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Queue Persistence

Hazelcast allows you to load and store the distributed queue items from/to a persistent datastore using the interface QueueStore. If queue store is enabled, each item added to the queue will also be stored at the configured queue store. When the number of items in the queue exceeds the memory limit, the subsequent items are persisted in the queue store, they are not stored in the queue memory.

QueueStore interface enables you to store, load, and delete items with methods like store, storeAll, load and delete. The following example class includes all of the QueueStore methods.

public class TheQueueStore implements QueueStore<Item> {
    @Override
    public void delete(Long key) {
        System.out.println("delete");
    }

    @Override
    public void store(Long key, Item value) {
        System.out.println("store");
    }

    @Override
    public void storeAll(Map<Long, Item> map) {
        System.out.println("store all");
    }

    @Override
    public void deleteAll(Collection<Long> keys) {
        System.out.println("deleteAll");
    }

    @Override
    public Item load(Long key) {
        System.out.println("load");
        return null;
    }

    @Override
    public Map<Long, Item> loadAll(Collection<Long> keys) {
        System.out.println("loadAll");
        return null;
    }

    @Override
    public Set<Long> loadAllKeys() {
        System.out.println("loadAllKeys");
        return null;
    }

Item must be serializable. Following is an example queue store configuration.

<queue-store>
  <class-name>com.hazelcast.QueueStoreImpl</class-name>
  <properties>
    <property name="binary">false</property>
    <property name="memory-limit">1000</property>
    <property name="bulk-load">500</property>
  </properties>
</queue-store>

Let's explain the properties.

  • Binary: By default, Hazelcast stores the queue items in serialized form in memory. Before it inserts the queue items into datastore, it deserializes them. But if you will not reach the queue store from an external application, you might prefer that the items be inserted in binary form. You can get rid of the de-serialization step; this would be a performance optimization. The binary feature is disabled by default.

  • Memory Limit: This is the number of items after which Hazelcast will store items only to datastore. For example, if the memory limit is 1000, then the 1001st item will be put only to datastore. This feature is useful when you want to avoid out-of-memory conditions. The default number for memory-limit is 1000. If you want to always use memory, you can set it to Integer.MAX_VALUE.

  • Bulk Load: When the queue is initialized, items are loaded from QueueStore in bulks. Bulk load is the size of these bulks. By default, bulk-load is 250.