org.apache.accumulo.core.client

Interface BatchScanner

All Superinterfaces:

Iterable<Map.Entry<Key,Value>>, ScannerBase

All Known Implementing Classes:

MockBatchDeleter, MockBatchScanner, TabletServerBatchDeleter, TabletServerBatchReader

public interface BatchScanner
extends ScannerBase

In exchange for possibly returning scanned entries out of order, BatchScanner implementations may scan an Accumulo table more efficiently by

• Looking up multiple ranges in parallel. Parallelism is constrained by the number of threads available to the BatchScanner, set in its constructor.
• Breaking up large ranges into subranges. Often the number and boundaries of subranges are determined by a table's split points.
• Combining multiple ranges into a single RPC call to a tablet server.

The above techniques lead to better performance than a Scanner in use cases such as

• Retrieving many small ranges
• Scanning a large range that returns many entries
• Running server-side iterators that perform computation, even if few entries are returned from the scan itself

To re-emphasize, only use a BatchScanner when you do not care whether returned data is in sorted order. Use a Scanner instead when sorted order is important.

A BatchScanner instance will use no more threads than provided in the construction of the BatchScanner implementation. Multiple invocations of iterator() will all share the same resources of the instance. A new BatchScanner instance should be created to use allocate additional threads.

Method Summary

Methods

Modifier and Type	Method and Description
void	close() Cleans up and finalizes the scanner
void	setRanges(Collection<Range> ranges) Allows scanning over multiple ranges efficiently.
void	setTimeout(long timeout, TimeUnit timeUnit) Sets a timeout threshold for a server to respond.

Methods inherited from interface org.apache.accumulo.core.client.ScannerBase

addScanIterator, clearColumns, clearScanIterators, fetchColumn, fetchColumnFamily, getTimeout, iterator, removeScanIterator, updateScanIteratorOption

Method Detail

setRanges

void setRanges(Collection<Range> ranges)

Allows scanning over multiple ranges efficiently.

Parameters:

ranges - specifies the non-overlapping ranges to query

close

void close()

Cleans up and finalizes the scanner

Specified by:

close in interface ScannerBase

setTimeout

void setTimeout(long timeout,
              TimeUnit timeUnit)

Sets a timeout threshold for a server to respond. The batch scanner will accomplish as much work as possible before throwing an exception. BatchScanner iterators will throw a TimedOutException when all needed servers timeout. Setting the timeout to zero or Long.MAX_VALUE and TimeUnit.MILLISECONDS means no timeout.

If not set, there is not timeout. The BatchScanner will retry forever.

Specified by:

setTimeout in interface ScannerBase

Parameters:

timeUnit - determines how timeout is interpreted

Since:

1.5.0