hive Context

Driver：
  public int compile(String command) {
        ctx = new Context(conf); //
      

  }




  public Context(Configuration conf) throws IOException {
    this(conf, generateExecutionId());
  }

  /**
   * Generate a unique executionId.  An executionId, together with user name and
   * the configuration, will determine the temporary locations of all intermediate
   * files.
   *
   * In the future, users can use the executionId to resume a query.
   */
  public static String generateExecutionId() {
    Random rand = new Random();
    SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd_HH-mm-ss_SSS");
    String executionId = "hive_" + format.format(new Date()) + "_"
        + Math.abs(rand.nextLong());
    return executionId;
  }
executionId的例子是：hive_2011-08-21_00-02-22_445_7799135143086468923
                                             hive_yyyy-MM-dd_HH-mm-ss_SSS_长整型随机数

  /**
   * Create a Context with a given executionId.  ExecutionId, together with
   * user name and conf, will determine the temporary directory locations.
   */
  public Context(Configuration conf, String executionId) throws IOException {
    this.conf = conf;
    this.executionId = executionId; //hive_2011-08-21_00-02-22_445_7799135143086468923

    // non-local tmp location is configurable. however it is the same across
    // all external file systems
    nonLocalScratchPath =
        new Path(HiveConf.getVar(conf, HiveConf.ConfVars.SCRATCHDIR),
                executionId);  // /tmp/hive-tianzhao/hive_2011-08-21_00-02-22_445_7799135143086468923
    // HiveConf   SCRATCHDIR("hive.exec.scratchdir", "/tmp/" + System.getProperty("user.name") + "/hive"),
   hive-default.xml文件中默认的配置是：
   <property>
        <name>hive.exec.scratchdir</name>
        <value>/tmp/hive-${user.name}</value>
        <description>Scratch space for Hive jobs</description>
   </property>
  

    // local tmp location is not configurable for now
    localScratchDir = System.getProperty("java.io.tmpdir")
           + Path.SEPARATOR + System.getProperty("user.name") + Path.SEPARATOR
           + executionId;  // /tmp/tianzhao/hive_2011-08-21_00-02-22_445_7799135143086468923
  }
  // 本地临时目录 System.getProperty("java.io.tmpdir") = /tmp
  // System.getProperty("user.name") = tianzhao



  /**
   * Get a tmp path on local host to store intermediate data.
   *
   * @return next available tmp path on local fs
   */
  public String getLocalTmpFileURI() {
    return getLocalScratchDir(true) + Path.SEPARATOR + LOCAL_PREFIX +
      nextPathId();  // file:/tmp/tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461/-local-10003
  }

  private String getScratchDir(String scheme, String authority,
                               boolean mkdir, String scratchDir) {

    String fileSystem =  scheme + ":" + authority;  // file:null
    String dir = fsScratchDirs.get(fileSystem); //

    if (dir == null) {
      Path dirPath = new Path(scheme, authority, scratchDir);
      if (mkdir) {  // true
        try {
          FileSystem fs = dirPath.getFileSystem(conf);
          dirPath = new Path(fs.makeQualified(dirPath).toString());
          if (!fs.mkdirs(dirPath)) {
            throw new RuntimeException("Cannot make directory: "
                                       + dirPath.toString());
          }
        } catch (IOException e) {
          throw new RuntimeException (e);
        }
      }
      dir = dirPath.toString(); // file:/tmp/tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461
      fsScratchDirs.put(fileSystem, dir);  // {file:null=file:/tmp/tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461, hdfs:localhost:54310=hdfs://localhost:54310/tmp/hive-tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461}
    }
    return dir;  //file:/tmp/tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461
  }




  /**
   * Get a path to store map-reduce intermediate data in.
   *
   * @return next available path for map-red intermediate data
   */
  public String getMRTmpFileURI() {
    return getMRScratchDir() + Path.SEPARATOR + MR_PREFIX +
      nextPathId();  // hdfs://localhost:54310/tmp/hive-tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461/-mr-10004
  }


  public String getMRScratchDir() {

    // if we are executing entirely on the client side - then
    // just (re)use the local scratch directory
    if(isLocalOnlyExecutionMode()) {
      return getLocalScratchDir(!explain);
    }

    try {
      // nonLocalScratchPath=/tmp/hive-tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461
      Path dir = FileUtils.makeQualified(nonLocalScratchPath, conf); // hdfs://localhost:54310/tmp/hive-tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461
      URI uri = dir.toUri();// hdfs://localhost:54310/tmp/hive-tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461
      return getScratchDir(uri.getScheme(), uri.getAuthority(),
                           !explain, uri.getPath());

    } catch (IOException e) {
      throw new RuntimeException(e);
    } catch (IllegalArgumentException e) {
      throw new RuntimeException("Error while making MR scratch "
          + "directory - check filesystem config (" + e.getCause() + ")", e);
    }
  }


  /**
   * Get a tmp directory on specified URI
   *
   * @param scheme Scheme of the target FS
   * @param authority Authority of the target FS
   * @param mkdir create the directory if true
   * @param scratchdir path of tmp directory
   */
  private String getScratchDir(String scheme, String authority,
                               boolean mkdir, String scratchDir) {

    String fileSystem =  scheme + ":" + authority;  //hdfs:localhost:54310
    String dir = fsScratchDirs.get(fileSystem); // hdfs://localhost:54310/tmp/hive-tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461

    if (dir == null) {
      Path dirPath = new Path(scheme, authority, scratchDir);
      if (mkdir) {
        try {
          FileSystem fs = dirPath.getFileSystem(conf);
          dirPath = new Path(fs.makeQualified(dirPath).toString());
          if (!fs.mkdirs(dirPath)) {
            throw new RuntimeException("Cannot make directory: "
                                       + dirPath.toString());
          }
        } catch (IOException e) {
          throw new RuntimeException (e);
        }
      }
      dir = dirPath.toString();
      fsScratchDirs.put(fileSystem, dir);
    }
    return dir; //hdfs://localhost:54310/tmp/hive-tianzhao/hive_2011-09-06_03-39-09_416_8623373648168603461
  }