Performing Fast In-Memory Map Operations

Hazelcast supports entry processing. An entry processor is a function that executes your code on a map entry in an atomic way.

An entry processor is a good option if you perform bulk processing on an IMap. Usually you perform a loop of keys-- executing IMap.get(key), mutating the value, and finally putting the entry back in the map using IMap.put(key,value). If you perform this process from a client or from a member where the keys do not exist, you effectively perform two network hops for each update: the first to retrieve the data and the second to update the mutated value.

If you are doing the process described above, you should consider using entry processors. An entry processor executes a read and updates upon the member where the data resides. This eliminates the costly network hops described above.

NOTE: Entry processor is meant to process a single entry per call. Processing multiple entries and data structures in an entry processor is not supported as it may result in deadlocks.

An entry processor enables fast in-memory operations on your map without you having to worry about locks or concurrency issues. You can apply it to a single map entry or to all map entries. Entry processors support choosing target entries using predicates. You do not need any explicit lock on entry thanks to the isolated threading model: Hazelcast runs the EntryProcessor for all entries on a partitionThread so there will NOT be any interleaving of the EntryProcessor and other mutations.

Hazelcast sends the entry processor to each cluster member and these members apply it to map entries. Therefore, if you add more members, your processing completes faster.

Using Indexes

Entry processors can be used with predicates. Predicates help to process a subset of data by selecting eligible entries. This selection can happen either by doing a full-table scan or by using indexes. To accelerate entry selection step, you can consider to add indexes. If indexes are there, entry processor will automatically use them.

Using OBJECT In-Memory Format

If entry processing is the major operation for a map and if the map consists of complex objects, you should use OBJECT as the in-memory-format to minimize serialization cost. By default, the entry value is stored as a byte array (BINARY format). When it is stored as an object (OBJECT format), then the entry processor is applied directly on the object. In that case, no serialization or deserialization is performed. However, if there is a defined event listener, a new entry value will be serialized when passing to the event publisher service.

NOTE: When in-memory-format is OBJECT, the old value of the updated entry will be null.

Entry Processing with IMap Methods

The methods below are in the IMap interface for entry processing.

executeOnKey processes an entry mapped by a key.
executeOnKeys processes entries mapped by a collection of keys.
submitToKey processes an entry mapped by a key while listening to event status.
executeOnEntries processes all entries in a map.
executeOnEntries can also process all entries in a map with a defined predicate.

/**
 * Applies the user defined EntryProcessor to the entry mapped by the key.
 * Returns the object which is the result of the process() method of EntryProcessor.
 */
Object executeOnKey( K key, EntryProcessor entryProcessor );

/**
 * Applies the user defined EntryProcessor to the entries mapped by the collection of keys.
 * Returns the results mapped by each key in the collection.
 */
Map<K, Object> executeOnKeys( Set<K> keys, EntryProcessor entryProcessor );

/**
 * Applies the user defined EntryProcessor to the entry mapped by the key with
 * specified ExecutionCallback to listen to event status and return immediately.
 */
void submitToKey( K key, EntryProcessor entryProcessor, ExecutionCallback callback );


/**
 * Applies the user defined EntryProcessor to all entries in the map.
 * Returns the results mapped by each key in the map.
 */
Map<K, Object> executeOnEntries( EntryProcessor entryProcessor );
	   
/**
 * Applies the user defined EntryProcessor to the entries in the map which satisfies 
 provided predicate.
 * Returns the results mapped by each key in the map.
 */
Map<K, Object> executeOnEntries( EntryProcessor entryProcessor, Predicate predicate );

NOTE: Entry Processors run via Operation Threads that are dedicated to specific partitions. Therefore, with long running Entry Processor executions, other partition operations such as map.put(key) cannot be processed. With this in mind, it is good practice to make your Entry Processor executions as quick as possible.

Distributed Computing Entry Processor Performing Fast In-Memory Map Operations