【从入门到放弃-MySQL】数据库连接过程分析

前言

上周出现了几次连接超时、连接池满还有dbc连接事务模板失败的问题。所以有必要深入了解下MySQL的连接过程。

正好，上周研究了怎么用Clion调试MySQL源码，接下来通过调试来研究一下吧。

服务端

启动

sql/main.cc

123	extern int (int argc, char **argv);int main(int argc, char **argv) { return mysqld_main(argc, argv); }

• main：入口文件，仅调用了mysqld_main函数

sql/mysqld.cc

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int (int argc, char **argv){if (my_init()) { ... } ... if (load_defaults(MYSQL_CONFIG_NAME, load_default_groups, &argc, &argv, &argv_alloc)) { ... } mysqld_socket_acceptor->connection_event_loop(); mysqld_exit(signal_hand_thr_exit_code);}

• mysql_main：MySQL服务端启动逻辑的主要处理函数
• my_init：系统库和线程初始化
• load_defaults：加载my.cnf各参数
• connection_event_loop：循环监听套接字。

sql/conn_handler/connection_acceptor.h

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/** Connection acceptor loop to accept connections from clients.*/void connection_event_loop() { Connection_handler_manager *mgr = Connection_handler_manager::get_instance(); while (!connection_events_loop_aborted()) { Channel_info *channel_info = m_listener->listen_for_connection_event(); if (channel_info != NULL) mgr->process_new_connection(channel_info); }}

• connection_event_loop：通过socket_connection.cc::listen_for_connection_event循环监听，直到有新的连接，开始connection_handler_manager.cc::process_new_connection新连接的处理过程。

  新连接

  服务端一直处于监听状态，当有新连接请求时，调用process_new_connection处理新连接。

sql/conn_handler/connection_handler_manager.cc

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void Connection_handler_manager::process_new_connection( Channel_info *channel_info) { if (connection_events_loop_aborted() || !check_and_incr_conn_count(channel_info->is_admin_connection())) { channel_info->send_error_and_close_channel(ER_CON_COUNT_ERROR, 0, true); delete channel_info; return; } if (m_connection_handler->add_connection(channel_info)) { inc_aborted_connects(); delete channel_info; }}

• connection_events_loop_aborted：先判断是否已取消监听
• check_and_incr_conn_count：再判断（会加锁）是否现有连接数是否大于连接最大值（连接池满），未满，则将线程数加一，满了则拒绝连接。（注意，这里的判断逻辑使MySQL的实际最大连接数是max_connections + 1）
• add_connection：调用add_connection添加连接

sql/conn_handler/connection_handler_pre_thread.cc

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bool Per_thread_connection_handler::add_connection(Channel_info *channel_info) { int error = 0; my_thread_handle id; DBUG_ENTER("Per_thread_connection_handler::add_connection"); // Simulate thread creation for test case before we check thread cache DBUG_EXECUTE_IF("fail_thread_create", error = 1; goto handle_error;); if (!check_idle_thread_and_enqueue_connection(channel_info)) DBUG_RETURN(false); /* There are no idle threads avaliable to take up the new connection. Create a new thread to handle the connection */ channel_info->set_prior_thr_create_utime(); error = mysql_thread_create(key_thread_one_connection, &id, &connection_attrib, handle_connection, (void *)channel_info);#ifndef DBUG_OFFhandle_error:#endif // !DBUG_OFF if (error) { ... //错误处理，略 } Global_THD_manager::get_instance()->inc_thread_created(); DBUG_PRINT("info", ("Thread created")); DBUG_RETURN(false);}

• 调用check_idle_thread_and_enqueue_connection查看是否有空闲的线程，有则将本次连接信息加入等待队列，并给空闲线程发送唤醒信号；否则新建线程处理本次连接
• 在新线程中，调用handle_connection函数开始进行逻辑处理。

