Web Caching

High Availability

Unfortunately, computer systems do occasionally fail. If 100% availability is important, you should build some redundancy into your caching service. Redundancy can be a complicated business, and many organizations take it to extremes. Because computer systems have so many different ways to fail, you’ll be faced with numerous options.

Power is one of the most obvious and common causes of failure. An uninterruptible power supply (UPS) is a good idea and relatively inexpensive. Even a small UPS that provides enough power for a few minutes will get you through most power outages. Many caching products and general-purpose servers have the option of redundant power supplies. These help guard against failure of the power supply itself and against power outages if you have separate power feeds.

Disk drives are also prone to failures. This is especially true for caching proxies that are busy 24 hours a day, 7 days a week. Some people like to use RAID systems to provide high-availability storage. In my experience, caches perform noticeably worse with RAID than without. If you really want to use RAID, you should probably use mirroring (RAID level 1) only. The importance of reliable storage depends on your location and the quality of your Internet connection. If your connection is good, the loss of a disk is not a big deal. However, if your connection is poor, the data on disk is very valuable, and RAID makes more sense.

In Chapter 9, I mentioned a number of ways that clusters ...