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# Including Extensions

Druid uses a module system that allows for the addition of extensions at runtime.

## Specifying extensions

Druid extensions can be specified in the `common.runtime.properties`. There are two ways of adding druid extensions currently.

### Add to the classpath

If you add your extension jar to the classpath at runtime, Druid will load it into the system.  This mechanism is relatively easy to reason about, but it also means that you have to ensure that all dependency jars on the classpath are compatible.  That is, Druid makes no provisions while using this method to maintain class loader isolation so you must make sure that the jars on your classpath are mutually compatible.

### Add to the extension directory

If you don't want to fiddle with classpath, you can create an extension directory and tell Druid to load extensions from there.

To let Druid load your extensions, follow the steps below

1) Specify `druid.extensions.directory` (root directory for normal Druid extensions). If you don' specify it, Druid will use their default value, see [Configuration](../configuration/index.html).

2) Prepare normal extension directories under root extension directory.  Under the root extension directory, you should create sub-directories for each extension you might want to load.  Inside each sub-directory, you can put extension related files in it.  (If you don't want to manually setup the extension directory, Druid also provides a [pull-deps](../pull-deps.html) tool that can help you genereate these directories automatically)

Example:

Suppose you specify `druid.extensions.directory=/usr/local/druid/extensions`, and want Druid to load normal extensions ```druid-examples```, ```druid-kafka-eight``` and ```mysql-metadata-storage```.

Then under ```extensions```, it should look like this,

```
extensions/
├── druid-examples
│   ├── commons-beanutils-1.8.3.jar
│   ├── commons-digester-1.8.jar
│   ├── commons-logging-1.1.1.jar
│   ├── commons-validator-1.4.0.jar
│   ├── druid-examples-0.8.0-rc1.jar
│   ├── twitter4j-async-3.0.3.jar
│   ├── twitter4j-core-3.0.3.jar
│   └── twitter4j-stream-3.0.3.jar
├── druid-kafka-eight
│   ├── druid-kafka-eight-0.7.3.jar
│   ├── jline-0.9.94.jar
│   ├── jopt-simple-3.2.jar
│   ├── kafka-clients-0.8.2.1.jar
│   ├── kafka_2.10-0.8.2.1.jar
│   ├── log4j-1.2.16.jar
│   ├── lz4-1.3.0.jar
│   ├── metrics-core-2.2.0.jar
│   ├── netty-3.7.0.Final.jar
│   ├── scala-library-2.10.4.jar
│   ├── slf4j-log4j12-1.6.1.jar
│   ├── snappy-java-1.1.1.6.jar
│   ├── zkclient-0.3.jar
│   └── zookeeper-3.4.6.jar
└── mysql-metadata-storage
    ├── jdbi-2.32.jar
    ├── mysql-connector-java-5.1.34.jar
    └── mysql-metadata-storage-0.8.0-rc1.jar
```

As you can see, under ```extensions``` there are three sub-directories ```druid-examples```, ```druid-kafka-eight``` and ```mysql-metadata-storage```, each sub-directory denotes an extension that Druid might load.

3) Tell Druid which extensions to load.  Now you have prepared your extension directories, if you want Druid to load a specific list of extensions under root extension directory, you need to specify `druid.extensions.loadList`. Using the example above, if you want Druid to load ```druid-kafka-eight``` and ```mysql-metadata-storage```, you can specify `druid.extensions.loadList=["druid-kafka-eight", "mysql-metadata-storage"]`.

If you specify `druid.extensions.loadList=[]`, Druid won't load any extension from file system.

If you don't specify `druid.extensions.loadList`, Druid will load all the extensions under root extension directory.