org.apache.drill.exec.store

Class PluginHandle

java.lang.Object

org.apache.drill.exec.store.PluginHandle

public class PluginHandle
extends Object

Represents a storage plugin, defined by a (name, config) pair. The config implies a connector definition, including a way to create the plugin instance (StoragePlugin. Storage plugins are created lazily to avoid long Drillbit start times. Plugin creation is synchronized as is closing.

A handle has a type used to determine which operations are allowed on the handle. For example, inrinsic (system) plugins cannot be deleted or disabled.

Caveats

Note that race conditions are still possible:

• User 1 submits a query that refers to plugin p. The registry creates an instance of plugin p and returns it. The planner proceeds to use it.
• User 2 disables p, causing its entry to be updated in persistent storage (but not yet in this entry)
• User 3 submits a query that refers to plugin p. The registry notices that the plugin is now disabled, removes it from the plugin list and closes the plugin.
• User 1 suffers a failure when the planner references the now-closed plugin p.

This issue has existed for some time and cannot be fixed here. The right solution is to introduce a reference count: each query which uses the plugin should hold a reference count until the completion of the plan or fragment. If we implement such a reference count, this is the place to maintain the count, and to close the plugin when the count goes to zero.

Constructor Summary

Constructors

Constructor and Description
PluginHandle(StoragePlugin plugin, ConnectorHandle connector, org.apache.drill.exec.store.PluginHandle.PluginType type)
PluginHandle(String name, StoragePluginConfig config, ConnectorHandle connector)
PluginHandle(String name, StoragePluginConfig config, ConnectorHandle connector, org.apache.drill.exec.store.PluginHandle.PluginType type)

Method Summary

Modifier and Type	Method and Description
void	close() Close the plugin.
StoragePluginConfig	config()
boolean	hasInstance()
boolean	isIntrinsic()
boolean	isStored()
String	name()
StoragePlugin	plugin() Retrieve the storage plugin instance, creating it if needed.
String	toString()
PluginHandle	transfer(org.apache.drill.exec.store.PluginHandle.PluginType type) Atomically transfer the plugin instance, if any, to a new handle of the given type.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

PluginHandle

public PluginHandle(String name,
                    StoragePluginConfig config,
                    ConnectorHandle connector)

PluginHandle

public PluginHandle(String name,
                    StoragePluginConfig config,
                    ConnectorHandle connector,
                    org.apache.drill.exec.store.PluginHandle.PluginType type)

PluginHandle

public PluginHandle(StoragePlugin plugin,
                    ConnectorHandle connector,
                    org.apache.drill.exec.store.PluginHandle.PluginType type)

Method Detail

name

public String name()

config

public StoragePluginConfig config()

isStored

public boolean isStored()

isIntrinsic

public boolean isIntrinsic()

plugin

public StoragePlugin plugin()

Retrieve the storage plugin instance, creating it if needed. Creation can take time if the plugin creates a connection to another system, especially if that system suffers timeouts.

Parameters:

context - the context to use for creating a new instance, if needed

Returns:

the initialized storage plugin

Throws:

UserException - if the storage plugin creation failed due to class errors (unlikely) or external system errors (more likely)

hasInstance

public boolean hasInstance()

close

public void close()

Close the plugin. Can occur when the handle is evicted from the loading cache where we must not throw an exception. Also called on shutdown.

transfer

public PluginHandle transfer(org.apache.drill.exec.store.PluginHandle.PluginType type)

Atomically transfer the plugin instance, if any, to a new handle of the given type. Avoids race conditions when transferring a plugin from/to ephemeral storage and the plugin cache, since those two caches are not synchronized as a whole. Ensures that only one of the threads in a race condition will transfer the actual plugin instance.

By definition, a plugin becomes disabled if it moves to ephemeral, enabled if it moves from ephemeral into stored status.

toString

public String toString()

Overrides:

toString in class Object