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static void *handle_connection(void *arg) { Global_THD_manager *thd_manager = Global_THD_manager::get_instance(); Connection_handler_manager *handler_manager = Connection_handler_manager::get_instance(); Channel_info *channel_info = static_cast<Channel_info *>(arg); bool pthread_reused MY_ATTRIBUTE((unused)) = false; if (my_thread_init()) { ... //错误处理，略 } for (;;) { THD *thd = init_new_thd(channel_info); if (thd == NULL) { ... //错误处理，略 }#ifdef HAVE_PSI_THREAD_INTERFACE if (pthread_reused) { ... //错误处理，略 }#ifdef HAVE_PSI_THREAD_INTERFACE /* Find the instrumented thread */ PSI_thread *psi = PSI_THREAD_CALL(get_thread)(); /* Save it within THD, so it can be inspected */ thd->set_psi(psi);#endif /* HAVE_PSI_THREAD_INTERFACE */ mysql_thread_set_psi_id(thd->thread_id()); mysql_thread_set_psi_THD(thd); mysql_socket_set_thread_owner( thd->get_protocol_classic()->get_vio()->mysql_socket); thd_manager->add_thd(thd); if (thd_prepare_connection(thd)) handler_manager->inc_aborted_connects(); else { while (thd_connection_alive(thd)) { if (do_command(thd)) break; } end_connection(thd); } close_connection(thd, 0, false, false); thd->get_stmt_da()->reset_diagnostics_area(); thd->release_resources(); // Clean up errors now, before possibly waiting for a new connection.#ifndef HAVE_WOLFSSL#if OPENSSL_VERSION_NUMBER < 0x10100000L ERR_remove_thread_state(0);#endif /* OPENSSL_VERSION_NUMBER < 0x10100000L */ thd_manager->remove_thd(thd); Connection_handler_manager::dec_connection_count();#ifdef HAVE_PSI_THREAD_INTERFACE /* Delete the instrumentation for the job that just completed. */ thd->set_psi(NULL); PSI_THREAD_CALL(delete_current_thread)();#endif /* HAVE_PSI_THREAD_INTERFACE */ delete thd; // Server is shutting down so end the pthread. if (connection_events_loop_aborted()) break; channel_info = Per_thread_connection_handler::block_until_new_connection(); if (channel_info == NULL) break; pthread_reused = true; if (connection_events_loop_aborted()) { ... //错误处理，略 } } my_thread_end(); my_thread_exit(0); return NULL;}

• 会对连接进行thd_prepare_connection预处理操作，没问题后继续下面的逻辑。
• 当连接未被关闭，就会一直do_command处理请求。
• 当连接关闭，则走下面关闭逻辑

执行

sql/sql_parse.cc

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bool do_command(THD *thd) { bool return_value; int rc; NET *net = NULL; enum enum_server_command command; COM_DATA com_data; DBUG_ENTER("do_command"); DBUG_ASSERT(thd->is_classic_protocol()); /* indicator of uninitialized lex => normal flow of errors handling (see my_message_sql) */ thd->lex->set_current_select(0); /* XXX: this code is here only to clear possible errors of init_connect. Consider moving to prepare_new_connection_state() instead. That requires making sure the DA is cleared before non-parsing statements such as COM_QUIT. */ thd->clear_error(); // Clear error message thd->get_stmt_da()->reset_diagnostics_area(); /* This thread will do a blocking read from the client which will be interrupted when the next command is received from the client, the connection is closed or "net_wait_timeout" number of seconds has passed. */ net = thd->get_protocol_classic()->get_net(); my_net_set_read_timeout(net, thd->variables.net_wait_timeout); net_new_transaction(net); /* Synchronization point for testing of KILL_CONNECTION. This sync point can wait here, to simulate slow code execution between the last test of thd->killed and blocking in read(). The goal of this test is to verify that a connection does not hang, if it is killed at this point of execution. (Bug#37780 - main.kill fails randomly) Note that the sync point wait itself will be terminated by a kill. In this case it consumes a condition broadcast, but does not change anything else. The consumed broadcast should not matter here, because the read/recv() below doesn't use it. */ DEBUG_SYNC(thd, "before_do_command_net_read"); /* Because of networking layer callbacks in place, this call will maintain the following instrumentation: - IDLE events - SOCKET events - STATEMENT events - STAGE events when reading a new network packet. In particular, a new instrumented statement is started. See init_net_server_extension() */ thd->m_server_idle = true; rc = thd->get_protocol()->get_command(&com_data, &command); thd->m_server_idle = false; if (rc) { ... //错误处理，略 } char desc[VIO_DESCRIPTION_SIZE]; vio_description(net->vio, desc); DBUG_PRINT("info", ("Command on %s = %d (%s)", desc, command, command_name[command].str)); DBUG_PRINT("info", ("packet: '%*.s'; command: %d", thd->get_protocol_classic()->get_packet_length(), thd->get_protocol_classic()->get_raw_packet(), command)); if (thd->get_protocol_classic()->bad_packet) DBUG_ASSERT(0); // Should be caught earlier // Reclaim some memory thd->get_protocol_classic()->get_output_packet()->shrink( thd->variables.net_buffer_length); /* Restore read timeout value */ my_net_set_read_timeout(net, thd->variables.net_read_timeout); return_value = dispatch_command(thd, &com_data, command); thd->get_protocol_classic()->get_output_packet()->shrink( thd->variables.net_buffer_length);out: /* The statement instrumentation must be closed in all cases. */ DBUG_ASSERT(thd->m_digest == NULL); DBUG_ASSERT(thd->m_statement_psi == NULL); DBUG_RETURN(return_value);}

