundrobin: Distributes requests sequentially. Ideally suited for stateless web servers.
• option httpclose: Critical for PHP applications. It closes the connection to the backend after the response, freeing up Apache slots immediately.
• check: This tells HAProxy to health-check the server. If web01 fails (returns 500 or times out), HAProxy instantly removes it from rotation. No downtime for the user.

Pro Tip: Don't rely on the default kernel parameters for high-traffic load balancers. Edit /etc/sysctl.conf and increase the port range: net.ipv4.ip_local_port_range = 1024 65000. This prevents port exhaustion during heavy connection churn.

Latency and Sovereignty: The Norwegian Context

Why host this in Norway? Aside from the obvious SEO benefits for local domains (.no), latency is the killer of user experience. If your target audience is in Oslo or Bergen, routing traffic through a data center in Frankfurt adds unnecessary milliseconds.

More importantly, we have the Personal Data Act (Personopplysningsloven) and the Datatilsynet to consider. Keeping customer data within Norwegian borders simplifies compliance significantly compared to navigating the complex Safe Harbor agreements required when hosting in the US.

When measuring latency to the NIX (Norwegian Internet Exchange) in Oslo, our CoolVDS instances typically show <2ms ping times. This responsiveness is critical for database-heavy applications where every millisecond of wait time compounds.

Hardware Isolation Matters

You might be tempted to run this on shared hosting or cheap VPS providers overselling their nodes. Don't.

In a load balancing scenario, network I/O is your bottleneck. If your VPS provider is running 500 containers on a single SATA RAID array, your packet processing will stutter. At CoolVDS, we utilize Enterprise SAS 15k RPM drives in RAID-10 and strict Xen isolation. This ensures that the I/O throughput you pay for is the I/O throughput you get. While SSDs are beginning to enter the enterprise market (like the Intel X25-E), a well-tuned SAS RAID-10 is still the reliability king for write-heavy workloads.

The Verdict

Scaling isn't just about adding more servers; it's about adding intelligence to your infrastructure. HAProxy provides that intelligence, and a dedicated, isolated virtualization platform provides the muscle.

Stop praying your single server holds up. Build a cluster that can handle the load.

Ready to architect for uptime? Deploy a high-performance Xen VDS in Norway today with CoolVDS.