[[multimap_cache]]
== Multimap Cache

MutimapCache is a type of {brandname} Cache that maps keys to values in which each key can contain multiple values.

=== Installation and configuration

.pom.xml
include::topics/dependencies/multimap.adoc[]

=== MultimapCache API

MultimapCache API exposes several methods to interact with the Multimap Cache.
All these methods are non-blocking in most of the cases. See [limitations]

[source,java]
----
public interface MultimapCache<K, V> {

   CompletableFuture<Void> put(K key, V value);

   CompletableFuture<Collection<V>> get(K key);

   CompletableFuture<Boolean> remove(K key);

   CompletableFuture<Boolean> remove(K key, V value);

   CompletableFuture<Void> remove(Predicate<? super V> p);

   CompletableFuture<Boolean> containsKey(K key);

   CompletableFuture<Boolean> containsValue(V value);

   CompletableFuture<Boolean> containsEntry(K key, V value);

   CompletableFuture<Long> size();

   boolean supportsDuplicates();

}
----

==== CompletableFuture<Void> put(K key, V value)
Puts a key-value pair in the multimap cache.

[source,java]
----
MultimapCache<String, String> multimapCache = ...;

multimapCache.put("girlNames", "marie")
             .thenCompose(r1 -> multimapCache.put("girlNames", "oihana"))
             .thenCompose(r3 -> multimapCache.get("girlNames"))
             .thenAccept(names -> {
                          if(names.contains("marie"))
                              System.out.println("Marie is a girl name");

                           if(names.contains("oihana"))
                              System.out.println("Oihana is a girl name");
                        });
----
The output of this code is :

[source, txt]
----
Marie is a girl name
Oihana is a girl name
----

==== CompletableFuture<Collection<V>> get(K key)

Asynchronous that returns a view collection of the values associated with key in this multimap cache, if any. Any changes to the retrieved collection won't change the values in this multimap cache.
When this method returns an empty collection, it means the key was not found.

==== CompletableFuture<Boolean> remove(K key)
Asynchronous that removes the entry associated with the key from the multimap cache, if such exists.

==== CompletableFuture<Boolean> remove(K key, V value)
Asynchronous that removes a key-value pair from the multimap cache, if such exists.

==== CompletableFuture<Void> remove(Predicate<? super V> p)
Asynchronous method. Removes every value that match the given predicate.

==== CompletableFuture<Boolean> containsKey(K key)
Asynchronous that returns true if this multimap contains the key.

==== CompletableFuture<Boolean> containsValue(V value)
Asynchronous that returns true if this multimap contains the value in at least one key.

==== CompletableFuture<Boolean> containsEntry(K key, V value)

Asynchronous that returns true if this multimap contains at least one key-value pair with the value.

==== CompletableFuture<Long> size()
Asynchronous that returns the number of key-value pairs in the multimap cache. It doesn't return the distinct number of keys.

==== boolean supportsDuplicates()
Asynchronous that returns true if the multimap cache supports duplicates. This means that the content of the multimap can be
'a' -> ['1', '1', '2']. For now this method will always return false, as duplicates are not yet supported.
The existence of a given value is determined by 'equals' and `hashcode' method's contract.

=== Creating a Multimap Cache

Currently the MultimapCache is configured as a regular cache. This can be done either by code or XML configuration.
See how to configure a regular Cache in the section link to [configure a cache].

==== Embedded mode

[source,java]
----
// create or obtain your EmbeddedCacheManager
EmbeddedCacheManager cm = ... ;

// create or obtain a MultimapCacheManager passing the EmbeddedCacheManager
MultimapCacheManager multimapCacheManager = EmbeddedMultimapCacheManagerFactory.from(cm);

// define the configuration for the multimap cache
multimapCacheManager.defineConfiguration(multimapCacheName, c.build());

// get the multimap cache
multimapCache = multimapCacheManager.get(multimapCacheName);
----

//==== Server mode
//TODO

=== Limitations

In almost every case the Multimap Cache will behave as a regular Cache, but some limitations exist in the current version.

==== Support for duplicates
Duplicates are not supported yet. This means that the multimap won't contain any duplicate key-value pair.
Whenever put method is called, if the key-value pair already exist, this key-value par won't be added.
Methods used to check if a key-value pair is already present in the Multimap are the `equals` and `hashcode`.

==== Eviction

For now, the eviction works per key, and not per key-value pair.
This means that whenever a key is evicted, all the values associated with the key will be evicted too.
Eviction per key-value could be supported in the future.

==== Transactions

Implicit transactions are supported through the auto-commit and all the methods are non blocking.
Explicit transactions work without blocking in most of the cases.
Methods that will block are `size`, `containsEntry` and `remove(Predicate<? super V> p)`
