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If you have hundreds or thousands of hosts, you want automatic detection and recovery from failures.
Each software deployment also needs to be managed carefully.
If a host crashes say once a year on average, then with 365 hosts you can a crash every day. Never have only one host able to satisfy the request.
An update to a product generates a Varnish PURGE request to remove all cache entries tagged with the specified product ID.
Caching can give one to two orders of magnitude improvement for customers hitting a web site when the cache hit rate is high, hence caching is a very useful and effective way to cope with increases in site traffic.
So for any particular site, different combinations of approaches may be appropriate to solve the scaling problem for the site.
This post explores common and effective techniques to scale.