Hazelcast distributed map implementation is an in-memory data store but
it can be backed by any type of data store such as RDBMS, OODBMS, or simply
a file based data store.
IMap.get(key) normally returns the value that is available in-memory. If the entry
doesn't exist in-memory, Hazelcast returns null. If a Loader implementation
is provided then, instead of returning null, Hazelcast will load the unknown entry by
calling the implementation's load (key) or loadAll(keys) methods. Loaded entries
will be placed into the distributed map and they will stay in-memory until they are
explicitly removed or implicitly evicted (if eviction is configured).
Loads the value of a given key. If distributed map doesn't contain the value
for the given key then Hazelcast will call implementation's load (key) method
to obtain the value. Implementation can use any means of loading the given key;
such as an O/R mapping tool, simple SQL or reading a file etc.