Package org.apache.druid.metadata.segment.cache

Interface SegmentMetadataCache

All Known Implementing Classes:

HeapMemorySegmentMetadataCache, NoopSegmentMetadataCache

public interface SegmentMetadataCache

Cache for metadata of pending segments and used segments maintained by the Overlord to improve performance of segment allocation and other task actions.

Not to be confused with AbstractSegmentMetadataCache which is used by Brokers to cache row signature, number of rows, etc. to aid with Druid query performance.

Nested Class Summary

Nested Classes

Modifier and Type	Interface	Description
static interface	SegmentMetadataCache.Action<T>	Represents a thread-safe read or write action performed on the cache within required locks.
static enum	SegmentMetadataCache.UsageMode	Cache usage modes.

Method Summary

Modifier and Type	Method	Description
void	awaitNextSync(long timeoutMillis)	Waits until the cache finishes the next sync with the metadata store or until the timeout elapses, whichever is sooner.
void	becomeLeader()	Refreshes the cache once the service is elected leader.
DataSourcesSnapshot	getDataSourcesSnapshot()
boolean	isEnabled()
boolean	isSyncedForRead()
<T> T	readCacheForDataSource(String dataSource, SegmentMetadataCache.Action<T> readAction)	Performs a thread-safe read action on the cache for the given datasource.
void	start()	Starts the cache on service start.
void	stop()	Stops the cache on service stop.
void	stopBeingLeader()	Notifies the cache that the service has lost leadership.
<T> T	writeCacheForDataSource(String dataSource, SegmentMetadataCache.Action<T> writeAction)	Performs a thread-safe write action on the cache for the given datasource.

Method Details

start

void start()

Starts the cache on service start.

stop

void stop()

Stops the cache on service stop.

becomeLeader

void becomeLeader()

Refreshes the cache once the service is elected leader.

stopBeingLeader

void stopBeingLeader()

Notifies the cache that the service has lost leadership.

isEnabled

boolean isEnabled()

Returns:

true if the cache is enabled

isSyncedForRead

boolean isSyncedForRead()

Returns:

true if the cache is enabled and synced with the metadata store. Reads can be done from the cache only if it is synced but writes can happen even before the sync has finished.

awaitNextSync

void awaitNextSync(long timeoutMillis)

Waits until the cache finishes the next sync with the metadata store or until the timeout elapses, whichever is sooner.

getDataSourcesSnapshot

DataSourcesSnapshot getDataSourcesSnapshot()

Returns:

Latest snapshot of the datasources with all their used segments.

readCacheForDataSource

<T> T readCacheForDataSource(String dataSource,
 SegmentMetadataCache.Action<T> readAction)

Performs a thread-safe read action on the cache for the given datasource. Read actions can be concurrent with other reads but are mutually exclusive from other write actions on the same datasource.

writeCacheForDataSource

<T> T writeCacheForDataSource(String dataSource,
 SegmentMetadataCache.Action<T> writeAction)

Performs a thread-safe write action on the cache for the given datasource. Write actions are mutually exclusive from other writes or reads on the same datasource.