When using a forward web proxy cache, it is possible to encounter the dog-pile effect. When a new page suddenly becomes very popular or when a popular page expires from the cache, the proxy will receive a large number of requests for the page at the same time. There are two ways to handle this:
- Since the page is not already in the cache, the proxy forwards each request to the origin server, OR
- The proxy forwards the first request to the origin server, and queues the others till the response to the first request fills the cache.
Option 1 leads to the dog-pile effect. The origin server is rapidly bombarded with a large number of requests. This is usually problematic. The server slows down and the requests keep piling up.
Option 2 is called connection collapsing or collapsed forwarding. Squid supports this feature - http://www.squid-cache.org/Doc/config/collapsed_forwarding/. However, Apache Traffic Server currently does not support it (Thanks to the super-responsive folks on the traffic-server IRC channel for confirming this). It used to be supported , but was removed since the implementation was buggy : http://mail-archives.apache.org/mod_mbox/trafficserver-commits/201102.mbox/%3C20110209171030.3247A23889BF@eris.apache.org%3E
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