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；