辅助进程类别
在postgresql中，有很多辅助进程，各个辅助进程完成特定的功能，支撑数据库系统运行和管理工作，辅助进程的类别定义如下：
/*
* Auxiliary-process type identifiers. These used to be in bootstrap.h
* but it seems saner to have them here, with the ProcessingMode stuff.
* The MyAuxProcType global is defined and set in bootstrap.c.
*/
typedef enum
{
NotAnAuxProcess = -1,
CheckerProcess = 0,
BootstrapProcess,
StartupProcess, //启动辅助进程，完成启动前的数据回放等工作
BgWriterProcess, //后台写进程，将脏页写入磁盘
CheckpointerProcess, //检查点进程
WalWriterProcess, //WAL日志写进程
WalReceiverProcess,
NUM_AUXPROCTYPES /* Must be last! */
} AuxProcType;
辅助进程启动方式
一般在主进程postmastermain中，启动辅助进程，各个辅助进程功能不同，但是典型的启动方式类似，如下图所示，StartChildProcess作为多个辅助进程的启动入口，内部通过fork()创建子进程，并封装子进程参数，在AuxiliaryProcessMain接口中根据参数不同，执行对应子进程分支。
#define StartupDataBase() StartChildProcess(StartupProcess)
#define StartBackgroundWriter() StartChildProcess(BgWriterProcess)
#define StartCheckpointer() StartChildProcess(CheckpointerProcess)
#define StartWalWriter() StartChildProcess(WalWriterProcess)
#define StartWalReceiver() StartChildProcess(WalReceiverProcess)
捕获.PNG
AuxiliaryProcessMain
AuxiliaryProcessMain作为辅助进程的入口，主要依序完成一下功能：
- 获取基本信息，如进程id，时间戳，可执行文件路径等；
- 遍历解析接口参数，使用接口
getopt，根据解析出的参数进行配置； - 设置辅助进程在ps命令下的显示名称
init_ps_display； - 对子进程的信号量，共享内存等资源进行配置
BaseInit(),InitAuxiliaryProcess()； - 针对解析出的子进程类型仅从分支处理，关键代码如下图所示，参数类型通过-x参数传递，在此处分支执行后，各个辅助进程正式开始各自的任务，至于各个辅助进程的具体内容，今后再做详细介绍。
switch (MyAuxProcType) //根据参数解析出的辅助进程类型
{
case CheckerProcess:
/* don't set signals, they're useless here */
CheckerModeMain();
proc_exit(1); /* should never return */
case BootstrapProcess:
/*
* There was a brief instant during which mode was Normal; this is
* okay. We need to be in bootstrap mode during BootStrapXLOG for
* the sake of multixact initialization.
*/
SetProcessingMode(BootstrapProcessing);
bootstrap_signals();
BootStrapXLOG();
BootstrapModeMain();
proc_exit(1); /* should never return */
case StartupProcess:
/* don't set signals, startup process has its own agenda */
StartupProcessMain();
proc_exit(1); /* should never return */
case BgWriterProcess:
/* don't set signals, bgwriter has its own agenda */
BackgroundWriterMain();
proc_exit(1); /* should never return */
case CheckpointerProcess:
/* don't set signals, checkpointer has its own agenda */
CheckpointerMain();
proc_exit(1); /* should never return */
case WalWriterProcess:
/* don't set signals, walwriter has its own agenda */
InitXLOGAccess();
WalWriterMain();
proc_exit(1); /* should never return */
case WalReceiverProcess:
/* don't set signals, walreceiver has its own agenda */
WalReceiverMain();
proc_exit(1); /* should never return */
default:
elog(PANIC, "unrecognized process type: %d", (int) MyAuxProcType);
proc_exit(1);
}
