作者：特贰的大妞 | 来源：互联网 | 2023-09-16 19:11
分拆TableSplit让多个mapper同时读取默认情况下，一个region是一个tableSplit，对应一个mapper进行读取，但单mapper读取速度较慢，因此想着把默认一个tables
分拆TableSplit 让多个mapper同时读取
默认情况下，一个region是一个tableSplit，对应一个mapper进行读取，但单mapper读取速度较慢，因此想着把默认一个table split分拆成多个split，这样hadoop就能通过多个mapper读取。
由于HBase不能像hadoop一样通过以下参数调整split大小，而实现多个mapper读取Java代码 
- mapred.min.split.size
- mapred.max.split.size
所以目前想到的方法有两种，一是修改TableInputFormatBase，把默认的一个TableSplit分拆成多个，另外一种方法是，通过Coprocessor处理。这里选择修改TableInputFormatBase类。
HBase权威指南里面有介绍怎么把HBase与MR结合，通过需要用到一下的辅助类实现把HBase表作为数据来源，读取数据：
Java代码
- TableMapReduceUtil.initTableMapperJob(table[0].getBytes(), scan,
- UserViewHisMapper2.class, Text.class, Text.class,
- genRecommendations);
而这个方法，最终是调用以下方法进行初始化设置的：
Java代码
- public static void initTableMapperJob(byte[] table, Scan scan,
- Classextends TableMapper> mapper,
Classextends WritableComparable> outputKeyClass,
Classextends Writable> outputValueClass, Job job,
boolean addDependencyJars)
throws IOException {
initTableMapperJob(Bytes.toString(table), scan, mapper, outputKeyClass,
outputValueClass, job, addDependencyJars, TableInputFormat.class);
}
所以，思路就应该修改TableInputFormat这个类。而这个类的核心方法是继承了TableInputFormatBase：
Java代码
- public class TableInputFormat extends TableInputFormatBase
- implements Configurable
最终要修改的则是TableInputFormatBase这个类，修改其以下方法：
Java代码
- public List getSplits(JobContext context) throws IOException {}
这个方法的核心是，获得table对应所有region的起始row，把每个region作为一个tableSplit：
Java代码
- public List getSplits(JobContext context) throws IOException {
- f (table == null) {
- throw new IOException("No table was provided.");
-
- Pair<byte[][], byte[][]> keys = table.getStartEndKeys();
- if (keys == null || keys.getFirst() == null ||
- keys.getFirst().length == 0) {
- throw new IOException("Expecting at least one region.");
- }
- int count = 0;
- List splits = new ArrayList(keys.getFirst().length);
- for (int i = 0; i < keys.getFirst().length; i++) {
- if ( !includeRegionInSplit(keys.getFirst()[i], keys.getSecond()[i])) {
- continue;
- }
- String regionLocation = table.getRegionLocation(keys.getFirst()[i]).
- getHostname();
- byte[] startRow = scan.getStartRow();
- byte[] stopRow = scan.getStopRow();
-
- if ((startRow.length == 0 || keys.getSecond()[i].length == 0 ||
- Bytes.compareTo(startRow, keys.getSecond()[i]) < 0) &&
- (stopRow.length == 0 ||
- Bytes.compareTo(stopRow, keys.getFirst()[i]) > 0)) {
- byte[] splitStart = startRow.length == 0 ||
- Bytes.compareTo(keys.getFirst()[i], startRow) >= 0 ?
- keys.getFirst()[i] : startRow;
- byte[] splitStop = (stopRow.length == 0 ||
- Bytes.compareTo(keys.getSecond()[i], stopRow) <= 0) &&
- keys.getSecond()[i].length > 0 ?
- keys.getSecond()[i] : stopRow;
- InputSplit split = new TableSplit(table.getTableName(),
- splitStart, splitStop, regionLocation);
- splits.add(split);
- if (LOG.isDebugEnabled())
- LOG.debug("getSplits: split -> " + (count++) + " -> " + split);
- }
- }
- return splits;
- }
这里要做的就是，把本来属于一个tableSplit的row在细分，分成自己希望的多个小split。但没有找到轻巧的实现，唯有不断迭代，把一个tableSplit的row全部取出，再拆分了，有点蛮力。
以下是我的实现方法：
Java代码
- public List getSplits(JobContext context) throws IOException {
- if (table == null) {
- throw new IOException("No table was provided.");
- }
- Pair<byte[][], byte[][]> keys = table.getStartEndKeys();
- if (keys == null || keys.getFirst() == null
- || keys.getFirst().length == 0) {
- throw new IOException("Expecting at least one region.");
- }
- int count = 0;
- List splits = new ArrayList(
- keys.getFirst().length);
- for (int i = 0; i < keys.getFirst().length; i++) {
- if (!includeRegionInSplit(keys.getFirst()[i], keys.getSecond()[i])) {
- continue;
- }
- String regionLocation = table.getRegionLocation(keys.getFirst()[i],true)
- .getHostname();
- byte[] startRow = scan.getStartRow();
- byte[] stopRow = scan.getStopRow();
-
- if ((startRow.length == 0 || keys.getSecond()[i].length == 0 || Bytes
- .compareTo(startRow, keys.getSecond()[i]) < 0)
- && (stopRow.length == 0 || Bytes.compareTo(stopRow,
- keys.getFirst()[i]) > 0)) {
- byte[] splitStart = startRow.length == 0
- || Bytes.compareTo(keys.getFirst()[i], startRow) >= 0 ? keys
- .getFirst()[i] : startRow;
- byte[] splitStop = (stopRow.length == 0 || Bytes.compareTo(
- keys.getSecond()[i], stopRow) <= 0)
- && keys.getSecond()[i].length > 0 ? keys.getSecond()[i]
- : stopRow;
-
- Scan scan1 = new Scan();
- scan1.setStartRow(splitStart);
- scan1.setStopRow(splitStop);
- scan1.setFilter(new KeyOnlyFilter());
- scan1.setBatch(500);
-
- ResultScanner resultscanner = table.getScanner(scan1);
-
-
- List rows = new ArrayList();
-
-
- for(Result rs : resultscanner)
- {
- if(rs.isEmpty())
- continue;
- rows.add(new String(rs.getRow()));
- }
-
- int splitSize = rows.size() / mappersPerSplit;
-
- for (int j = 0; j < mappersPerSplit; j++) {
- TableSplit tablesplit = null;
- if (j == mappersPerSplit - 1)
- tablesplit = new TableSplit(table.getTableName(),
- rows.get(j * splitSize).getBytes(),
- rows.get(rows.size() - 1).getBytes(),
- regionLocation);
- else
- tablesplit = new TableSplit(table.getTableName(),
- rows.get(j * splitSize).getBytes(),
- rows.get(j * splitSize + splitSize).getBytes(), regionLocation);
- splits.add(tablesplit);
- if (LOG.isDebugEnabled())
- LOG.debug((new StringBuilder())
- .append("getSplits: split -> ").append(i++)
- .append(" -> ").append(tablesplit).toString());
- }
- resultscanner.close();
- }
- }
- return splits;
- }
通过配置设置需要拆分的split数。
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