Go to hazelcast.org/documentation

Eviction

Unless you delete the map entries manually or use an eviction policy, they will remain in the map. Hazelcast supports policy based eviction for distributed maps. Currently supported policies are LRU (Least Recently Used) and LFU (Least Frequently Used). There are also other properties as shown in the below sample declarative configuration. 

<hazelcast>
  <map name="default">
    ...
    <time-to-live-seconds>0</time-to-live-seconds>
    <max-idle-seconds>0</max-idle-seconds>
    <eviction-policy>LRU</eviction-policy>
    <max-size policy="PER_NODE">5000</max-size>
    <eviction-percentage>25</eviction-percentage>
    ...
  </map>
</hazelcast>

Let's describe each property.

  • time-to-live: Maximum time in seconds for each entry to stay in the map. If it is not 0, entries that are older than and not updated for this time are evicted automatically. Valid values are integers between 0 and Integer.MAX VALUE. Default value is 0 and it means infinite. Moreover, if it is not 0, entries are evicted regardless of the set eviction-policy.
  • max-idle-seconds: Maximum time in seconds for each entry to stay idle in the map. Entries that are idle for more than this time are evicted automatically. An entry is idle if no get, put or containsKey is called. Valid values are integers between 0 and Integer.MAX VALUE. Default value is 0 and it means infinite.

  • eviction-policy: Valid values are described below.

    • NONE: Default policy. If set, no items will be evicted and the property max-size will be ignored. Of course, you still can combine it with time-to-live-seconds and max-idle-seconds.
    • LRU: Least Recently Used.
    • LFU: Least Frequently Used.

  • max-size: Maximum size of the map. When maximum size is reached, map is evicted based on the policy defined. Valid values are integers between 0 and Integer.MAX VALUE. Default value is 0. If you want max-size to work, eviction-policy property must be set to a value other than NONE. Its attributes are described below.

    • PER_NODE: Maximum number of map entries in each JVM. This is the default policy.

      <max-size policy="PER_NODE">5000</max-size>

    • PER_PARTITION: Maximum number of map entries within each partition. Storage size depends on the partition count in a JVM. So, this attribute may not be used often. If the cluster is small it will be hosting more partitions and therefore map entries, than that of a larger cluster.

      <max-size policy="PER_PARTITION">27100</max-size>

    • USED_HEAP_SIZE: Maximum used heap size in megabytes for each JVM.

      <max-size policy="USED_HEAP_SIZE">4096</max-size>

    • USED_HEAP_PERCENTAGE: Maximum used heap size percentage for each JVM. If, for example, JVM is configured to have 1000 MB and this value is 10, then the map entries will be evicted when used heap size exceeds 100 MB.

      <max-size policy="USED_HEAP_PERCENTAGE">10</max-size>

  • eviction-percentage: When max-size is reached, specified percentage of the map will be evicted. If 25 is set for example, 25% of the entries will be evicted. Setting this property to a smaller value will cause eviction of small number of map entries. So, if map entries are inserted frequently, smaller percentage values may lead to overheads. Valid values are integers between 0 and 100. Default value is 25.

Sample Eviction Configuration

<map name="documents">
  <max-size policy="PER_NODE">10000</max-size>
  <eviction -policy>LRU</eviction -policy> 
  <max-idle-seconds>60</max-idle-seconds>
</map>

In the above sample, documents map starts to evict its entries from a member when the map size exceeds 10000 in that member. Then, the entries least recently used will be evicted. And, the entries not used for more than 60 seconds will be evicted as well.

Evicting Specific Entries

Above explained eviction policies and configurations apply to all the entries of a map. The entries that meet the specified eviction conditions are evicted.

But, you may particularly want to evict some specific map entries. In this case, you can use the ttl and timeunit parameters of the method map.put(). A sample code line is given below.

myMap.put( "1", "John", 50, TimeUnit.SECONDS )

So, the map entry with the key "1" will be evicted in 50 seconds after it is put into myMap.