• 主要的处理逻辑为dispatch_command，根据不同的command类型进行分发。

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/** Perform one connection-level (COM_XXXX) command. @param thd connection handle @param command type of command to perform @param com_data com_data union to store the generated command @todo set thd->lex->sql_command to SQLCOM_END here. @todo The following has to be changed to an 8 byte integer @retval 0 ok @retval 1 request of thread shutdown, i. e. if command is COM_QUIT*/bool dispatch_command(THD *thd, const COM_DATA *com_data, enum enum_server_command command) { ... //太长不看 switch (command) { case ... //太长不看 case COM_QUERY: { ... //太长不看 mysql_parse(thd, &parser_state); ... //太长不看 DBUG_PRINT("info", ("query ready")); break; } case ... //太长不看 default: my_error(ER_UNKNOWN_COM_ERROR, MYF(0)); break; }}

• 主要看COM_QUERY这个逻辑，我们要用到的DDL、DML都会走这个流程，这个流程中主要是调用mysql_parse方法

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/** Parse a query. @param thd Current session. @param parser_state Parser state.*/void mysql_parse(THD *thd, Parser_state *parser_state) { ... //太长不看 mysql_reset_thd_for_next_command(thd); if (!err) { err = parse_sql(thd, parser_state, NULL); ... //太长不看 } if (!err) { mysql_rewrite_query(thd); ... //太长不看 } if (!err) { ... error = mysql_execute_command(thd, true); ... }}

• 主要是SQL语法解析和执行
• mysql_reset_thd_for_next_command是对下一次执行做准备，重置线程各变量
• mysql_rewrite_query看着像是SQL优化？待定 还没追进去，记个TODO
• 词法解析前不应该有缓存吗？没有找到缓存的逻辑，记个TODO（后续：原来MySQL8.0取消了query cache，详见：https://mysqlserverteam.com/mysql-8-0-retiring-support-for-the-query-cache/）

关闭连接

sql/conn_handler/connection_handler_pre_thread.cc

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Channel_info *Per_thread_connection_handler::block_until_new_connection() { Channel_info *new_conn = NULL; mysql_mutex_lock(&LOCK_thread_cache); if (blocked_pthread_count < max_blocked_pthreads && !shrink_cache) { /* Don't kill the pthread, just block it for reuse */ DBUG_PRINT("info", ("Blocking pthread for reuse")); /* mysys_var is bound to the physical thread, so make sure mysys_var->dbug is reset to a clean state before picking another session in the thread cache. */ DBUG_POP(); DBUG_ASSERT(!_db_is_pushed_()); // Block pthread blocked_pthread_count++; while (!connection_events_loop_aborted() && !wake_pthread && !shrink_cache) mysql_cond_wait(&COND_thread_cache, &LOCK_thread_cache); blocked_pthread_count--; if (shrink_cache && blocked_pthread_count <= max_blocked_pthreads) { mysql_cond_signal(&COND_flush_thread_cache); } if (wake_pthread) { wake_pthread--; if (!waiting_channel_info_list->empty()) { new_conn = waiting_channel_info_list->front(); waiting_channel_info_list->pop_front(); DBUG_PRINT("info", ("waiting_channel_info_list->pop %p", new_conn)); } else { DBUG_ASSERT(0); // We should not get here. } } } mysql_mutex_unlock(&LOCK_thread_cache); return new_conn;}

• 如果阻塞的线程数小于最大阻塞线程数，则此线程不回收，而是进入阻塞状态（等待），等待新连接来的时候重复使用。
• 否则关闭线程。

客户端

【从入门到放弃-MySQL】数据库连接过程分析-客户端

参考文献：
https://www.cnblogs.com/FateTHarlaown/p/8676166.html

原文:大专栏  【从入门到放弃-MySQL】数据库连接过程分析

来源：https://www.cnblogs.com/wangziqiang123/p/11645271.html