com.hazelcast.map.MapLoader Maven / Gradle / Ivy
/*
* Copyright (c) 2008-2021, Hazelcast, Inc. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.hazelcast.map;
import com.hazelcast.core.HazelcastInstance;
import com.hazelcast.core.OperationTimeoutException;
import java.io.Closeable;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
/**
* 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.
*
* {@link IMap#get(Object)} normally returns the value that
* is available in-memory. If the entry doesn't exist in-memory, Hazelcast
* returns {@code null}. If a Loader implementation is provided then, instead
* of returning {@code null}, Hazelcast will attempt to load the unknown entry
* by calling the implementation's {@link #load(Object)} or
* {@link #loadAll(Collection)} 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).
*
* MapLoader implementations are executed by a partition thread, therefore care
* should be taken not to block the thread with an expensive operation or an
* operation that may potentially never return, the partition thread does not
* time out the operation. Whilst the partition thread is executing the
* MapLoader it is unable to respond to requests for data on any other
* structure that may reside in the same partition, or to respond to other
* partitions mapped to the same partition thread. For example a very slow
* MapLoader for one map could block a request for data on another map, or even
* a queue. It is therefore strongly recommended not to use MapLoader to call
* across a WAN or to a system which will take on average longer than a few
* milliseconds to respond.
*
* MapLoaders should not be used to perform cascading operations on other data
* structures via a {@link HazelcastInstance}, the MapLoader should only
* concern itself with the operation on the assigned map. If the MapLoader
* attempts to access another data structure on a different partition to the
* key used in the MapLoader, a {@link java.lang.IllegalThreadStateException}
* is thrown. A MapLoader can only interact with other data structures that
* reside on the same partition.
*
* If a blocked partition thread is called from a Hazelcast Client the caller
* will also block indefinitely, for example
* {@link IMap#get(Object)}. If the same call is made from
* another cluster member the operation will eventually timeout with a
* {@link OperationTimeoutException}.
*
* @param type of the MapLoader key
* @param type of the MapLoader value
*/
public interface MapLoader {
/**
* 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.
*
* @param key, cannot be {@code null}
* @return value of the key; returning {@code null} value signals value missing in the underlying store
*/
V load(K key);
/**
* Loads given keys. This is batch load operation so that implementation can
* optimize the multiple loads.
*
* For any key in the input keys, there should be a single mapping in the resulting map. Also the resulting
* map should not have any keys that are not part of the input keys.
*
* The given collection should not contain any {@code null} keys.
* The returned Map should not contain any {@code null} keys or values.
*
* Loading other items than what provided in keys
* prevents the map from being filled from the map store.
*
* @param keys keys of the values entries to load
* @return map of loaded key-value pairs.
*/
Map loadAll(Collection keys);
/**
* Loads all of the keys from the store. The returned {@link Iterable} may return the keys lazily
* by loading them in batches. The {@link Iterator} of this {@link Iterable} may implement the
* {@link Closeable} interface in which case it will be closed once iteration is over.
* This is intended for releasing resources such as closing a JDBC result set.
*
* The returned Iterable should not contain any {@code null} keys.
*
* @return all the keys. Keys inside the Iterable cannot be {@code null}.
*/
Iterable loadAllKeys();
}