I’ve been a part of some late night release procedures and they’re never fun. You’ve got QA, Dev, IT and a handful of managers sitting in their jammies in a group IM (or worse, a conference call) from 2:00 AM until way too early in the morning. Everyone’s grumpy and sleepy, causing the release to be more difficult and take longer. Sometimes the dreaded “rollback!” is yelled. All this because you’re running a high profile website that needs to be accessible 24/7, and 2:00 AM - 5:00 AM downtime is better than daytime downtime.

If you're a site that doesn't have 10s of thousands to drop on a real http load balancer, use this strategy to release software during business hours with no downtime using apache's mod_proxy_balancer....

Adapted From the website:

HAProxy is a free, very fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for web sites crawling under very high loads while needing persistence or Layer7 processing. Supporting tens of thousands of connections is clearly realistic with todays hardware. Its mode of operation makes its integration into existing architectures very easy and riskless, while still offering the possibility not to expose fragile web servers to the Net.