Periodic refresh of cache

Question

Hi
Will the below ensure that distributed (internal local cache scheme) is refresh at regular intervals by setting high-units, low-units to zero.
The use case is to expire all entries in cache at regular intervals, and reload them from cache store.
<distributed-scheme>
<scheme-name>example-distributed</scheme-name>
<service-name>DistributedCache1</service-name>
<backing-map-scheme>
     <read-write-backing-map-scheme>
     <scheme-name>DBCacheLoaderScheme</scheme-name>
     <internal-cache-scheme>
     <local-scheme>
<high-units>0</high-units>
<low-units>0</low-units>
                         <expiry-delay>60s</expiry-delay>
     </local-scheme>
     </internal-cache-scheme>
          <cachestore-scheme>
          <class-scheme>
               <class-name>com.test.DBCacheStore</class-name>
               <init-params>
                              <init-param>
                                   <param-type>java.lang.String</param-type>
                                   <param-value>locations</param-value>
                              </init-param>
                              <init-param>
                                   <param-type>java.lang.String</param-type>
                                   <param-value>{cache-name}</param-value>
                              </init-param>
               </init-params>                     
               </class-scheme>
          </cachestore-scheme>
          <read-only>true</read-only>
          <refresh-ahead-factor>0.5</refresh-ahead-factor>     
     </read-write-backing-map-scheme>
     </backing-map-scheme>
     <thread-count>10</thread-count>
<autostart>true</autostart>
</distributed-scheme>
I see with above config, that all objects are being expired at 60s(expiry delay) * .5 (refresh-ahead-factor) = 30 secs, (as low-units and max-units is set to zero). Please clarify if this configuration works in all cases.
Thanks
sunder

Answer 1

Hi sunder,
refresh-ahead does not on its own guarantees refresh.
Refresh-ahead is only ever consulted if there was a get()-like operation (read operation specifically directed against the entry... queries are not suitable for triggering refresh-ahead) against the entry within the interval designated by the refresh-ahead-factor.
Let's suppose you have refresh-ahead set to 0.25, and expiry-delay set to 1 minute, then if you have a get against the entry in the last 15 second (= 0.25 * 1 minute) before it expires, then there is going to be an asynchronous fetch of the entry from the cache store. This means that the triggering read will see the old value, but once the asynchronous fetch completes, it will replace the old value with the newly fetched value in the cache.
But if there are no triggering reads, then it will simply expire instead of refreshing.
Best regards,
Robert
Edited by: robvarga on May 9, 2011 1:56 PM