[[plugin-authors]]
== Help for plugin authors

:plugin-properties-files: {elasticsearch-root}/buildSrc/src/main/resources

The Elasticsearch repository contains examples of:

* a https://github.com/elastic/elasticsearch/tree/master/plugins/examples/custom-settings[Java plugin]
  which contains a plugin with custom settings.
* a https://github.com/elastic/elasticsearch/tree/master/plugins/examples/rest-handler[Java plugin]
  which contains a plugin that registers a Rest handler.
* a https://github.com/elastic/elasticsearch/tree/master/plugins/examples/rescore[Java plugin]
  which contains a rescore plugin.
* a https://github.com/elastic/elasticsearch/tree/master/plugins/examples/script-expert-scoring[Java plugin]
  which contains a script plugin.

These examples provide the bare bones needed to get started.  For more
information about how to write a plugin, we recommend looking at the plugins
listed in this documentation for inspiration.

[discrete]
=== Plugin descriptor file

All plugins must contain a file called `plugin-descriptor.properties`.
The format for this file is described in detail in this example:

["source","properties"]
--------------------------------------------------
include::{plugin-properties-files}/plugin-descriptor.properties[]
--------------------------------------------------

Either fill in this template yourself or, if you are using Elasticsearch's Gradle build system, you
can fill in the necessary values in the `build.gradle` file for your plugin.

[discrete]
==== Mandatory elements for plugins


[cols="<,<,<",options="header",]
|=======================================================================
|Element                    | Type   | Description

|`description`              |String  | simple summary of the plugin

|`version`                  |String  | plugin's version

|`name`                     |String  | the plugin name

|`classname`                |String  | the name of the class to load, fully-qualified.

|`java.version`             |String  | version of java the code is built against.
Use the system property `java.specification.version`. Version string must be a sequence
of nonnegative decimal integers separated by "."'s and may have leading zeros.

|`elasticsearch.version`    |String  | version of Elasticsearch compiled against.

|=======================================================================

Note that only jar files at the root of the plugin are added to the classpath for the plugin!
If you need other resources, package them into a resources jar.

[IMPORTANT]
.Plugin release lifecycle
==============================================

You will have to release a new version of the plugin for each new Elasticsearch release.
This version is checked when the plugin is loaded so Elasticsearch will refuse to start
in the presence of plugins with the incorrect `elasticsearch.version`.

==============================================


[discrete]
=== Testing your plugin

When testing a Java plugin, it will only be auto-loaded if it is in the
`plugins/` directory.  Use `bin/elasticsearch-plugin install file:///path/to/your/plugin`
to install your plugin for testing.

You may also load your plugin within the test framework for integration tests.
Read more in {ref}/integration-tests.html#changing-node-configuration[Changing Node Configuration].


[discrete]
[[plugin-authors-jsm]]
=== Java Security permissions

Some plugins may need additional security permissions. A plugin can include
the optional `plugin-security.policy` file containing `grant` statements for
additional permissions. Any additional permissions will be displayed to the user
with a large warning, and they will have to confirm them when installing the
plugin interactively. So if possible, it is best to avoid requesting any
spurious permissions!

If you are using the Elasticsearch Gradle build system, place this file in
`src/main/plugin-metadata` and it will be applied during unit tests as well.

Keep in mind that the Java security model is stack-based, and the additional
permissions will only be granted to the jars in your plugin, so you will have
write proper security code around operations requiring elevated privileges.
It is recommended to add a check to prevent unprivileged code (such as scripts)
from gaining escalated permissions. For example:

[source,java]
--------------------------------------------------
// ES permission you should check before doPrivileged() blocks
import org.elasticsearch.SpecialPermission;

SecurityManager sm = System.getSecurityManager();
if (sm != null) {
  // unprivileged code such as scripts do not have SpecialPermission
  sm.checkPermission(new SpecialPermission());
}
AccessController.doPrivileged(
  // sensitive operation
);
--------------------------------------------------

See https://www.oracle.com/technetwork/java/seccodeguide-139067.html[Secure Coding Guidelines for Java SE]
for more information.
