Tephra —— HBase 全局一致性事务支持

• Tephra 在 Apache HBase 的基础上提供了全局一致性的事务支持。HBase 提供了强一致性的基于行和区域的 ACID 操作支持，但是牺牲了在跨区域操作的支持。这就要求应用开发者花很大力气来确保区域边界上操作的一致性。而 Tephra 提供了全局事务支持，可以夸区域、跨表以及多个 RPC 上简化了应用的开发。

示例代码：

/**
   * A Transactional SecondaryIndexTable.
   */
  public class SecondaryIndexTable {
    private byte[] secondaryIndex;
    private TransactionAwareHTable transactionAwareHTable;
    private TransactionAwareHTable secondaryIndexTable;
    private TransactionContext transactionContext;
    private final TableName secondaryIndexTableName;
    private static final byte[] secondaryIndexFamily =
      Bytes.toBytes("secondaryIndexFamily");
    private static final byte[] secondaryIndexQualifier = Bytes.toBytes('r');
    private static final byte[] DELIMITER  = new byte[] {0};

    public SecondaryIndexTable(TransactionServiceClient transactionServiceClient,
                               HTable hTable, byte[] secondaryIndex) {
      secondaryIndexTableName =
            TableName.valueOf(hTable.getName().getNameAsString() + ".idx");
      HTable secondaryIndexHTable = null;
      HBaseAdmin hBaseAdmin = null;
      try {
        hBaseAdmin = new HBaseAdmin(hTable.getConfiguration());
        if (!hBaseAdmin.tableExists(secondaryIndexTableName)) {
          hBaseAdmin.createTable(new HTableDescriptor(secondaryIndexTableName));
        }
        secondaryIndexHTable = new HTable(hTable.getConfiguration(),
                                          secondaryIndexTableName);
      } catch (Exception e) {
        Throwables.propagate(e);
      } finally {
        try {
          hBaseAdmin.close();
        } catch (Exception e) {
          Throwables.propagate(e);
        }
      }

      this.secondaryIndex = secondaryIndex;
      this.transactionAwareHTable = new TransactionAwareHTable(hTable);
      this.secondaryIndexTable = new TransactionAwareHTable(secondaryIndexHTable);
      this.transactionContext = new TransactionContext(transactionServiceClient,
                                                       transactionAwareHTable,
                                                       secondaryIndexTable);
    }

    public Result get(Get get) throws IOException {
      return get(Collections.singletonList(get))[0];
    }

    public Result[] get(List<Get> gets) throws IOException {
      try {
        transactionContext.start();
        Result[] result = transactionAwareHTable.get(gets);
        transactionContext.finish();
        return result;
      } catch (Exception e) {
        try {
          transactionContext.abort();
        } catch (TransactionFailureException e1) {
          throw new IOException("Could not rollback transaction", e1);
        }
      }
      return null;
    }

    public Result[] getByIndex(byte[] value) throws IOException {
      try {
        transactionContext.start();
        Scan scan = new Scan(value, Bytes.add(value, new byte[0]));
        scan.addColumn(secondaryIndexFamily, secondaryIndexQualifier);
        ResultScanner indexScanner = secondaryIndexTable.getScanner(scan);

        ArrayList<Get> gets = new ArrayList<Get>();
        for (Result result : indexScanner) {
          for (Cell cell : result.listCells()) {
            gets.add(new Get(cell.getValue()));
          }
        }
        Result[] results = transactionAwareHTable.get(gets);
        transactionContext.finish();
        return results;
      } catch (Exception e) {
        try {
          transactionContext.abort();
        } catch (TransactionFailureException e1) {
          throw new IOException("Could not rollback transaction", e1);
        }
      }
      return null;
    }

    public void put(Put put) throws IOException {
      put(Collections.singletonList(put));
    }


    public void put(List<Put> puts) throws IOException {
      try {
        transactionContext.start();
        ArrayList<Put> secondaryIndexPuts = new ArrayList<Put>();
        for (Put put : puts) {
          List<Put> indexPuts = new ArrayList<Put>();
          Set<Map.Entry<byte[], List<KeyValue>>> familyMap = put.getFamilyMap().entrySet();
          for (Map.Entry<byte [], List<KeyValue>> family : familyMap) {
            for (KeyValue value : family.getValue()) {
              if (value.getQualifier().equals(secondaryIndex)) {
                byte[] secondaryRow = Bytes.add(value.getQualifier(),
                                                DELIMITER,
                                                Bytes.add(value.getValue(),
                                                DELIMITER,
                                                value.getRow()));
                Put indexPut = new Put(secondaryRow);
                indexPut.add(secondaryIndexFamily, secondaryIndexQualifier, put.getRow());
                indexPuts.add(indexPut);
              }
            }
          }
          secondaryIndexPuts.addAll(indexPuts);
        }
        transactionAwareHTable.put(puts);
        secondaryIndexTable.put(secondaryIndexPuts);
        transactionContext.finish();
      } catch (Exception e) {
        try {
          transactionContext.abort();
        } catch (TransactionFailureException e1) {
          throw new IOException("Could not rollback transaction", e1);
        }
      }
    }
  }

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