WebSphere eXtreme Scale Best Practices for Operation and Management

116 WebSphere eXtreme Scale Best Practices for Operation and Management

6.2.1 Default (time-to-live) evictor

A default evictor is created with every backing map that removes entries based on a

time-to-live (TTL) concept. The TTL evictor is not active by default but can be enabled

programmatically using the BackingMap interface, or it can be enabled through the

ObjectGrid descriptor XML file.

When enabled, the TTL evictor uses an algorithm based on an expiration delay (time-to-live),

which is added to a reference time. The reference time can be the creation time of an entry in

the map or the last time that an entry was accessed or updated. This behavior is defined by

the TTLtype attribute (programmatically) or the ttlEvictorType attribute in the XML file. The

TTL type is one of the following types:

򐂰 NONE

Specifies that entries never expire and, therefore, are never removed from the map,

effectively disabling the evictor

򐂰 CREATION_TIME

Specifies that entries are evicted, depending upon when they were created

򐂰 LAST_ACCESS_TIME

Specifies that entries are evicted, depending upon when they were last accessed and

whether they were read or updated at the time

򐂰 LAST_UPDATE_TIME

Specifies that entries are evicted, depending upon when they were last updated

To enable the TTL evictor for a specific map, set the TTL type and a time-to-live value in

seconds. This value is the expiration delay, which will be used to determine when to evict

entries.

Example 6-1 shows an example of enabling TTL evictors for each of three maps using the

ObjectGrid descriptor XML file.

Example 6-1 Enabling TTL evictors

<?xml version="1.0" encoding="UTF-8"?>

<objectGridConfig xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://ibm.com/ws/objectgrid/config ../objectGrid.xsd"

xmlns="http://ibm.com/ws/objectgrid/config">

<objectGrids>

<objectGrid name="grid1">

<backingMap name="map1" ttlEvictorType="NONE" />

<backingMap name="map2" ttlEvictorType="LAST_ACCESS_TIME"

timeToLive="1800" />

<backingMap name="map3" ttlEvictorType="CREATION_TIME"

timeToLive="1200" />

</objectgrid>

</objectGrids>

If you use CREATION_TIME, an entry is evicted when its time from creation equals its

timeToLive value. If you set the timeToLive value to 10 seconds, the entry is evicted

automatically 10 seconds after it was inserted. Take caution when setting this value. This

evictor type is best used when reasonably high numbers of additions to the cache exist that

are used only for a set amount of time. With this strategy, anything that is created is removed

after the set amount of time. CREATION_TIME is useful in scenarios, such as refreshing