上篇文章介绍了Metastore Thrift服务端，接下来浅析一下Metastore Thrift客户端。

1、IMetaStoreClient接口

 IMetaStoreClient接口定义了Metastore的thrift api，该接口中定义了操作元数据的各种方法，如下图所示。

IMetaStoreClient

2、HiveMetaStoreClient

Hive中IMetaStoreClient的实现类是HiveMetaStoreClient
Hive.getMSC() ➔ createMetaStoreClient()
  ↳ RetryingMetaStoreClient.getProxy()//动态代理类创建代理对象
  ↳RetryingMetaStoreClient.RetryingMetaStoreClient()
  ↳MetaStoreUtils.newInstance()//反射实例化对象
  ↳SessionHiveMetaStoreClient.SessionHiveMetaStoreClient()
  ↳HiveMetaStoreClient.HiveMetaStoreClient()
  ↳HiveMetaStoreClient.open()

3、建立Thrift连接

  private void open() throws MetaException {
    isConnected = false;
    TTransportException tte = null;
    boolean useSSL = conf.getBoolVar(ConfVars.HIVE_METASTORE_USE_SSL);
    boolean useSasl = conf.getBoolVar(ConfVars.METASTORE_USE_THRIFT_SASL);
    boolean useFramedTransport = conf.getBoolVar(ConfVars.METASTORE_USE_THRIFT_FRAMED_TRANSPORT);
    boolean useCompactProtocol = conf.getBoolVar(ConfVars.METASTORE_USE_THRIFT_COMPACT_PROTOCOL);
    int clientSocketTimeout = (int) conf.getTimeVar(
        ConfVars.METASTORE_CLIENT_SOCKET_TIMEOUT, TimeUnit.MILLISECONDS);

    for (int attempt = 0; !isConnected && attempt < retries; ++attempt) {
      for (URI store : metastoreUris) {
        LOG.info("Trying to connect to metastore with URI " + store);

        try {
          if (useSasl) {
            // Wrap thrift connection with SASL for secure connection.
            try {
              HadoopThriftAuthBridge.Client authBridge =
                ShimLoader.getHadoopThriftAuthBridge().createClient();

              // check if we should use delegation tokens to authenticate
              // the call below gets hold of the tokens if they are set up by hadoop
              // this should happen on the map/reduce tasks if the client added the
              // tokens into hadoop's credential store in the front end during job
              // submission.
              String tokenSig = conf.get("hive.metastore.token.signature");
              // tokenSig could be null
              tokenStrForm = Utils.getTokenStrForm(tokenSig);
              transport = new TSocket(store.getHost(), store.getPort(), clientSocketTimeout);

              if(tokenStrForm != null) {
                // authenticate using delegation tokens via the "DIGEST" mechanism
                transport = authBridge.createClientTransport(null, store.getHost(),
                    "DIGEST", tokenStrForm, transport,
                        MetaStoreUtils.getMetaStoreSaslProperties(conf));
              } else {
                String principalConfig =
                    conf.getVar(HiveConf.ConfVars.METASTORE_KERBEROS_PRINCIPAL);
                transport = authBridge.createClientTransport(
                    principalConfig, store.getHost(), "KERBEROS", null,
                    transport, MetaStoreUtils.getMetaStoreSaslProperties(conf));
              }
            } catch (IOException ioe) {
              LOG.error("Couldn't create client transport", ioe);
              throw new MetaException(ioe.toString());
            }
          } else {
            if (useSSL) {
              ......
          }

          final TProtocol protocol;
          if (useCompactProtocol) {
            protocol = new TCompactProtocol(transport);
          } else {
            protocol = new TBinaryProtocol(transport);
          }
          client = new ThriftHiveMetastore.Client(protocol);
          try {
            if (!transport.isOpen()) {
              transport.open();
              LOG.info("Opened a connection to metastore, current connections: " + connCount.incrementAndGet());
            }
            isConnected = true;
          } catch (TTransportException e) {
            tte = e;
            if (LOG.isDebugEnabled()) {
              LOG.warn("Failed to connect to the MetaStore Server...", e);
            } else {
              // Don't print full exception trace if DEBUG is not on.
              LOG.warn("Failed to connect to the MetaStore Server...");
            }
          }

          if (isConnected && !useSasl && conf.getBoolVar(ConfVars.METASTORE_EXECUTE_SET_UGI)){
            ......
          }
        } catch (MetaException e) {
          LOG.error("Unable to connect to metastore with URI " + store
                    + " in attempt " + attempt, e);
        }
        if (isConnected) {
          break;
        }
      }
      // Wait before launching the next round of connection retries.
      if (!isConnected && retryDelaySeconds > 0) {
        try {
          LOG.info("Waiting " + retryDelaySeconds + " seconds before next connection attempt.");
          Thread.sleep(retryDelaySeconds * 1000);
        } catch (InterruptedException ignore) {}
      }
    }

    if (!isConnected) {
      throw new MetaException("Could not connect to meta store using any of the URIs provided." +
        " Most recent failure: " + StringUtils.stringifyException(tte));
    }

    snapshotActiveConf();

    LOG.info("Connected to metastore.");
  }