// Licensed to the Apache Software Foundation (ASF) under one or more
// contributor license agreements.  See the NOTICE file distributed with
// this work for additional information regarding copyright ownership.
// The ASF licenses this file to You 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.
= Near Caches

A near cache is a local cache that stores the most recently or most frequently accessed data on the local node. Let's say your application launches a client node and regularly queries reference data, such as country codes. Because client nodes do not store data, these queries always fetch data from the remote nodes. You can configure a near cache to keep the country codes on the local node while your application is running.

A near cache is configured for a specific regular cache and holds data for that cache only.

Near caches store data in the on-heap memory. You can configure the maximum size of the cache and eviction policy for near cache entries.

[NOTE]
====
Near caches are fully transactional and get updated or invalidated automatically whenever the data changes on the server nodes.
====

== Configuring Near Cache

You can configure a near cache for a particular cache in the cache configuration.

:javaCodeFile: {javaCodeDir}/NearCache.java
:xmlFile: code-snippets/xml/near-cache-config.xml

[tabs]
--
tab:XML[]
[source,xml]
----
include::{xmlFile}[tag=cache-with-near-cache,indent=0]
----
tab:Java[]
[source,java]
----
include::{javaCodeFile}[tag=nearCacheConfiguration,indent=0]
----
tab:C#/.NET[]
[source,csharp]
----
include::code-snippets/dotnet/NearCaches.cs[tag=nearCacheConf,indent=0]
----
tab:C++[unsupported]
--

Once configured in this way, the near cache is created on any node that requests data from the underlying cache, including both server nodes and client nodes.
When you get an instance of the cache, as shown in the following example, the data requests go through the near cache.

[tabs]
--
tab:Java[]
[source,java]
----
IgniteCache<Integer, Integer> cache = ignite.cache("myCache");

int value = cache.get(1);
----
--

Most configuration parameters available in the cache configuration that make sense for the near cache are inherited from the underlying cache configuration.
For example, if the underlying cache has an link:configuring-caches/expiry-policies[expiry policy] configured, entries in the near cache are expired based on the same policy.

The parameters listed in the table below are not inherited from the underlying cache configuration.

[cols="1,3,1",opts="autowidth.stretch,header"]
|===
|Parameter | Description | Default Value
|nearEvictionPolicy| The eviction policy for the near cache. See the link:memory-configuration/eviction-policies[Eviction policies] page for details. | none
|nearStartSize| The initial capacity of the near cache (the number of entries it can hold). | 375,000
|===

== Creating Near Cache Dynamically On Client Nodes
When making request from a client node to a cache that hasn't been configured to use a near cache, you can create a near cache for that cache dynamically.
This increases performance by storing "hot" data locally on the client side.
This cache is operable only on the node where it was created.

To do this, create a near cache configuration and pass it as an argument to the method that gets the instance of the cache.

[tabs]
--
tab:Java[]
[source,java]
----
include::{javaCodeFile}[tag=createNearCacheDynamically,indent=0]
----
tab:C#/.NET[]
[source,csharp]
----
include::code-snippets/dotnet/NearCaches.cs[tag=nearCacheClientNode,indent=0]
----
tab:C++[unsupported]
--