在Kyuubi服务源码解析：FrontendService一文解析HiveConnection的构造函数时，有一行代码是openSession()，现在来解析一下。
Client端OpenSession解析
HiveConnection.java：
- openSession()方法
private void openSession() throws SQLException {
TOpenSessionReq openReq = new TOpenSessionReq();
***省略部分代码***
try {
//Client远程调用Server端的OpenSession方法
TOpenSessionResp openResp = client.OpenSession(openReq);
// validate connection
Utils.verifySuccess(openResp.getStatus());
if (!supportedProtocols.contains(openResp.getServerProtocolVersion())) {
throw new TException("Unsupported Hive2 protocol");
}
protocol = openResp.getServerProtocolVersion();
sessHandle = openResp.getSessionHandle();
***省略部分代码***
} catch (TException e) {
LOG.error("Error opening session", e);
throw new SQLException("Could not establish connection to "
+ jdbcUriString + ": " + e.getMessage(), " 08S01", e);
}
isClosed = false;
}
TOpenSessionResp openResp = client.OpenSession(openReq);
这句代码中，client的类型是TCLIService.Iface，TOpenSessionResp和TCLIService都是由Thrift文件生成的类：
Thrift是跨语言的RPC框架，也就是能够远程调用函数。
client.OpenSession(openReq)实际上调用的是Server端的OpenSession()方法，这个Server端也就是Kyuubi(当然也可以是Hiveserver2)。接下来我们看一下Server端的OpenSession方法。
Server端OpenSession解析
FrontendService.scala:
再看一下FrontendService的类声明：
private[kyuubi] class FrontendService private(name: String, beService: BackendService)
extends AbstractService(name) with TCLIService.Iface with Runnable with Logging
FrontendService实现了TCLIService.Iface接口，上文client.OpenSession(openReq)这个client的类型也是TCLIService.Iface。然后看一下FrontendService的OpenSession方法：
- OpenSession()方法
override def OpenSession(req: TOpenSessionReq): TOpenSessionResp = {
info("Client protocol version: " + req.getClient_protocol)
val resp = new TOpenSessionResp
try {
val sessionHandle = getSessionHandle(req, resp)
resp.setSessionHandle(sessionHandle.toTSessionHandle)
// resp.setConfiguration(new Map[String, String]())
resp.setStatus(OK_STATUS)
val context = currentServerContext.get
.asInstanceOf[FrontendService#FeServiceServerContext]
if (context != null) {
context.setSessionHandle(sessionHandle)
}
} catch {
case e: Exception =>
warn("Error opening session: ", e)
resp.setStatus(KyuubiSQLException.toTStatus(e))
}
resp
}
实际上FrontendService重写了TCLIService.Iface的OpenSession方法，JDBC client调用的也正是Server端这个重写的OpenSession方法。
关于Thrift远程调用不理解的同学可以参考这篇博文：thrift远程调用示例。
