Don't Let the "Slashdot Effect" Kill Your Servers

It’s 3:00 AM. Your monitoring system is screaming. Your client just got linked on a major news outlet or, heaven forbid, the front page of Digg. Traffic is spiking vertically. If you are running a standard LAMP stack on a single box, you are already dead. Apache processes are spawning out of control, eating up every megabyte of RAM until the kernel invokes the OOM killer.

I saw this happen last month during the Eyjafjallajökull ash cloud crisis. Travel sites hosted on shared setups crumbled under the desperate refreshing of stranded passengers. The ones that stayed up? They weren't just lucky. They were architected correctly.

You don't need a massive budget. You need HAProxy. Specifically, the new version 1.4.

Why Apache mod_proxy Isn't Enough

Many sysadmins try to use Apache as their load balancer. It works, but it’s heavy. Apache uses a process or thread per connection. When you have 5,000 concurrent users, you do not want 5,000 heavy Apache processes just to route packets.

HAProxy is different. It uses an event-driven model (similar to Nginx). It can saturate a Gigabit link with thousands of simultaneous connections while barely touching the CPU. It acts as the bouncer at the door, only letting valid requests through to your backend application servers.

The Architecture: Decoupling for Survival

Here is the setup we deploy for high-availability clients at CoolVDS:

• Frontend (The Shield): One CoolVDS VPS running HAProxy. This handles the public IP.
• Backend (The Workers): Two or more web servers (Apache/Lighttpd) on a private network (LAN).
• Database: A dedicated MySQL box, separated from the web heads.

This setup allows you to take down a web server for patching (CentOS updates, anyone?) without the user noticing. HAProxy detects the failure and routes traffic to the remaining healthy node.

Configuration: The haproxy.cfg Boilerplate

With the release of HAProxy 1.4 earlier this year, we finally got native client-side keep-alive support. This reduces latency significantly for the end user. Here is a battle-tested config snippet for /etc/haproxy/haproxy.cfg:

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

frontend http_front
    bind *:80
    default_backend web_cluster

backend web_cluster
    mode http
    balance roundrobin
    option httpchk HEAD /health_check.php HTTP/1.0
    server web1 10.0.0.10:80 check
    server web2 10.0.0.11:80 check

Pro Tip: Never rely on a simple TCP check. Create a health_check.php file that actually queries the local database. If MySQL fails on web1, HAProxy should stop sending traffic there immediately.

The "Steal Time" Trap: Why Visualization Matters

You might configure HAProxy perfectly, but if your underlying host is garbage, you will still fail. This is common with oversold OpenVZ providers.

If you run top and see high %st (steal time), your provider is overcrowding the physical server. The CPU cycles you paid for are being stolen by a neighbor. For a load balancer, this introduces jitter and latency. In Norway, where users expect near-instant loads due to our high-speed fiber infrastructure, 500ms of latency is noticeable.

At CoolVDS, we prioritize Xen virtualization. Xen offers better resource isolation. We don't overcommit memory. When you buy 512MB RAM, it’s yours. This stability is critical for HAProxy, which relies on consistent memory buffers to manage connections.

Data Privacy in 2010: The Norwegian Context

Hosting outside of Norway is becoming a headache. With the Personopplysningsloven (Personal Data Act) and the strict oversight of Datatilsynet, moving data across borders—especially to the US—requires navigating the Safe Harbor principles carefully.

By keeping your load balancer and backends in our Oslo data center, you reduce latency to the NIX (Norwegian Internet Exchange) to under 2ms for local users. More importantly, you ensure that your customer data remains under Norwegian jurisdiction, simplifying compliance significantly.

Key Takeaways for the Pragmatic Admin

1. Upgrade to 1.4: The keep-alive support is a requirement for modern interactive web apps.
2. Monitor IO Wait: If your current VPS provider has slow disks, your logs will block the load balancer. We use high-performance RAID 10 SAS arrays (and are testing early Enterprise SSDs) to eliminate this bottleneck.
3. Test Failure: Yank the network cable (virtually) on one web server. If your site goes down, you configured HAProxy wrong.

Building a high-availability cluster used to require expensive hardware load balancers like F5 Big-IP. Now, you can do it with open-source software and reliable managed hosting.

Don't wait for the next traffic spike to find out your architecture is weak. Spin up a CoolVDS instance today and configure your first load balancer. Your uptime depends on it.