进程架构
1.1 进程父子关系
PostgreSQL 的进程架构由多个后端进程组成，其父进程是 postmaster。进程 ID 记录在 {PGDATA}/postmaster.pid 文件中。当实例成功关闭时，该文件将被删除。数据库客户端与 postmaster 进程正在侦听的端口建立连接。
图1 进程父子关系
在下面的示例中，进程 ID 2680 是 postmaster 进程。很明显，所有其他进程是 postmaster 的子进程。 Postmaster 进程接收来自客户端的连接并对其进行身份验证。然后postmaster启动postgres进程作为子进程执行SQL
示例1 处理父子关系
$ ps -ef | grep postgres | grep -v grep
postgres 2680 1 0 10:25 ? 00:00:00 /usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data
postgres 2681 2680 0 10:25 ? 00:00:00 postgres: logger process
postgres 2683 2680 0 10:25 ? 00:00:00 postgres: checkpointer process
postgres 2684 2680 0 10:25 ? 00:00:00 postgres: writer process
postgres 2685 2680 0 10:25 ? 00:00:00 postgres: wal writer process
postgres 2686 2680 0 10:25 ? 00:00:00 postgres: autovacuum launcher process
postgres 2687 2680 0 10:25 ? 00:00:00 postgres: stats collector process
1.2 进程名称
PostgreSQL 实例如上所述配置了多个进程。通过将参数 update_process_title 指定为“on”（默认值：“on”），部分进程名称会发生变化。 ps 命令中引用的每个进程的名称如下表所示。
表 1 进程名称
| 进程 | 进程名称 |
|---|---|
| postmaster | {INSTALL}/bin/postgres -D |
| logger | postgres: logger process |
| checkpointer | postgres: checkpointer process |
| writer | postgres: writer process |
| wal writer | postgres: wal writer process |
| autovacuum launcher | postgres: autovacuum launcher process |
| autovacuum worker | postgres: autovacuum worker process |
| archiver | postgres: archiver process last was |
| stats collector | postgres: stats collector process |
| postgres (local) | postgres: {PGUSER} {PGDATABASE} [local] |
| postgres (remote) | postgres: {PGUSER} {PGDATABASE} {TCP/IP (PORT)} |
| wal sender | postgres: wal sender process {PGUSER} {TCP/IP (PORT)} streaming |
| postgres: wal sender process {PGUSER} {TCP/IP (PORT)} sending backup "{BACKUP_LABEL}" | |
| wal receiver | postgres: wal receiver process streaming |
| startup process | postgres: startup process recovering |
| bgworker | postgres: bgworker: |
| parallel worker | postgres: bgworker: parallel worker for PID |
如果指定参数 cluster_name，这是 PostgreSQL 9.5 的新功能，则输出已指定为进程名称一部分的字符串。以下是将“cluster1”指定为参数 cluster_name 时的示例
示例2 cluster_name 参数
$ ps –ef | grep postgres
postgres 12364 1 0 06:14 pts/0 00:00:00 /usr/local/pgsql/bin/postgres -D data
postgres 12365 12364 0 06:14 ? 00:00:00 postgres: cluster1: logger process
postgres 12367 12364 0 06:14 ? 00:00:00 postgres: cluster1: checkpointer process
postgres 12368 12364 0 06:14 ? 00:00:00 postgres: cluster1: writer process
postgres 12369 12364 0 06:14 ? 00:00:00 postgres: cluster1: wal writer process
postgres 12370 12364 0 06:14 ? 00:00:00 postgres: cluster1: autovacuum launcher process
从这个例子可以看出，在postmaster的进程名中并没有输出cluster name进程。参数 cluster_name 中可以指定的字符仅限于 ASCII 字符串 (0x20 ~ 0x7E)。其他代码在转换为问号（？）后输出。
1.3 过程与信号
您可以通过向后端进程发送特定信号来执行操作，后端进程配置实例。这里验证了对几个信号的响应动作。
- SIGKILL 信号
当 postmaster 进程收到一个杀死信号。这次没有删除 postmaster.pid 文件。实例重启后虽然有如下日志记录，但实例正常启动。
示例3 异常终止后重启日志
LOG: database system was interrupted; last known up at 2017-02-11 11:12:03 JST
LOG: database system was not properly shut down; automatic recovery in progress
LOG: redo starts at 0/155E118
FATAL: the database system is starting up
FATAL: the database system is starting up
LOG: invalid record length at 0/5A5C050: wanted 24, got 0
LOG: redo done at 0/5A5C018
LOG: last completed transaction was at log time 2017-02-11 12:25:15.443492+09
LOG: MultiXact member wraparound protections are now enabled
LOG: autovacuum launcher started
LOG: database system is ready to accept connections
当postgres进程异常终止（包括接收到KILL信号），以及适当的进程，它将重置从客户端连接的所有会话。实例上运行的所有事务都被回滚，成为接收错误信号后的所有 SQL 语句。要安全地停止 postgres 进程，执行 pg_cancel_backend 函数（发送 SIGINT 信号）或 pg_terminate_backend 函数（发送 SIGTERM 信号）。
示例4 收到KILL信号后记录
LOG: server process (PID 3416) was terminated by signal 9: Killed
LOG: terminating any other active server processes
LOG: archiver process (PID 3404) exited with exit code 1
WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and repeat your command.
