org.apache.druid.segment.loading

Interface SegmentCacheManager

All Known Implementing Classes:

SegmentLocalCacheManager

public interface SegmentCacheManager

A class to fetch segment files to local disk and manage the local cache. Implementations must be thread-safe.

Method Summary

Modifier and Type	Method and Description
void	cleanup(DataSegment segment) Cleanup the cache space used by the segment.
File	getSegmentFiles(DataSegment segment) This method fetches the files for the given segment if the segment is not downloaded already.
boolean	isSegmentCached(DataSegment segment) Checks whether a segment is already cached.
boolean	release(DataSegment segment) Reverts the effects of reserve(DataSegment) (DataSegment)} by releasing the location reserved for this segment.
boolean	reserve(DataSegment segment) Tries to reserve the space for a segment on any location.

Method Detail

isSegmentCached

boolean isSegmentCached(DataSegment segment)

Checks whether a segment is already cached. It can return false even if reserve(DataSegment) has been successful for a segment but is not downloaded yet.

getSegmentFiles

File getSegmentFiles(DataSegment segment)
              throws SegmentLoadingException

This method fetches the files for the given segment if the segment is not downloaded already. It is not required to reserve(DataSegment) before calling this method. If caller has not reserved the space explicitly via reserve(DataSegment), the implementation should reserve space on caller's behalf. If the space has been explicitly reserved already - implementation should use only the reserved space to store segment files. - implementation should not release the location in case of download erros and leave it to the caller.

Throws:

SegmentLoadingException - if there is an error in downloading files

reserve

boolean reserve(DataSegment segment)

Tries to reserve the space for a segment on any location. When the space has been reserved, getSegmentFiles(DataSegment) should download the segment on the reserved location or fail otherwise. This function is useful for custom extensions. Extensions can try to reserve the space first and if not successful, make some space by cleaning up other segments, etc. There is also improved concurrency for extensions with this function. Since reserve is a cheaper operation to invoke till the space has been reserved. Hence it can be put inside a lock if required by the extensions. getSegment can't be put inside a lock since it is a time-consuming operation, on account of downloading the files.

Parameters:

segment - - Segment to reserve

Returns:

True if enough space found to store the segment, false otherwise

release

boolean release(DataSegment segment)

Reverts the effects of reserve(DataSegment) (DataSegment)} by releasing the location reserved for this segment. Callers, that explicitly reserve the space via reserve(DataSegment), should use this method to release the space. Implementation can throw error if the space is being released but there is data present. Callers are supposed to ensure that any data is removed via cleanup(DataSegment)

Parameters:

segment - - Segment to release the location for.

Returns:

- True if any location was reserved and released, false otherwise.

cleanup

void cleanup(DataSegment segment)

Cleanup the cache space used by the segment. It will not release the space if the space has been explicitly reserved via reserve(DataSegment)