============
SPI Overview
============

When you implement a new Presto plugin, you implement interfaces and
override methods defined by the SPI.

Plugins can provide additional :doc:`connectors`, :doc:`types`,
:doc:`functions` and :doc:`system-access-control`.
In particular, connectors are the source of all data for queries in
Presto: they back each catalog available to Presto.

Code
----

The SPI source can be found in the ``presto-spi`` directory in the
root of the Presto source tree.

Plugin Metadata
---------------

Each plugin identifies an entry point: an implementation of the
``Plugin`` interface. This class name is provided to Presto via
the standard Java ``ServiceLoader`` interface: the classpath contains
a resource file named ``io.prestosql.spi.Plugin`` in the
``META-INF/services`` directory. The content of this file is a
single line listing the name of the plugin class:

.. code-block:: none

    com.example.plugin.ExamplePlugin

For a built-in plugin that is included in the Presto source code,
this resource file is created whenever the ``pom.xml`` file of a plugin
contains the following line:

.. code-block:: none

    <packaging>presto-plugin</packaging>

Plugin
------

The ``Plugin`` interface is a good starting place for developers looking
to understand the Presto SPI. It contains access methods to retrieve
various classes that a Plugin can provide. For example, the ``getConnectorFactories()``
method is a top-level function that Presto calls to retrieve a ``ConnectorFactory`` when Presto
is ready to create an instance of a connector to back a catalog. There are similar
methods for ``Type``, ``ParametricType``, ``Function``, ``SystemAccessControl``, and
``EventListenerFactory`` objects.

Building Plugins via Maven
--------------------------

Plugins depend on the SPI from Presto:

.. code-block:: xml

    <dependency>
        <groupId>io.prestosql</groupId>
        <artifactId>presto-spi</artifactId>
        <scope>provided</scope>
    </dependency>

The plugin uses the Maven ``provided`` scope because Presto provides
the classes from the SPI at runtime and thus the plugin should not
include them in the plugin assembly.

There are a few other dependencies that are provided by Presto,
including Slice and Jackson annotations. In particular, Jackson is
used for serializing connector handles and thus plugins must use the
annotations version provided by Presto.

All other dependencies are based on what the plugin needs for its
own implementation. Plugins are loaded in a separate class loader
to provide isolation and to allow plugins to use a different version
of a library that Presto uses internally.

For an example ``pom.xml`` file, see the example HTTP connector in the
``presto-example-http`` directory in the root of the Presto source tree.

Deploying a Custom Plugin
-------------------------

In order to add a custom plugin to a Presto installation, create a directory
for that plugin in the Presto plugin directory and add all the necessary jars
for the plugin to that directory. For example, for a plugin called
``my-functions``, you would create a directory ``my-functions`` in the Presto
plugin directory and add the relevant jars to that directory.

By default, the plugin directory is the ``plugin`` directory relative to the
directory in which Presto is installed, but it is configurable using the
configuration variable ``catalog.config-dir``. In order for Presto to pick up
the new plugin, you must restart Presto.

Plugins must be installed on all nodes in the Presto cluster (coordinator and workers).