LOG: all server processes terminated; reinitializing
每个后端进程收到信号时的行为如下。 SIG_IGN 表示信号忽略，并且 SIG_DFL 显示 Linux 进程的默认行为。
- postgres进程接收信号时的行为
postgres进程接收信号时的操作如下。
表 2 postgres 进程的行为
| Signal | Signal Handler function | Behavior |
|---|---|---|
| SIGHUP | SigHupHandler | Reload configuration file |
| SIGINT | StatementCancelHandler | Destruction of the running transactions (Processing of pg_cancel_backend function) |
| SIGTERM | die | Destruction of the running transactions and exit process (Processing of pg_terminate_backend function) |
| SIGQUIT | quickdie or die | Forced termination |
| SIGALRM | handle_sig_alarm | Timeout occurrence notification |
| SIGPIPE | SIG_IGN | |
| SIGUSR1 | procsignal_sigusr1_handler | Database recovery |
| SIGUSR2 | SIG_IGN | |
| SIGFPE | FloatExceptionHandler | Output ERROR log |
| SIGCHLD | SIG_DFL |
- postmaster 进程接收信号时的行为
postmaster进程接收信号时的操作如下：
表 3 postmaster 进程的行为
| Signal | Signal Handler function | Behavior |
|---|---|---|
| SIGHUP | SIGHUP_handler | Reload configuration file Send SIGHUP signal to the child process |
| SIGINT | pmdie | FAST shutdown |
| SIGTERM | pmdie | SMART shutdown |
| SIGQUIT | pmdie | IMMEDIATE shutdown |
| SIGALRM | SIG_IGN | |
| SIGPIPE | SIG_IGN | |
| SIGUSR1 | sigusr1_handler | Signal reception processing from the child process |
| SIGUSR2 | dummy_handler | Does nothing |
| SIGCHLD | reaper | Processing at the end of a child process Restart the back-end process |
| SIGTTIN | SIG_IGN | |
| SIGTTOU | SIG_IGN | |
| SIGXFSZ | SIG_IGN |
当你向 postmaster 进程发送 SIGHUP 信号时，postgresql.conf 文件（postgresql.auto.conf 和 pg_*.conf 文件，以及）被重新加载。这与执行 pg_ctl reload 命令的行为相同。输出以下日志。
示例5 重新读取配置文件的日志输出
LOG: received SIGHUP, reloading configuration files
1.4 启动和停止进程
Checkpointer, writer, and stats collector进程始终启动。其他进程的启动/停止时序如下。 postmaster 的子进程定期检查其父进程的存在，postmaster 和他们在检测到 postmaster 进程停止时终止自己的进程。
表 4 启动和停止进程
| Process | Start / stop timing |
|---|---|
| logger | Start-up in the case where parameter logging_collector to "on" (default: "off") |
| autovacuum launcher | Start-up in the case where parameter autovacuum to "on" (default: "on") |
| autovacuum worker | Autovacuum launcher process is started with the interval specified by the parameter autovacuum_naptime (default: "1min"); it stops after completing the work |
| archiver | Start-up in the case of stand-alone or in replication environment of the master instance, parameter archive_mode to "on" or "always" (default:"off"). In slave instance of replication environment, start-up only if the archive_mode to "always" |
| postgres (local) | Start with the local connection of the client, and stop on disconnect. |
| postgres (remote) | Start with the remote connection of the client, and stop on disconnect. |
| wal sender | It starts in the master instance of streaming replication environment. Starts when slave instances come to connect, and stops when slave instances disconnect. Starts during the backup by pg_basebackup command, and stop with the completion of the backup. |
| wal receiver | Starts in the slave instance of streaming replication environment. When the master instance is stopped, it automatically stops. Restarts when the master instance is restarted. |
| startup process | Always start in the slave instance of streaming replication environment. |
| wal writer | It does not start in the slave instance of the replication environment. Except for this situation, it always starts. |
| bgworker | Behavior is changed according to the specifications of the custom process. |
| parallel worker | Starting at the time of Parallel Query run, stop at the time of SQL execution is completed. |
- autovacuum worker进程数
从它的进程名称可以看出，每个数据库都启动了 autovacuum worker 进程。启动进程的最大数量由参数 autovacuum_max_workers 决定（默认值：3）。每个工作进程逐个表执行处理。
- postgres进程数
当客户端连接时，Postgres 进程自动启动。 postgres的最大数量受限于参数 max_connections（默认值：100）。没有SUPERUSER权限的一般用户可以连接的连接数是“max_connections ‑ superuser_reserved_connections（默认：3）”的计算结果。以下日志输出超过此限制的连接请求。
示例6 普通用户连接数超标
FATAL: remaining connection slots are reserved for non-replication superuser connections
例子7 超过参数max_connections中指定的连接数
FATAL: sorry, too many clients already
