Objects Increasingly Refreshed Several Hours After Restart Of Web Cache (Doc ID 1300325.1)

Last updated on MARCH 08, 2017

Applies to:

Web Cache - Version: 10.1.2.3.0 and later   [Release: AS10gR2 and later ]
IBM AIX on POWER Systems (64-bit)

Symptoms

In a Web Cache cluster, after several hours of load (after a restart) Web Cache refreshes cached objects, at an increasing rate, from the Origin Server.

After Web Cache refreshes cache objects, it does not set the Age (in the HTTP Header) of that document to 0 (zero). After a period of time (hours or days) the 'Age' remains at or very close to the 'Max-Age' causing objects to be refreshed at an even increasing rate from the Origin Server.

The event log then cycles with the following Trace level messages:
"The cached object is stale and an origin server will be contacted for a fresh copy"


The symptoms can be confirmed by monitoring the HTTP header and comparing Age and Max-Age:

(i) Make an initial request to cache the object into Web Cache -
wget -S --no-proxy 'http://host.domain:32790/test.html'

(ii) Now make several requests to the same object like this -
wget -S --no-proxy 'http://host.domain:32790/test.html'

(iii)  Review the header response and ensure that 'Age' is counting up  the seconds from zero towards 'Max-Age' which means the document is indeed sourced from the cache. Then, only after 'Age' reaches 'Max-Age' the cached object should be refreshed from the Origin Server and 'Age' reset to zero. The access log for the Origin Server can also be monitored to ensure requests are only made to objects which have become stale in Web Cache,

Cause

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