以3.11.4为基础
Cassandra 启动流程从bin下面的cassandra脚本开始进去看，会涉及到cassandra-env.sh进行进程启动的一些参数设置比如堆上内存分配，日志参数配置，基本环境设置等，我们直接看到启动的进程，也就是launch_service 这个函数,然后看到里面的exec 执行JAVA的进程，也就是class（launch_service参数传进来的classname为org.apache.cassandra.service.CassandraDaemon类）；大概的流程是：
cassandra -> lauch_service -> JAVA启动org.apache.cassandra.service.CassandraDaemon；接下来就是看到org.apache.cassandra.service.CassandraDaemon这个类，直接在这个类里面通过main函数找到启动的入口：
public static void main(String[] args)
{
instance.activate();
}
在active函数里面有3个主要要看的函数：
// Do not put any references to DatabaseDescriptor above the forceStaticInitialization call.
try
{
applyConfig();
MBeanWrapper.instance.registerMBean(new StandardMBean(new NativeAccess(), NativeAccessMBean.class), MBEAN_NAME, MBeanWrapper.OnException.LOG);
if (FBUtilities.isWindows)
{
// We need to adjust the system timer on windows from the default 15ms down to the minimum of 1ms as this
// impacts timer intervals, thread scheduling, driver interrupts, etc.
WindowsTimer.startTimerPeriod(DatabaseDescriptor.getWindowsTimerInterval());
}
setup();
String pidFile = System.getProperty("cassandra-pidfile");
if (pidFile != null)
{
new File(pidFile).deleteOnExit();
}
start();
}
catch (Throwable e)
{
boolean logStackTrace =
e instanceof ConfigurationException ? ((ConfigurationException)e).logStackTrace : true;
System.out.println("Exception (" + e.getClass().getName() + ") encountered during startup: " + e.getMessage());
if (logStackTrace)
{
if (runManaged)
logger.error("Exception encountered during startup", e);
// try to warn user on stdout too, if we haven't already detached
e.printStackTrace();
exitOrFail(3, "Exception encountered during startup", e);
}
else
{
if (runManaged)
logger.error("Exception encountered during startup: {}", e.getMessage());
// try to warn user on stdout too, if we haven't already detached
System.err.println(e.getMessage());
exitOrFail(3, "Exception encountered during startup: " + e.getMessage());
}
}
applyConfig();主要是用来初始化cassandra.yaml配置文件里的基本的配置，setup主要是做一些单节点初始化的时候的一些初始化工作，比如load本地的sstable元信息到内存，清除无用的sstable文件，启动的时候没有commit的transaction 执行或回滚，commitlog 的启动以及回放，集群元信息的效验，启动compaction，启动7000端口内部交流server等等事情；
start函数主要是启动9042的client监听端口，主要是netty的server，当然如果是thrift的话，也启动9160 端口服务；
