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Server functionality for HBase
/**
*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.hbase.regionserver;
import java.io.IOException;
import java.util.SortedSet;
import org.apache.hadoop.hbase.classification.InterfaceAudience;
import org.apache.hadoop.hbase.KeyValue;
import org.apache.hadoop.hbase.client.Scan;
/**
* Scanner that returns the next KeyValue.
*/
@InterfaceAudience.Private
public interface KeyValueScanner {
/**
* Look at the next KeyValue in this scanner, but do not iterate scanner.
* @return the next KeyValue
*/
KeyValue peek();
/**
* Return the next KeyValue in this scanner, iterating the scanner
* @return the next KeyValue
*/
KeyValue next() throws IOException;
/**
* Seek the scanner at or after the specified KeyValue.
* @param key seek value
* @return true if scanner has values left, false if end of scanner
*/
boolean seek(KeyValue key) throws IOException;
/**
* Reseek the scanner at or after the specified KeyValue.
* This method is guaranteed to seek at or after the required key only if the
* key comes after the current position of the scanner. Should not be used
* to seek to a key which may come before the current position.
* @param key seek value (should be non-null)
* @return true if scanner has values left, false if end of scanner
*/
boolean reseek(KeyValue key) throws IOException;
/**
* Get the sequence id associated with this KeyValueScanner. This is required
* for comparing multiple files to find out which one has the latest data.
* The default implementation for this would be to return 0. A file having
* lower sequence id will be considered to be the older one.
*/
long getSequenceID();
/**
* Close the KeyValue scanner.
*/
void close();
/**
* Allows to filter out scanners (both StoreFile and memstore) that we don't
* want to use based on criteria such as Bloom filters and timestamp ranges.
* @param scan the scan that we are selecting scanners for
* @param columns the set of columns in the current column family, or null if
* not specified by the scan
* @param oldestUnexpiredTS the oldest timestamp we are interested in for
* this query, based on TTL
* @return true if the scanner should be included in the query
*/
boolean shouldUseScanner(
Scan scan, SortedSet columns, long oldestUnexpiredTS
);
// "Lazy scanner" optimizations
/**
* Similar to {@link #seek} (or {@link #reseek} if forward is true) but only
* does a seek operation after checking that it is really necessary for the
* row/column combination specified by the kv parameter. This function was
* added to avoid unnecessary disk seeks by checking row-column Bloom filters
* before a seek on multi-column get/scan queries, and to optimize by looking
* up more recent files first.
* @param forward do a forward-only "reseek" instead of a random-access seek
* @param useBloom whether to enable multi-column Bloom filter optimization
*/
boolean requestSeek(KeyValue kv, boolean forward, boolean useBloom)
throws IOException;
/**
* We optimize our store scanners by checking the most recent store file
* first, so we sometimes pretend we have done a seek but delay it until the
* store scanner bubbles up to the top of the key-value heap. This method is
* then used to ensure the top store file scanner has done a seek operation.
*/
boolean realSeekDone();
/**
* Does the real seek operation in case it was skipped by
* seekToRowCol(KeyValue, boolean) (TODO: Whats this?). Note that this function should
* be never called on scanners that always do real seek operations (i.e. most
* of the scanners). The easiest way to achieve this is to call
* {@link #realSeekDone()} first.
*/
void enforceSeek() throws IOException;
/**
* @return true if this is a file scanner. Otherwise a memory scanner is
* assumed.
*/
boolean isFileScanner();
// Support for "Reversed Scanner"
/**
* Seek the scanner at or before the row of specified KeyValue, it firstly
* tries to seek the scanner at or after the specified KeyValue, return if
* peek KeyValue of scanner has the same row with specified KeyValue,
* otherwise seek the scanner at the first KeyValue of the row which is the
* previous row of specified KeyValue
*
* @param key seek KeyValue
* @return true if the scanner is at the valid KeyValue, false if such
* KeyValue does not exist
*
*/
public boolean backwardSeek(KeyValue key) throws IOException;
/**
* Seek the scanner at the first KeyValue of the row which is the previous row
* of specified key
* @param key seek value
* @return true if the scanner at the first valid KeyValue of previous row,
* false if not existing such KeyValue
*/
public boolean seekToPreviousRow(KeyValue key) throws IOException;
/**
* Seek the scanner at the first KeyValue of last row
*
* @return true if scanner has values left, false if the underlying data is
* empty
* @throws IOException
*/
public boolean seekToLastRow() throws IOException;
/**
* @return the next key in the index (the key to seek to the next block)
* if known, or null otherwise
*/
public byte[] getNextIndexedKey();
}